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AP Physics C: Mech

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Unit 2

2.4 MC Answers and Review

8 min read•december 22, 2021

AP Physics C: Mechanics ⚙️

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Answers and Review for Multiple Choice Practice on Newton's Laws of Motion

Image courtesy of Pixabay

⛔STOP⛔ Before you look at the answers make sure you gave this practice quiz a try so you can assess your understanding of the concepts covered in Unit 2. Click here for the practice questions: AP Physics C: Mechanics Unit 2 Multiple Choice Questions. Facts about the test: The AP Physics C: Mechanics exam has 35 multiple choice questions and you will be given 45 minutes to complete the section. That means it should take you around 15 minutes to complete 12 questions.

The following questions were not written by College Board and, although they cover information outlined in the AP Physics C: Mechanics Course and Exam Description, the formatting on the exam may be different.

1. Which of the following situations can be represented by the free body diagram pictured?

A. An egg falls from a nest towards the Earth in the presence of air resistance.

B. A cart slides down a ramp due to the force of gravity

C. A wagon is pulled at an angle by its handle along the rough sidewalk.

D. A sign hangs from a rope off of the edge of a building.

Answer: The key is the force vector labeled "F" that points at an upward angle. The only situation that fits is the wagon, because any of the other applied forces in the other situations are not at a logical angle.

📄 Study AP Physics C: Mechanics, Unit 2.1 : Newton's Laws of Motion: First and Second Law

2. Three blocks (A, B, and C) are connected together at rest on a frictionless surface as pictured. The mass of block A is 5kg, the mass of block B is 2kg, and the mass of block C is 1kg. Which rope exerts the biggest force as the blocks are accelerated to the right?

A. A

B. B

C. C

D. All experience the same force.

Answer: Think about it in terms of Newton's 2nd Law. All blocks accelerate at the same rate sine they are connected. Rope C has to pull the largest mass (8kg total), so it must exert the biggest force.

📄 Study AP Physics C: Mechanics, Unit 2.1 : Newton's Laws of Motion: First and Second Law

3. Which of the following is equivalent to a unit of force?

A. kg(m/s)

B. kg(m/s^2)

C. kg(m^2/s^2)

D. m/s^3 Answer: The easiest way to think about this is that F = ma. If Newtons are the unit for force, they must be equal to the unit for mass multiplied by the unit for acceleration. Kilograms is the standard unit for mass and m/s^2 is the standard unit for acceleration.

📄 Study AP Physics C: Mechanics, Unit 2.1 : Newton's Laws of Motion: First and Second Law

4. A 5kg ball on a 1.5m string swings in a vertical circle with a uniform linear speed of 2 m/s. At the top of the circle, which value is the closest to the magnitude of the tension force in the string?

A. 35 N

B. 42 N

C. 56 N

D. 62 N

Answer: At the top of the circle, the tension force + the weight of the ball is equal to the centripetal force. Write a net force equation and solve for tension. T = (mv^2)/R - mg and plug in values.

📄 Study AP Physics C: Mechanics, Unit 2.2 : Circular Motion

5. The goal of a spin cycle for a washing machine is to remove the excess water from the clothes. Which of the following statements is true about how a washing machine accomplishes this?

A. The washer container exerts a centripetal force on the water pushing it out of the machine.

B. The inertia of the clothes pushes the water out of the machine.

C. The centrifugal force of the spinning motion pulls the clothes into the center of the machine more than the water, because of the bigger mass of the clothes.

D. The circular wall in the washer exerts a centripetal force on the clothes, but the holes in the wall don't apply force, allowing the water to keep moving in a straight line out of the container.

Answer: The circular wall exerts centripetal force inward on the clothes, keeping them in the machine. The holes in the machine don't exert force (because there's no wall there), so the water keeps going straight out because of its inertia.

📄 Study AP Physics C: Mechanics, Unit 2.2 : Circular Motion

6. A car is turning left around a flat curve with radius R. What is the correct expression for the maximum speed that the car can turn without slipping?

A. v = (coefficient of static friction * g * R)

B. v = sqrt(coefficient of static friction*g*R)

C. v = g*R

D. v = g*R^2

Answer: Set the formula for centripetal force (v^2/R) equal to the force of static friction. Remember that static frictional force = (coefficient of static friction)(normal force). Plug in mg for the normal force, since the road is flat and it's equal to the gravitational force, and solve for v.

📄 Study AP Physics C: Mechanics, Unit 2.2 : Circular Motion

7. A toy yo-yo moves in uniform circular motion in a vertical circle shown in the image. Which of the arrows depicts the direction of the net force on the yo-yo?

A. A

B. B

C. C

D. D

Answer: When an object is in uniform circular motion, the net force of the object is always equal to the centripetal force. The centripetal force always points towards the center of the circle.

📄 Study AP Physics C: Mechanics, Unit 2.2 : Circular Motion

8. Jack Sparrow shoots a cannon from a cannonball and hits an enemy ship. Which of the following statements is always true?

A. The ship exerts an equal and opposite force on the cannon, through the interaction with the cannonball.

B. The cannon exerts an equal force that the cannonball exerts on the ship, just in the other direction.

C. The cannon pushes the cannonball forwards, and the cannonball exerts an equal force on the ship it hits.

D. The cannonball exerts an equal force that pushes the cannon backwards as it is being propelled forwards out of the cannon.

Answer: This is a direct application of Newton's 3rd law. As the cannon shoots the cannonball, the cannonball MUST exert an equal and opposite force on the cannon. That's why it slides backwards as it shoots!

📄 Study AP Physics C: Mechanics, Unit 2.3 : Newton's Laws of Motion: Third Law

9. Two identical masses hang at rest over a massless, frictionless pulley as seen in the image. What is the correct expression for the frictional force exerted on mass 1 by the table in order to keep the masses at rest?

A. m1g

B. m2g

C. g(m2 + m1)

D. g(m2 - m1)

Answer: Draw FBDs for each mass and see that the only external forces changing the acceleration of the masses are friction from the table and the weight of mass 2. Assuming that down is positive, sum of forces = mg - friction. Since the masses are at rest, the sum of forces is zero and mg = friction.

📄 Study AP Physics C: Mechanics, Unit 2.1 : Newton's Laws of Motion: First and Second Law

10. A 62kg human is standing on a human-sized Newton scale in an elevator accelerating upward at 2 m/s^2. What is the closest approximation to the scale reading?

A. 124N

B. 480N

C. 608N

D. 730N

Answer: The scale reading is equal to the normal force in the situation. Write a net force equation and apply Newton's 2nd law to get: N - W = ma. Solve for the normal force.

📄 Study AP Physics C: Mechanics, Unit 2.1 : Newton's Laws of Motion: First and Second Law

11. A woman is pulling a 25kg shopping cart at an angle at a constant velocity. She exerts a force of 20N and the frictional force on the shopping cart is 10N. At what angle is she pulling?

A. 0 degrees

B. 30 degrees

C. 35 degrees

D. 60 degrees

Answer: The frictional force of the floor has to be equal and opposite to the horizontal component of her pulling force. Set up a net force equation in the horizontal direction, and solve for the angle.

📄 Study AP Physics C: Mechanics, Unit 2.3 : Newton's Laws of Motion: Third Law

12. A 10 kg ball rotates in uniform circular motion at a speed of 2 m/s on the end of 4 m long string. If the ball is moving in a horizontal circle, what is its centripetal acceleration?

A. 0.5 m/s^2

B. 1 m/s^2

C. 2 m/s^2

D. 10 m/s^2

Answer: There's extra info in this problem - don't let it distract you. To solve for centripetal acceleration, just use v^2/R.

📄 Study AP Physics C: Mechanics, Unit 2.2 : Circular Motion

13. Jayden uses the same net force to push two shopping carts along the same patch of parking lot. If the Green Cart has four times the acceleration of the Blue Cart, which of the following is true?

A. There's not enough information to tell.

B. The Blue Cart and the Green Cart have the same mass

C. The Blue Cart has one fourth the mass of the Green Cart.

D. The Blue Cart has four times the mass as the Green Cart.

Answer: Use Newton's 2nd law to answer this question. If the net force is the same, and the Green Cart has four times the acceleration, then it must have 1/4 the mass.

📄 Study AP Physics C: Mechanics, Unit 2.1 : Newton's Laws of Motion: First and Second Law

14. A passenger is in a moving car, facing forward. They suddenly jerk forwards and are kept in place by their seatbelt. This is an indication of which of the following descriptions of the motion of the car?

A. The car slowed down quickly

B. The car sped up quickly

C. The car turned left

D. The car turned right

Answer: This is an application of Newton's 1st Law. When the car stops suddenly, the passenger keeps going because they have inertia. The external force of the seatbelt keeps them in place. That's why it's important to wear one, y'all!

📄 Study AP Physics C: Mechanics, Unit 2.1 : Newton's Laws of Motion: First and Second Law

15. In the diagram, two blocks with masses m and 3m respectively are in contact on a horizontal frictionless surface. If they are being pushed with a force F as shown. What is a correct expression for the acceleration of the 3m block?

A. F/3m

B. F/4m

C. 3F/m

D. 4F/m

Answer: Both blocks accelerate at the same rate since they are in contact. Use Newton's 2nd Law and the combined mass of the system to solve for a.

📄 Study AP Physics C: Mechanics, Unit 2.1 : Newton's Laws of Motion: First and Second Law

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