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# AP Physics 1 review of Centripetal Forces

In this video David explains each concept for centripetal motion and solves an example problem for each concept. Created by David SantoPietro.

## Want to join the conversation?

• Hi all. Here's a dropbox link to download the 2D motion notes if you want them. Good luck on the exam!

https://www.dropbox.com/s/kuzxjp1d2x7rj33/Centripetal%20forces%20review.pdf?dl=0 • For the example question on centripetal force, why is it Mg - M(S^2/R) rather than Mg + M(S^2/R)? My thought process was that if normal force equals to the forces in the opposite direction and that since the weight of the ball and the centripetal force both point inward, the normal force would equal the weight + centripetal force rather than minus. What is the problem to my thinking? • How do we know that all planets are spherical?
(1 vote) • we dont. And they are not all spherical.

However, it makes sense that most things in the universe would be spherical. At least when they were intially formed. Why? a) because most things would be 'fluid' in nature when formed. Either because they are very hot or because they are formed from gas, dust etc that would be able to flow.
b) gravitational fields are radial. This means they tend to pull those fluids towards a center. All the bits of fluid are trying to get close to that centre so the lowest energy arrangement of the whole thing is spherical.

I guess if bodies are not spherical, it because they have had their shape changed after they have been formed. By impact or strong gravitational fields (eg moons of Jupiter)
• at he says Ms but writes Mp. Am I overlooking something?
(1 vote) • Acceleration = (Velocity)' = (position)''. Typically using the power rule, subsequent derivatives lead to lower exponents. However, this is just how the numbers work out. Another method is to divide by dt (A=(dv/dt). Given these trends, why does it increase when we talk about rotating objects. Here, ||P(t)|| = r, ||V(t)|| = rw, ||A(t)|| = r(w^2). Why do the subsequent derivatives get an extra w? NOTE: rw/(dt) = rα = A (but r/dt is nothing). I am a little hazy on the difference between Ac and A. However, nevertheless, why is there that correlation (multiplying by w) as seen above.
(1 vote) • At he says that the normal force has to be less than the force due to gravity.

But how is that possible? If the object is at rest, the object is obviously not sinking thus the normal force is equal to the gravitational force. Are we just saying that when it is moving then the normal force isn't necessarily the same?

I thought like this;
1. Object is moving past the top part of the circle - Normal Force is keeping it from moving down.
2. Object moved past the top part of the circle and is not colliding with it anymore, so gravity moves it down.
3. Depending on how fast the object is moving and how much acceleration is due from gravity, the object will move along the circle, if not actually colliding with it. If it does collide with it then the normal force will actually prevent it from moving down farther at that moment.

Where is my logic wrong?

Thanks!
(1 vote) • Well, first of all, the object is not at rest. It is moving at a constant speed along the top of the hill. If an object undergoes circular motion, that means that the net force must point towards the center of the circular path, this means that the normal force must be less than the weight of the car. As for your logic: There is no point where the car loses contact with the hill. If the car is to momentarily lose contact, its speed must be greater than a certain threshold. We do not have any information about the car being at this threshold, therefore, we can infer that the car does not lose contact with the hill. Also, the normal force doesn't always point straight up, it points perpendicularly out of the hill. So the normal force stops the car from going into the hill but it does not stop the car from traveling down the hill. Hope this helps!
(1 vote)
• At , he cancels out the R^3 in numerator with the R^2 in denominator, leaving R. But aren't these R's actually different variables? Isn't the R^3 in the numerator the radius of the planet (from the volume of sphere), and isn't the R^2 in denominator the distance of the object from the mass in the gravitational field? If I am not mistaken, then these two can't cancel each other out....
(1 vote) • At , for acceleration B why was the 2S not squared to become 4S^2?
(1 vote) • At , we say the object accelerate down because the net force is downward (The Mg is more than Fn). What about the moment in which the object is going upward? I am asking that because I believe the Fn can't be more than Mg. So what is causing the upward motion?
(1 vote) • Wouldn't it be better to refer to "v" as velocity instead o speed? 