Unit 3 FRQ (Circular Motion & Gravitation) Answers

AP Physics 1 Free Response Question Answers for Circular Motion & Gravitation

👋 Welcome to the AP Physics Unit 3 FRQ (Circular Motion & Gravitation) 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

A portion of a rollercoaster is shown above. At point a, the track has a radius of approximately 3.0 m and the cart has a speed of 2.2 m/s. The mass of the rider is approximately 60 kg. We will be ignoring the mass of the car.

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Questions with Answers & Rubric

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Part A

(i) Without manipulating equations, state whether the rider feels lighter, heavier, or the same as they normally would while standing still on earth. Briefly explain your reasoning. (2pts)

• 1pt: The rider feels lighter.

• 1pt: The normal force of the rider is smaller than the force due to gravity, since the rider is going over the hill and the net force is directed in the direction of the center of the circle (down, in this case). 📄 Additional Resources

• Study Guide: Applications of Newton’s Second Law

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(ii) On the dot below, draw and label a free body diagram for the rider at point a. (2pts)

📄 Additional Resources 

• Study Guide: Free-Body Diagrams for Objects in Uniform Circular Motion

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(iii) How heavy would a 60 kg passenger feel at point a? (What is their apparent weight?) Make sure you show ALL work! (3pts)

• 1pt: Fn = 503.2 N

📄 Additional Resources 

• Study Guide: Applications of Newton's Second Law

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Part B

At point b, the track has a radius of 13m and the rider feels four times heavier than normal. 

(i) Without manipulating equations, state whether the rider is moving faster, slower, or the same speed at point b as they were at point a. Briefly explain your reasoning. (2pts)

• 1pt: The rider is moving faster.

• 1pt: The rider has potential energy due to gravity at point a, which is then converted to kinetic energy at point b, and therefore must have more speed at b. 📄 Additional Resources

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

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(ii) On the dot below, draw a free body diagram for the rider. (2pts)

📄 Additional Resources

• Study Guide: Free-Body Diagrams for Objects in Uniform Circular Motion

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(iii) How fast is the rider traveling at point b? Make sure to show all your work. (4pts)

• 1pt: V = 19.7 m/s

📄 Additional Resources 

• Study Guide: Centripetal Acceleration and Centripetal Force

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Part C

At point c, the track and cart exert a force of 400 N on the 60 kg rider. Their velocity is 7.0 m/s.

(i) On the dot below, draw a free-body diagram for the rider at point c. (2pts)

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(ii) Determine the radius of the track at point c. Make sure to show all work. (2pts)

• 1pt: r = 2.94m

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Next Steps

• 🧠 Want to continue reinforcing your knowledge of Unit 3? Check out Unit 3 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 4 Resources.
• • 📚 Want to review multiple units? Check out all of the AP Physics 1 FRQs.