In Episode 6 of the Shedding Light on Motion series, Newton’s First Law, presenter Spiro Liacos is “thrown forward” in a head-on collision, “thrown backwards” when his tram takes off, and “thrown to the side” when his car suddenly turns a corner. But in fact none of these things actually happen! Using brilliant visuals, this episode looks at the fact that an object will remain stationary or move with a constant velocity unless a force acts on it. It also describes a number of different forces that affect our lives daily.
The Shedding Light on Motion series is a visual treasure trove of demonstrations, animations, and explanations of all things motion! To an extent we’re all familiar with motion because we all move and we see movement everywhere, but a detailed knowledge of motion has allowed us to build the wonderful modern world that we live in.
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The Transcript (which can be used as a textbook)
Part A: Introduction.
Part B: Force For Change: Why are we “thrown forward” when our car stops suddenly? Why are all modern cars fitted with head restraints? Why are we thrown to the left when our car turns to the right? (Well, we’re not actually but it certainly seems that way!) And what exactly is an unbalanced force?
Part C: May the Forces Be With You. At any given moment lots and lots of forces are acting on you. Some are normal, but there’s friction among them and some show a fair bit of resistance. Never-the-less, they can also give you a lift until you’re feeling buoyant.
Part D: Multiple Forces. So how do we make sense of all the different forces acting on us? In Part D, we’ll point you in the right direction.
Part A: Introduction
We know that things can speed up and slow down. But what makes things speed up or slow down? What makes things change direction or change shape? Well, the answer is “forces”. A force is basically a push or a pull.
If I jump off a 10-m diving platform and accelerate downwards, there has to be a force acting on me. In this case it’s the Earth’s gravity. When athletes accelerate at the start of a sprint, the force causing the acceleration comes from their muscles. And when a car comes to a stop, the force causing the deceleration comes from friction.
About 300 years ago, the famous English scientist Sir Isaac Newton developed three Laws of Motion
which together can help us explain how forcesaffect the way things move. In this video, we’re going to look at Newton’s First Law of Motion and look at some examples of common forces that affect our lives daily.
Part B: Force For Change
Newton’s First Law states that: An object will remain at rest or move in a straight line at a constant speed unless acted upon by an unbalanced force.
It says, basically, that nothing’s going to speed up or slow down or change direction unless a force acts on it. It’s pretty simple really but it governs the movement of every single thing in the universe.
According to Newton’s First Law, this ball, which is at rest, will continue to remain at rest, forever, and ever unless a force makes it accelerate. Why should it ever accelerate if some sort of force doesn’t act on it? And if something is moving, it’s going to keep on moving at the same speed and in the same direction until a force makes it change its speed or direction.
Let’s look at this collision where I’m standing on a skateboard which is on a trolley that is moving at about 2.5 m/s. To track the movement, I can place small dots on every second frame of the video on my head, on the front wheel of the skate board and in the middle of the trolley. When the trolley hits the mat, the mat applies a force on it and it slows down. The dots here are clearly closer together. However, the mat hasn’t applied a force on me or on the skateboard so we keep moving at the same speed that we were moving before the trolley hit the mat, and you can tell because the dots are still evenly spaced. Newton’s First Law in action: why should something slow down if a force doesn’t act on it. Then the skateboard and my lower legs hit the mat, and the mat applies a force on them and they too slow down; see how the dots here are closer together, indicating a reduced speed. But the mat hasn’t applied a force on my head, so my head, once again, keeps moving at the same speed as before, although then it starts falling because it isn’t being supported by my body anymore. It finally slows right down when it hits the mat.
We often say that we are “thrown forward” when our car stops suddenly. But are we really thrown forward? In a head on collision, the barrier in this case applies a huge force on the car which slows the car down, but that doesn’t mean that there’s a huge force on the occupants. Apart from a little bit of friction between the occupants’ legs and bottom and the seat, there’s basically no force acting in the horizontal direction on the occupants. They therefore keep moving forward at the same speed that they were travelling at before the collision, until, that is, a force acts on them as well. In this case it’s the force of the seatbelt and the air bag.
I can simulate a car crash on my trolley. Here the trolley slows down when the crash mat applies a stopping force on it, but there’s very little sideways force on me, just a little bit of friction, so even after the trolley starts slowing down, I continue moving forward at more or less at the same speed as before the collision until, of course, a force does act on me.
Seatbelts and air bags, are designed to provide a stopping force that slows the occupants down as slowly as possible and to spread that force over a wide part of the body. Without these safety measures, unrestrained occupants in a car that’s crashing continue to move forward with a constant velocity until a force acts on them when they hit, for example, the windscreen, the dashboard or other occupants, which can cause severe and unfortunately all too often lethal injuries.
Now from the car’s frame of reference, it does look as if things are thrown forward, and there’s nothing wrong with the expression “thrown forward” but it’s just a little misleading. We actually keep moving at the same speed until a force slows us down. We’ll look at seatbelts and other car safety features in a little more detail in our next episode.
This basketball seems to mysteriously start rolling forwards when the trolley slows down, but if I use dots to trace the path of the ball and the path of the trolley, you can see that even though the trolley starts slowing down, the ball travels at more or less the same speed until it hits the blue rubber divider, because until that time there is only the very very small force of friction acting on it.
The same thing happens in say a tram which suddenly slows down. It seems that we get pushed forward, but we’re not really pushed forward. When the trolley here slows down, my feet slow down as well because there’s friction between my shoes and the floor of the trolley, but there’s not much of a sideways force on my upper body, so it keeps going until it too experiences a force.
Leaning against the deceleration provides a sideways force as well as upwards force. A force that is applied upwards at an angle is applying both an upwards force and a sideways force, and the force is transferred from bone to bone all the way up to your head.
Newton’s First Law also applies when a tram accelerates, and we supposedly “fall backwards”. This is what the same situation looks like from outside the tram. Once again, Newton’s First Law says that nothing’s going speed up unless a force acts on it. Here the floor applies a force on my feet (because of friction), and so my feet accelerate with the vehicle. However, my upper body stays more or less stationary because there’s no sideways force acting on it. To maintain balance I instinctively walk backwards relative to the trolley, to try to get my feet back underneath the rest my body. When you’re in an accelerating vehicle it feels like you’re going backwards, and you are relative to the vehicle, but if I mark my starting position and my finishing position, you can see that relative to the Earth, I actually moved forward a little bit.
From the frame of reference of the trolley though, it does look as if I move backwards. If, while trying to regain your balance, something like a bag stops you from walking backwards, that is walking backwards relative to the vehicle, your feet basically get pulled out from underneath you, and relative to the vehicle you can actually fall backwards.
This basketball rolls backwards relative to the trolley as the trolley accelerates. However, relative to the Earth it doesn’t move backwards at all. It just doesn’t accelerate as much as the trolley does because there’s very little force acting on it.
Here, when the skateboard is pulled, my feet start moving to the side, because there’s a sideways force on them, but because there’s very little sideways force on my head, well, the sideways force is practically zero, my head moves very little, even 0.4 seconds later.
In a similar kind of way, if your car is hit from behind, your body is pushed forward by the seat, but your head stays more or less where it was before the impact—Newton’s First Law. Despite this car having moved forward some 40 cm or so, the crash-test dummy’s head doesn’t start moving until about now.
The sudden forward movement of the body relative to the head can cause an injury that is called whiplash, where any number of the ligaments that hold your neck bones together can be torn. Whiplash injuries used to be really common until these head restraints or headrests started being installed in cars. The headrests prevent the head from lagging too far behind the rest of the body. You have to make sure though, that they’re adjusted properly.
When people are boxing, a sudden blow to the head accelerates the skull, but the brain kind of gets left behind if the skull accelerates too quickly. The skull crashes into the brain and can cause severe and even permanent injuries. I can simulate a brain with a yellow balloon. Punching the skull forces it sideways, but the brain stays where it is and the skull crashes into it. Our brain is actually cushioned really well inside our skulls with fluids and connective tissues but there’s a limit to how much it can take.
Now when a car suddenly turns to the left everything seems to move towards the right. But does it really move towards the right? The trolley that I’m standing on is moving straight towards the camera, but it’s about to turn suddenly to its left. Will I be thrown to the right? Well, no. I actually keep moving in a straight line, although relative to the trolley I did move to its right.
What if I’m sitting in a chair as if I’m in a car? Well, once again, relative to the Earth I moved more or less in a straight line, and only relative to the trolley did I move to the right.
Newton’s First Law says that in order to speed up or slow down, or change direction, a force needs to be applied. The trolley turns because acting on it there’s a sideways force coming from the boy pushing the trolley and from the sideways friction between the wheels and the floor. However, there’s very little sideways force on me, so I just keep going in more or less a straight line.
In fact Newton’s First Law is often described by saying that things resist changes in their motion and just try to keep doing what they’re doing. It’s not actually a bad way of thinking about it.
When the trolley here turns to the right, the ball rolls to the left relative to the trolley. However it actually moves to the right, just not as much to the right as the trolley. Compare the path of the ball with the white line.
It certainly doesn’t seem that way though when you’re in a car. From inside the car it seems that you are thrown outwards and that you hit the window when the car turns suddenly. What actually happens, from the Earth’s Frame of Reference, is that your head (represented by the yellow circle of course), is moving kind of in a straight line and the car moves towards you until it hits your head and forces it to change direction!
So it’s only relative to the vehicle that you’re in that you get thrown forwards, or backwards, or to the side when a vehicle changes its velocity. When you make your observations from outside the vehicle you can see what’s actually going on.
So once again, Newton’s Law states that an object will remain at rest, or move in a straight line at a constant speed unless acted upon by an unbalanced force. But what exactly is an unbalanced force?
Well here, the Earth was pulling me downwards and I was accelerating. Here, though, I’m not accelerating, but that doesn’t mean that there are no forces acting on me. The Earth is still pulling me downwards, so why am I not accelerating downwards? In this situation, the floor I’m standing on must be providing an upwards force. The upwards force is called the “normal” force (FN) (that’s its actual name), normal in this case meaning perpendicular, or at right angles to the surface.
The normal force comes as a result of the atoms in the floor or the ground being compressed slightly and so they naturally push back up against the thing that’s pushing down on them. If I step up onto this playground see-saw, the atoms of the spring get pushed together slightly and so the spring naturally tries to spring back to its original shape. The force of gravity and the normal force are now balanced and so I’m not accelerating.
Here, the normal force comes as a result of the timber bending a little and so it naturally tries to spring back to its original shape. The force of the Earth pulling me downwards (the force of gravity in other words) is exactly the same size as the force of the timber pushing me upwards (the so-called normal force) and since the two forces are acting in the opposite direction, they balance out and so I’m not accelerating. In order to accelerate, an unbalanced force has to act. An unbalanced force can come from a ball, someone pushing me, or even from my own muscles.
The same kind of thing applies to these spring. As I hang weights on the springs, they stretch until they provide an upwards force that balances out the force of gravity. Since the weights aren’t accelerating, the force of gravity pulling downwards, must be exactly the same size as the force of the spring pulling them upwards. However, I should just point out that the force provided by the springs is not called the normal force.
Now you don’t normally think of concrete as being stretchy, but everything is stretchy, even concrete, so when you stand on concrete, or on any hard surface, it provides an upwards force, the normal force, that balances out gravity.
However, if the thing you’re standing on can’t provide a big enough normal force to balance out the force of the Earth’s gravity, then you will accelerate downwards.
In this case, the downwards force of gravity on the trolley was counteracted by the upwards normal force of the floor on the trolley. The two forces were balanced and therefore didn’t affect the velocity of the trolley at all. However, the mat provided a sideways force on the trolley which was unbalanced and therefore it slowed the trolley down. But there was no unbalanced force on me so I kept moving with the same velocity until an unbalanced force acted on me as well.
Let’s now actually look at some common forces and at how they affect things.
Part C: May the Forces Be With You.
Gravity is the force that pulls everything downwards. The force of the earth’s gravity on a 1 kg dumbbell is fairly small. On a 5 kg dumbbell it’s much greater, five time greater in fact. On a 10 kg dumbbell, it’s greater still.
No-one actually knows exactly how gravity works, but with the knowledge that we have, we’ve been able to build huge planes, launch satellites, build space stations, and send space probes to other planets.
Getting back to the dumbbells, when resting on the seat, the seat is applying a normal force upwards that is equal in size to the pull of Earth’s gravity downwards. A larger mass experiences more gravitational force than a smaller mass, and therefore requires a greater normal force to support it.
Since the size of a force and the direction that it acts in are both important, we typically represent forces in diagrams with arrows. The length of the arrow represents the size of the force and the direction of the force is of course indicated by the direction of the arrow. To simplify things we typically draw the forces acting from the “centre of mass” of the object, which is kind of the average point of all the atoms in the object.
Friction is the force that acts between two surfaces that are in contact with one another. After the puck is struck it slides along the floor but comes to rest fairly quickly. This is because of friction. A rolling ball has far less friction and so it will travel much further than a sliding puck.
Friction is caused by the fact that at the microscopic level, surfaces are rough. As one surface slides over the other, the rough bits smash into each other and tend to slow the object down. Friction always acts in the opposite direction to the direction that something is moving, or trying to move.
This puck is stationary and there are two forces acting on it: the force of the Earth’s gravity pulling it downwards and the normal force of the floor pushing it upwards. Both of these forces are balanced. As I strike the puck, there are three forces acting on it. The force of the stick on the puck is unbalanced, so this force makes the puck accelerate in the direction of the force. However, as soon as the puck starts moving, friction acts against the movement of the puck.
Once the stick and the puck are no longer in contact, the stick is obviously not applying any more force to the puck, but we now have the force of friction acting in the opposite direction to the direction that the puck is moving. The unbalanced frictional force slows the puck down. If there was no friction, the puck would slide forever! Newton’s First Law. Why should anything slow down if a force doesn’t act on it?
A space probe in space experiences no friction at all, so it just keeps moving forever. Rockets are needed to get it moving in the first place of course, but once the rockets have run out of fuel and therefore can’t provide any more force, the space probe just keeps going and going and going because there is no force acting on it to slow it down.
FUN FACT: Satellites that orbit the Earth also maintain more-or-less a constant speed as they orbit. However, even at the height of the International Space Station, about 400 km above the Earth’s surface, there’s a tiny tiny amount of atmosphere which slows the ISS down. Every so often, small rocket engines on the ISS have to fire to boost the Station’s speed.
The amount of friction between two surfaces depends on a number of factors. Firstly the type of materials in contact plays a big role. The friction between my cotton socks and a polished wooden floor is fairly low, but the friction between the rubber soles of my shoes and the polished wooden floor is quite large.
The amount of friction also depends on the surface area that is in contact. A fat racing car tyre has much more friction than a normal car tyre since more rubber is in contact with the road surface. This allows the racing car to turn faster without skidding off the track and to accelerate without spinning the wheels. The friction between shoes and the floor or between tyres and the road is often called “grip”. Rubber tyres have quite good grip because the rubber sinks down into the rough road surface, making it difficult for the rubber to slide. If I really really exaggerate, it’s kind of like this. There’s no way this tyre is going to slip.
It’s very similar to what gives football boots extra grip, since the studs sink down into the soil.
Friction also depends on the size of the force pushing the surfaces together. I can measure the amount of friction between the block of wood and the table surface by pulling on the wood with a spring balance. The friction between the two surfaces holds the wood in place until the sideways force of the spring balance moves it. The block of wood starts sliding when the reading on the spring balance is about 50 grams.
If I load up the block of wood with some weights, the two surfaces get pressed together more and friction increases. I now need to apply a larger sideways force to overcome the friction. The reading here is about 80 grams when the block starts moving. When I place even more weights onto the block of wood, even more sideways force is required to overcome friction.
Racing cars get extra grip on the track, not by carrying extra weight, extra weight would make it harder for the car to accelerate, but rather by using spoilers. These are like upside down wings which push the car down onto the road, which as I said results in a much better grip.
So, quite obviously friction can be very useful. The friction between the brake pads and the wheels of a bike allow the bike to stop. The friction between our shoes and the floor or ground allows us to walk or to run without slipping. When a vehicle turns, it’s the friction acting between the road and the tyres that pushes the car in the direction of the turn, in this case to its left. Changing direction on ice, where there’s very little friction, is very difficult, so as I said, friction can be very useful.
However, friction can also be troublesome because it often restricts the movement of things that you don’t want restricted. Wheels reduce friction because they roll. In simple wheel set ups the wheel slides on the axle, which is this piece of steel holding the wheel. Most wheels though, on cars for example, are attached to the axle with ball bearings. The steel surfaces don’t slide past each other,they roll along either on rollers or on little steel balls.
Motor oil is a “lubricant” which helps reduce friction in an engine. The motor oil gets in between all the parts that are rubbing together and so they move more easily and they don’t wear out as much. Water on the road acts as a lubricant so cars are more likely to slide out of control in wet conditions. If water gets trapped under the tyre as it rolls along, what’s called “aquaplaning” or “hydroplaning” occurs, where the car kind of skis along the surface of the water, much like a water skier does.
The tread on tyres allows water to squeeze out to the sides as the tyre rolls along, giving the tyres better grip and so the risk of aquaplaning decreases, but it’s still always a possibility if you’re driving too fast for the conditions. Driving in the wet with what are called bald tyres where the tread has worn away too much results in aquaplaning even at very low speeds. This tyre is totally unroadworthy.
Formula 1 cars use what are called “slicks”, that is, untreaded tyres, only in dry weather. Slicks increase the surface area of the rubber that is in contact with the road and so increase the grip of the tyre. However, slicks are completely useless when the track is wet. When the track is wet, treaded tyres are fitted to the cars.
Another force that we come across every day is air resistance which is also called drag. Air resistance is a little like friction in that it typically acts in the oppose direction to the movement of the object. It’s caused by the fact that as something moves through the air, it’s colliding with the trillions of particles that make up the air which each exert a small force on the object.
The shuttlecock here initially moves quite quickly, but it slows down because of air resistance. If we trace the path of the shuttlecock, it’s really obvious.
Air resistance is quite similar to water resistance. It’s hard to run through water, because as you move forward, you have to push the water out of the way, but at the same time the water pushes back on you and slows you down. Generally speaking, a larger cross-sectional area experiences more air resistance.
Because of air resistance, aeroplanes and rockets (and other vehicles) are always kind of pointy. We say that they are “streamlined”. We also use the word streamlined for things like sharks and dolphins.
Air resistance is obviously affected by the shape of an object, but it’s also affected by an object’s speed. At low speeds, air resistance is quite low. It increases, however as you get faster and faster. On this particular day, there was an 80 km/hr wind blowing towards the right of screen. The air resistance when I was trying to ride into the wind was huge.
A graph of air resistance vs velocity looks something like this. As velocity increases, air resistance increases, but doubling your velocity doesn’t result in a doubling of air resistance, the air resistance increases by a factor closer to 4, and tripling your velocity increases air resistance by a factor of about 9! I haven’t got any numbers on the graph because the actual force of air resistance is different for any given object.
The force that lifts an aeroplane as a result of the airflow over its wings is called “lift”. Wings are pretty complicated, but basically, as they move through the air at a slight angle to the direction of the plane, the air hits them from underneath and lifts them up. This wing design is being tested in a wind tunnel. Smoke trails pass over the wing and scientists can study the way the air flows over it. Though even a flat board can act as wing, wings typically have a curvy shape because the lift is improved and drag is reduced.
The force generated by a jet engine or a rocket engine is called thrust. For a vertical rocket to accelerate upwards, the thrust has to be greater than the downwards force of gravity. The word thrust is also often used for the forward force that drives any vehicle in the forward direction.
Buoyancy is the force that pushes things upwards when they’re in the water. The famous Ancient Greek scientist Archimedes worked out the principle behind buoyancy more than 2000 years ago. What we now call the Archimedes Principle states that the upwards buoyant force on something wholly or partially submerged in water is equal in size to the weight of the water displaced.
When something is immersed in water, it displaces an amount of water that is equal to the object’s volume. The volume of water that flowed out of the beaker here is equal to the volume of my fist. Let’s now actually make some measurements though.
If I place this retort stand and clamp onto a set of electronic scales, and switch on the scales, they read zero. When I hang the brass weight onto the clamp, the scales read 241 grams, which is actually the mass of the brass weight. Physically lifting the weight myself provides an upwards force on it, and the scales of course give a smaller reading.
Now if I place a beaker of water under the brass weight and lift it up, the scales once again produce a smaller reading. The water is providing an upwards force, which as I said is called buoyancy, which seems to reduce the weight of the brass weight. Once fully submerged, the reading on the scales is now only 213 grams, 28 grams less than before. So what does this have to do with the volume of the brass weight? Let’s see. It has a diameter of 3 cm which is a radius of 1.5 cm and a length of 4 centimetres.
The volume of a cylindrical prism (that’s what the brass weight is: a cylindrical prism) is given by the formula V = ?r2h, so the brass weight has a volume of 28 cm3. This is the same number as the difference in reading on the scales that we saw a few seconds ago, which is not a co-incidence! Let’s keep going. The 28 cm3 brass weight should displace 28 cm3 of water when it’s immersed in water. Let’s see if it does.
If I fill up this so-called overflow beaker until it can’t hold any more water, and then lower the brass weight into it, the water overflows into the measuring cylinder, and lo and behold, reading off the
bottom of the curved surface of the water, which is how measuring cylinders are designed, we can see that 28 cm3 of water were displaced.
Now the density of water is 1 gram/cm3, so 28 grams of water has a volume of 28 cm3. In fact, earlier I placed the measuring cylinder onto the scales which registered 84 grams, but the combined mass of the measuring cylinder and 28 cm3 of water is 112 grams, so if we do some simple mathematics it’s pretty clear that 28 cm3 of water has a mass of 28 grams.
This is the exact difference between what the spring balance registers for the brass cylinder in air and in water.
So, I’ll go over the Archimedes Principle one more time. If a 28 cm3 object is immersed in water, it will displace 28 cm3 of water. Water has a density of 1 g/cm3, so this volume of water has a mass of 28 grams. Since gravity is pulling on the water that’s been lifted up, it kind of pushes back up on the object. A buoyancy force equivalent to the weight of 28 grams of water will push up on the object, and so it looks as if the object has lost 28 grams. The upwards force of buoyancy that an object experiences is equal to the weight of the water that it displaces.
If the upwards force of buoyancy is greater than the downwards force of gravity, then the object will float. This occurs for any object that has a density of less than 1 g/cm3. However, just to be really accurate, a floating object will rise up out of the water until the force of buoyancy upwards is exactly equal to the force of gravity downwards.
If the force of gravity is greater than the force of buoyancy, the object will sink. This occurs for anything that has a density of greater than 1 g/cm3, like glass or plasticine or aluminium or brass. However, if you put air into the material, the overall density may end up being less than 1. It displaces more water and so it floats. Ships are made of steel, which has a density of about 8 g/cm3, but because of their shape the weight of the water they displace is equal to their own weight and so they float. Their overall density if you include all the air in the hull is actually less than 1!
The density of sea water is about 1.025 g/cm3, which means that for a given volume, there is more weight. Ships therefore float slightly higher up in sea water than they do in fresh water (like rivers and most lakes), since in sea water there is increased buoyancy.
Now while many forces have names like buoyancy, lift, gravity, thrust, the normal force, friction, and air resistance (or drag), we often simply describe a force by naming the object providing the force and the object that the force is being applied on.
For example, here we might label the force as Ffoot on ball, while here we might label the force of one car on the other as Fcar A on car B. We can even use this labelling system with forces that have names. Here we might label the force of gravity as Fearth on man, and here the force of drag as Fair on woman.
Now we saw earlier that when a vehicle turns one way, things seem to get pushed to the side the other way. They don’t really get pushed to the side, they initially travel kind of straightish, but this fictitious force seems so real inside the reference frame of the turning vehicle, that it’s actually been given a name: it’s called centrifugal force. There’s actually no such thing as centrifugal force because you’re not actually being forced to the opposite side by anything, but centrifugal force does exist as a concept because it seems so real when things are flying around from side to side. The actual force that pushes you inwards once you’ve (for example) hit the window, or rather, when the window has hit you is called centripetal force.
Part D: Multiple Forces
So, now that we know about Newton’s First Law, and about the way lots of different forces act, let’s look in a little more detail at some situations where multiple forces act on an object at the same time.
When you’re stationary on the ground, there are two forces acting on you: the force of gravity pulling you downwards, and the normal force of the ground pushing you upwards. These two forces are balanced and so you don’t accelerate at all. You remain stationary. I won’t draw them in from now on because they’re not going to change at all.
To accelerate, an unbalanced force has to act on you. When you are speeding up there is a forward force coming from your muscles, which transfers through the various bike parts down to the ground. This forward force of thrust is unbalanced and that’s why you accelerate. However, friction and air resistance, which I’ll put together in this explanation, are nearly always present and they stop you from accelerating as quickly as what you would have accelerated if they weren’t around. As we saw earlier, as you get faster and faster the air resistance increases.
When you get to a constant speed that you’re comfortable with, you reduce the size of the thrust so that it exactly matches air resistance and friction. Since after that the forwards force of thrust and the combined backwards force of friction and air resistance are the same size and therefore balanced, you don’t accelerate anymore. You just maintain a constant speed.
Now this point sometimes confuses students, who say, “if the forward force and the backward force are equal, shouldn’t the bike not move?” Well, no. At some point, to get to that speed, the forward force had to be greater than the force of air resistance, but Newton’s First Law tells us that when all the forces are balanced, the bike doesn’t stop moving, it just doesn’t get any faster or any slower. In other words, it doesn’t accelerate and it continues moving with the same constant velocity that it had when the forces became balanced.
When you decide to slow down, you can stop pedalling and let air resistance and friction slow you down, or you apply the brakes which increases friction even more so that you slow down more quickly.
The same kind of thing applies to an aeroplane. When it accelerates as it takes off, the forward force of thrust has to be greater than the force of air resistance. When a plane is in level flight and it’s moving with a constant velocity, the thrust of the engines has to be equal to the force of air resistance. If the thrust was greater the plane would speed up and if the air resistance was greater it would slow down. Of course the force of lift upwards also has to be equal in size to the force of gravity downwards.
When a plane is slowing down like it does when it’s landing, the thrust has to be smaller than the force of air resistance. Diagrams like these ones are called free-body diagrams. If the free-body diagram shows all the forces acting they can be used to determine which way an object will accelerate.
Now why is it that when I accelerate the trolley slowly, the basketball moves off with the trolley, but when I accelerate quickly, the basketball rolls backwards relative to the trolley? And why is it that when I pull on this tablecloth slowly, the dinner set moves with the table cloth, but when I pull on the tablecloth quickly, the dinner set stays more or less where it is?
Newton’s First Law tell us that forces are needed to accelerate things, but it doesn’t tell us how much force is required to accelerate any given object by a given amount. It doesn’t tell us for example how much force is required to accelerate the basketball at the same rate as the trolley, or to accelerate the dinner set at the same rate as the tablecloth.
Newton’s Second Law does and it’s what we’ll be looking at in our next episode. See you then.
A Note about the word “Inertia”.
You may have heard of the word “inertia”, which sometimes gets mentioned in relation to Newtons’ First Law. The word inertia comes from the Latin word inert which, as we saw in our Relative Motion program, means unchanging.
Inertia is defined either as:
- The tendency of an object to remain at rest or to move with a constant velocity unless a force acts on it (in other words the tendency to resist changes in its motion) OR
- a property of the object itself, which gives it an ability to resist changes in its motion.
So, sometimes people say “an object keeps going because of inertia” (definition A), while other times they say “an object keeps moving because it has inertia” (definition B).
Because of this ambiguity, I decided to not mention inertia in the video and simply rely on the basic description that an object will remain at rest or move with a constant velocity unless acted upon by an unbalanced force. (In other words, why would anything speed up or slow down or change direction if a force doesn’t act on it?)
Once we get into Newton’s Second Law, we will see that the more mass something has, the harder it is to accelerate it, so I prefer to use the word mass, not inertia.
However, if I can talk SERIOUS Physics for a moment, something that has lots of mass is difficult to accelerate (more about this in Episode 7 of our series) and produces a greater gravitational field. Scientists often therefore talk about “inertial mass” when they’re talking about mass and acceleration and “gravitational mass” when they’re talking about a mass’s gravitational field. And get this: inertial mass and gravitational mass are directly proportional to each other. The more inertial mass an object has, the more gravitational mass an object has. No-one really knows why this is the case!
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