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# AP Physics 1 review of Momentum and Impulse

In this video David quickly reviews the momentum and impulse topics on the AP Physics 1 exam and solves an example problem for each concept. Created by David SantoPietro.

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• At , I dont see why the answer is D. The problem asks for the velocity not the momentum. Since p=mv, shouldnt the velocity equal to p/m which is -20?? Thank you!
• Yes, you're right. I just totally forgot to divide by the mass at the end. My bad! I'll add an annotation.
• Where's the dropbox link?
• For the 2D collisions problem, how do we know that there are no net forces in the system?
(1 vote)
• That is an excellent question, Shawn.

You may consider the phrase "net force" as "the external force", or "the force left over after subtracting all individual forces in a system", according to Giancoli AP Physics. My suggestion is that you decide whether there is a third party force exerted, say, by an experimenter.

Consider the circumstance: A rocket and its additional tanks are fired up to the sky and after a while the tanks exert a repulsive force in the opposite direction of the rocket so they may separate with the rocket and fall. Will we consider the forces the tanks exert "net forces" to the whole system? Apparently no. They are simply regarded as "inner-forces" existed in the whole rocket-tank system. However, if we now expand a little bit and consider air resistance, will air resistance be a net force? Well, yes, since that is indeed a third party force that is not part of the system.

To reiterate, what to consider is merely the net third party force that does not belong to the original whole system you have been asked to consider. The sum of forces that are not exerted BY the system itself but are exerted ON the system.

Again, thank you for asking. And sorry for writing this long. A rookie here.

AP Center, High School Affiliated to RUC, Beijing, China
• can you link the document
• So then, where does the energy go in an inelastic collision?
• Heat.
• At The annotation needs to be changed. David solved for the "Final Momentum" instead of the final velocity. To say that he solved the problem for the change in momentum is the purpose of the of the example.
(1 vote)
• Is the answer of the question at wrong? Shouldn't we choose C instead of D? We need to consider about the unit of the velocity, right?
(1 vote)
• Yes, the correct answer is C since you would divide that -40 kg m/s by the mass (2kg) to get the velocity of -20 m/s.
(1 vote)
• At should the answer not be C because the Ke is conserved in that the momentum is the same before and after?
(1 vote)
• Energy is NOT in fact conserved in the collision: Some energy is transferred outside of the system and turned into thermal energy.
(1 vote)
• At , I would think that the answer is C, not D. I understand that point D. is correct in that IMMEDIATELY after the car arrives at rest, that would be a possible solution. However, the block is on ice so why wouldn´t the block slide up to position c? the momentum causes slight forward movement correct?
(1 vote)
• The question asks for the positions at the instant that the car's tires stop moving, not later on. Plus there should be no change in center of mass. In answer choice C, the center of mass is at 5, not 4, which was the original center of mass.
(1 vote)
• For the question at , is the momentum conserved?
(1 vote)
• Momentum, both linear and angular, is always conserved (unless, of course, there were external unbalanced forces).
(1 vote)