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## AP®︎/College Physics 1

### Course: AP®︎/College Physics 1>Unit 3

Lesson 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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• At , shouldn't the frictional force point in the opposite direction of the velocity? • 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.
• At , if the only force in the y-direction is downwards then why doesn't the car fall down? • 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.
• 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? • how can air friction be counteracted by frictional force ,and why is friction on the direction of motion? • 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.
• At , where does the extra Fn comes from? • 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? • Are we assuming that the cars' engines are turned off? So they are not exerting any driving force? • 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. • 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.  