AP®︎/College Physics 1
Course: AP®︎/College Physics 1 > Unit 3Lesson 6: Free-body diagrams for objects in uniform circular motion
Identifying centripetal force for cars and satellites
Identifying forces or force components acting as the centripetal force for a car driving in a horizontal circle, a car driving in a vertical circle, and a satellite in orbit.
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- At2:19, shouldn't the frictional force point in the opposite direction of the velocity?(33 votes)
- The frictional force is opposite to whatever force that is causing motion. In this case, it is the force of moving [a point] on the wheel of the car. To rotate the wheels, the wheels need to exert a force on the road, and to resist this motion (Newton's 3rd Law), the road pushes back on the wheel in the opposite direction. Therefore the car moves.(22 votes)
- At8:07, if the only force in the y-direction is downwards then why doesn't the car fall down?(16 votes)
- The car does accelerate downward at the moment, however after this acceleration downward, at the very next moment, the normal force is at a different angle, perpendicular to the surface. Due to this, the car continues in a circular motion. If the very top of the circle is the end of the car's path, then it will fall down.(12 votes)
- Why is there friction going forwards? I don't understand that part in the free body diagram? Isn't friction usually opposite of the motion?(13 votes)
- how can air friction be counteracted by frictional force ,and why is friction on the direction of motion?(5 votes)
- When a human is walking she is pushing on the ground or, more precisely, she is pushing the ground backward. Actually, you could go as far as to say that she is pushing the ground backward while trying to make her foot slide back... but the friction force prevents this from happening!
That's why friction force is exerted on the opposite direction making her body accelerate forward.
The situation is a bit more complicated when talking about wheels but the principle is the same.
About the second question, it is safe to assume that air friction slows you down no matter where you are going (the soundest argument for me is to say that you convert a fraction of your kinetic energy to heat).
So, if you want to keep moving through air, you need at the very least to counteract its tendency to slow you down. If i did a good job convincing you that friction is in the direction of motion (obvious exceptions apply, i can explain this further if you want me to) then it's now clear that frictional force actually counteracts air friction.(8 votes)
- At4:50, where does the extra Fn comes from?(6 votes)
- The normal force Fn is provided by the loop's surface itself. If you're wondering why/how is it larger than Fg, I guess it's because the 'loop-y ' surface is pushing the car towards the center of the circle at every point more than gravity is pulling it downwards.(2 votes)
- If the direction of the force of friction is always opposite to the direction of motion, why does it point inwards to the center of the circle?
The car is clearly not moving opposite to this force, isn't it?2:50(3 votes)
- the cars tyres use the frictional force to go forward, and if you turn the wheels left the frictional forces with the road will push you left- or if you turn the wheel towards the centre of the circular path the frictional forces will be the net force/centripetal force.(4 votes)
- Are we assuming that the cars' engines are turned off? So they are not exerting any driving force?(4 votes)
- We don't count the car's engine as a force being applied.(0 votes)
- At the top of the loop de loop, Both the normal force and the gravitational force are acting towards the center of the loop. So what prevents the car from falling downwards? Thank You.(2 votes)
- Gravity is taking the place of the centripetal force. In this case, we need something to counter gravity.
Think of the car, and recall Newton's First Law. The Car wants to move constantly in a straight line. But the shape of the loop de loop keeps it in its path. As it wants to keep moving in a straight line, this accounts for the ficticious centrifugal force. For the driver in the car, it seems like centrifugal force. But for us outside, we see that its just Newton's First Law pushing the car against the railing and canceling the forces of gravity.
So what prevents the car from falling downwards?
Its just centrifugal force (or Newton's First Law depending on what perspective you are looking a
If you are wondering how gravity is a centripetal force:
There is no force called "centripetal force" on its own, its just a recreation of other physical everday forces we experience in our everyday lives.
If you take a tennis ball and tie it to a string and whip it around your head, the tension forces act as the centripetal force, its not centripetal force on its own.
For a race car speeding down a curve, friction acts like the centripetal force, keeping it on its path.
For the particle, the magnetic field's force that acts like the centripetal force when it moves in a circular motion.
Likewise, same for other forces, gravity must be acting like the centripetal force in this case.(3 votes)
- Why is the normal force greater than the force of gravity at4:50.(2 votes)
- In order to keep the car moving upwards, the normal force has to be greater than gravity, which wants to keep the car on level ground. The inward, normal force has to be greater than the force of gravity when entering the loop.(3 votes)
- In our science class, it was explained to us that the moon orbits the Earth because of gravity and inertia. Why didn't Sal mention inertia?(2 votes)
- Inertia isn't a force; in fact, it's not even a quantity. This video focuses on identifying forces. Hope that I helped.(3 votes)
- So here we have something that you probably have done in the last, maybe in the last day. And if we're in a car and we're just making a turn, let's say at a constant speed, on a road that is flat. So it's not a banked race track or anything like that. What is keeping the car from just veering off in a straight line? And this one's a little bit less intuitive because we don't have any string here that's tethering the car to the center of the curve of our road. So what's keeping it from going in a straight line here? Pause the video and think about that. Well in this situation, and we could think about other forces that are at play. And once again I'll assume we're in a vacuum, although you could think about air resistance as well and think about what is counteracting the air resistance. It turns out that that's friction. But the other forces at play, you of course have the force of gravity pulling downward on the car, force of gravity. And that's being counteracted by the normal force, that's being counteracted by the normal force. Force, the normal force of the road on the car. But what's keeping the car going in a circle? And actually let's just do air resistance for fun. So the air resistance, the force of the air on the car, that's gonna be pushing in the direction opposite from the velocity of the car. So we could call that, let's just call that force of air. You can't read that, let me do it this. So, force of the air, that would be its magnitude. And then that's being counteracted by, and this is a little bit counterintuitive, and this will actually give us a clue on the centripital force, that is this component, that is being counteracted by this component of the friction, so force of friction, in the direction that the car is going. Think about it, if you didn't have-- if this was ice on ice, if the wheels didn't have traction, no matter how hard the engine went and no matter how fast the wheels sped, it wouldn't be able to overcome the air resistance, and then the car would decelerate. But these are all the forces that aren't acting in a radial direction, that aren't keeping the car on the road, so to speak, or keeping it going in that circular motion around the curve. The one there, is once again, the force of friction. So this is another, I guess you could say, another component of the force of friction. And that's happening where the tires, where the literally, the rubber meets the road. But this right over here, you have the force of friction, that is keeping, and maybe I'll call it force of friction radially, radially. We'll put it in parentheses. Force of friction, radially. That is keeping us going in a circular direction. And so, in this situation, that is our centripetal force. Let's do another example, and let's keep going with the theme of cars now. So let's say a scenario where, we are on a loop-de-loop, which is always fun, and kind of scary. I have dreams where I have to drive on loop-de-loop, for some reason, and I find it intimidating. But let's think about the car at different points of the loop-de-loop, and think about what is-- what is the centripetal force at different points. So let's first think about this point right over here. And, once again, we assume that we are dealing, we're on a planet, and so you have your force of gravity, right over here, force of gravity. And then, you also have your normal force. And I'm going to draw it a little bit larger, because in order to be moved, I guess you could say, upwards, to stay on the loop-de-loop, the normal force has to be larger, you have to have a net force inward. So this is "F," this is our normal force. And so, in this situation, the magnitude, the magnitude of our centripetal force, let me do this in a different color. The magnitude of our centripetal force is going to be the net radial inward radial force, for the magnitude of the net radial inward force. So this would be equal to the magnitude of our normal force minus, minus the magnitude of the force of gravity. If this wasn't net inward right over here, then you would not, this car would not be able to move in a circle. It would just, if this netted out to zero, it would go in a straight line that way. And if this netted out so that it was negative it would accelerate downwards. So, let's go at this point right over here. And we could also think about things like air-resistance and friction, where air-resistance is pushing back on the car, and friction is overcoming it, but we're going to focus just on the things that are driving us centripetally inward or outward right now. Now what about this point for the car? Well, we still have the force of gravity, you still have the force of gravity. And actually, I'll make this a little bit bigger. We could-- eh, let me put the air-resistance there, just to be complete. So this would be the air resistance, force of the air, and then that's being counteracted by force of friction, the traction that the car has with the road. Over here, this orange vector, this would now be the combination of the force of gravity. And actually, you could even consider it the force of gravity, plus the, plus the force, the air resistance, plus the force of the air pushing back on the car, the pressure of the air. And then that is being counteracted by the force of friction, so the force of friction of the tires of the tires pushing, pushing-- Or I guess, the force of friction of the tire, between the tire and the road. But neither of these are acting centripetally, acting radially inward. So, what's that going to be? Well, here you have the normal force of the track. The track is what's keeping this car going in this circular direction. And so you have, so you have-- Here, the inward force is the normal force, F normal. So in this situation, our centripetal force, the magnitude of our centripetal force is equal to the magnitude of our normal force. And these, actually, are even going to be the same, the exact same vectors. Now, let's consider one last scenario. When we are at the top of the loop-de-loop. Pause the video, and see if you can figure that out. Well, once again, we can do things like, we could say, hey look, there's probably some air-resistance that is keeping us, that is trying to decelerate us. So that, and then that's being, that's being netted out by the force of friction. But let's think about what's going on in the vertical direction. So here, pushing down this way, you're going to have, potentially several forces, and I want this to actually be at the top of the loop-de-loop, although it doesn't look quite like that, but actually let's just assume it is. We're at the top of the loop-de-loop. Pushing down, you're going to have the force of gravity. But what else are you going to have? Assuming you're going fast enough, the track is also pushing down. The force of gravity plus the normal force. The magnitude of this vector would be the magnitude, it would be the sum of the magnitudes of the gravitational force and the normal force. And that is what's providing your centripetal force there. And so, in this scenario, we would say the magnitude of our centripetal force is equal to the magnitude of our gravitational force plus the magnitude of our normal force. Or, we could even think about it as vectors. We could say, hey look, if we just add up these vectors, these two vectors, you're going to get your centripetal force vector. That's what keeps the car going in that circular motion. Now let's just do one last scenario, just for fun. Let's imagine that we have an object in orbit. So, this is our planet, or any planet really. And you have an object in orbit, some type of a satellite. I'll draw what we normally associate with as a satellite. But this could be even a natural satellite, a moon for the planet, and what I'm about to say applies to the moon as well. So here, we don't have air, we have very minimal air resistance, there might be a few molecules every here and there. But for the most part, this is in a vacuum, and it's in orbit, so what keeps-- So it's in a uniform circular motion, it's moving in a circular orbit around the planet. What keeps it going off in a straight line? Pause the video and think about it. Well here, you have the force of gravity. You have the force of gravity of the planet. So right there, you have the force of gravity. And at first people say, wait wait, gravity? I see these picture of astronauts floating when they're in orbit. Well, that's just because they're in free-fall. But the gravity at that point, if you're a few hundred miles above the surface of the Earth, is not that different than the gravity on the surface of the Earth. You just don't have air there, and if you are in orbit, you're in constant free-fall, so it feels, to you, like there is no gravity. But it's gravity that is keeping you on the orbital path, on that circular path, and keeps you from just going in a straight line out into space.