AP Physics Unit 4 FRQ Practice Prompts & Feedback

FAQ practice is one of the most important ways to prepare for the exam! Review FRQ student writing practice samples and corresponding feedback from Fiveable Teacher Peter Apps.

The Practice FRQ

Describe a physical situation and a system to which this energy bar chart could apply

Explain your reasoning.

FRQ Submission & Feedback

Student Submission

Initially, a block could be attached to a spring. This spring was compressed and released from the compressed position. At this point, the bar chart for the initial system energy demonstrates the block has reached the point where the spring is at its initially unstretched position, where it has maximum kinetic energy and the PE of the system is 0. The block then continues to travel beyond this point, however, on a surface with friction. This friction does negative work on the object, decreasing its speed and therefore its kinetic energy. While this will increases the GPE due to an increase in position, the box will slow to a stop, making the PE spring equal too. These conditions are demonstrated in the bar chart for the final system energy past the unstretched position of the string.

Teacher Feedback

Here’s a couple of tips about your answer: (1) Nice job interpreting that we need to have KE decrease, PEg increase and some outside force do work on the system. (2) Make sure at the beginning of your paragraph you clearly define the system. Are we talking about just the sphere (just KE allowed), sphere-Earth (KE and PEg allowed), or sphere-spring-Earth (KE, PEs, PEg allowed)? (3) PEg increasing means we need to have a change in height, but I didn’t see anything in your explanation that mentioned a hill or ramp or launching the sphere upwards. (4) Friction could reduce the KE of the object, but the overall work is positive, not negative, so there must be another force that will result in the net force being positive.

Student's Updated FRQ based on Feedback

In the sphere - spring - earth system, a black is initially attached to a spring on a flat surface. The block is then compressed and releases once the block reaches the springs unstretched position once again, the KE is at a maximum while the PE g and PE s is 0. This is demonstrated with the bar chart for the initial system energy. Next, the spring is brought up an incline with friction by the force of a hand bringing it up. The spring is repositioned so that it can be in an unstretched position on the ramp. The spring is then compressed and released. Once the spring reaches its unstretched position, there is GPE and KE, however, friction does do work on the black. The PE spring is at 0 because the block is at the spring's unstretched position.

More Teacher Feedback

This is better, but you’re still over complicating the scenario. Because PEs was 0 at the beginning and end, you don’t need a spring at all. It’s ok that you included it but then you need to jump through so many hoops to make it work.

I’d do something much simpler. We need to be moving at the beginning (KE), gain some energy (positive work), and end up with a bit less KE but more PEg at the end.

Anytime we’re looking at increasing PEg we need to change height. That means a ramp or hill or going upwards into the air.

Possible Responses

Here are some possible Scenarios:

1. A car driving down the road when it comes to a hill. The driver pushes the gas pedal to increase the car’s mechanical energy, but it’s not enough to keep the car’s speed constant as it goes up the hill.
2. A student is pushing a refrigerator into a moving truck. The fridge needs to go up a ramp to get into the truck, so the student pushes harder on it (does work). However the student can’t push hard enough to keep it moving at a constant speed.
3. A vertical lifting situation where 0 PEg is defined as the starting position of the moving object.