Unit 5 FRQ (Momentum) Answers

AP Physics 1 Free Response Question Answers for Momentum

👋 Welcome to the AP Physics Unit 5 FRQ (Momentum) Answers. Have your responses handy as you go through the rubrics to see how you did!

⏱ Remember, the AP Physics 1 exam has 5 free-response questions, and you will be given 90 minutes to complete the FRQ section. (This means you should give yourself ~18 minutes to go through each practice FRQ.)

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Setup

Identical blocks 1 and 2 are placed on a horizontal surface at points A and E, respectively, as shown. The surface is frictionless except for the region between points B and C, where the surface is rough. Beginning at time tA, Block 1 is pushed with a constant horizontal force from Point A to Point B by a mechanical plunger.

Upon reaching Point B, Block 1 loses contact with the plunger and continues moving to the right along the horizontal surface toward Block 2. Block 1 collides with and sticks to Block 2 at Point E, after which the two-block system continues moving across the surface, eventually passing Point F.

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Questions with Answers & Rubric

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Part A

(i) On the axes below, sketch the speed of the center of mass of the two-block system as a function of time, from time tA until the blocks pass Point F at time tF. The times at which Block 1 reaches Points A through F are indicated on the time axis. (3pts)

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Part B

The system is reset and Block 1 is again released. The following conditions now exist:

In addition, there is no friction on the horizontal surface except for between letters B and C.

(i) Determine the speed of Block 1 at Point B. (3pts)

📄 Additional Resources

• Study Guide: Conservation of Energy, the Work-Energy Principle, and Power

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(ii) Determine the speed of Block 1 at Point D. (4pts)

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(iii) Determine the speed of the combined Blocks 1 and 2 at Point F. (2pts)

📄 Additional Resources

• Study Guide: Momentum and Impulse

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Part C

(i) Was momentum and/or energy conserved from Point D to Point F? Briefly explain your reasoning for EACH. (2pts)

• 1pt: Momentum is conserved in a closed system for a perfectly inelastic collision.
• 1pt: Energy is NOT conserved in a perfectly inelastic collision.

NOTE: Points may be earned for correct explanations, even if the wrong boxes are checked.

📄 Additional Resources

• Study Guide: Conservation of Linear Momentum  

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Next Steps

• 🧠 Want to continue reinforcing your knowledge of Unit 5? Check out Unit 5 Trivia, either as a document or as a game.
• ⏭ Ready to move on to the next topic? Take a look at the collection of Unit 6 Resources.
• • 📚 Want to review multiple units? Check out all of the AP Physics 1 FRQs.