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AP Physics 1

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Unit 4

4.5 MC Answers and Review

7 min read•march 13, 2023

AP Physics 1 🎡

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Answers and Review for Multiple Choice Practice on Energy

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⛔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 4. Click here for the practice questions: AP Physics 1 Unit 4 Multiple Choice Questions. Facts about the test: The AP Physics 1 exam has 50 multiple choice questions (45 single-select and 5 multiple-select) and you will be given 90 minutes to complete the section. That means it should take you around 15 minutes to complete 8 questions.

The following questions were not written by College Board and, although they cover information outlined in the AP Physics 1 Course and Exam Description, the formatting on the exam may be different.

1. A 2 kg block is released from rest from the top of an inclined plane, as shown in the diagram. There is no friction between the block and the surface. How much work is done by the gravitational force on the block?

A. 20 J

B. 40 J

C. 60 J

D. 80 J

Explanation: The work is due to the change in potential energy of the block. The gravitational potential energy of the block at the top of the incline is Ug = mgh = 2*10*3 = 60 J. The gravitational potential energy at the bottom of the block is 0 J. Therefore, the work is the change in the potential energy, 60 J - 0 J = 60 J

📄 Study Unit 4.2: Work and Mechanical Energy

2. A 2 kg block is released from rest from the top of an inclined plane, as shown in the diagram. There is no friction between the block and the surface. hat is the speed of the block when it reaches the horizontal surface?

A. 3.2 m/s

B. 4.3 m/s

C. 5.8 m/s

D. 7.7 m/s

Explanation: Since the Ug at the top is equal to the K at the bottom, we can use 60 J = K. Next, we solve for v where 1/2mv^2=K and v = sqrt(2*60/2) = 7.7 m/s.

📄 Study Unit 4.2: Work and Mechanical Energy

3. A driver in a 2000 kg Porsche wishes to pass a slow moving school bus on a 4 lane road. What is the average power in watts required to accelerate the sports car from 30 m/s to 60 m/s in 9 seconds?

A. 1,800 J

B. 5,000 J

C. 100,000 J

D. 300,000 J

Explanation: We know that P = W/t and work = change in kinetic energy. P = W/t = .5*2000*(60^2-30^2)=300,000.

📄 Study Unit 4.2: Work and Mechanical Energy

4. A force of 20 N compresses a spring with a spring constant 50 N/m. How much energy is stored in the spring?

A. 2 J

B. 4 J

C. 5 J

D. 6 J

Explanation: The energy of a spring is Us = 1/2kx^2 but we don't have x! We need to start with the force of a spring, Fs = kx and solve for the stretch. x = Fs/k = 20/50 = .4 m. Now we can use Us = 1/2(50)(.4^2) = 4 J.

📄 Study Unit 4.2: Work and Mechanical Energy

5. An object with a mass of 2.0 kg is initially at rest at a position x = 0. A non constant force F is applied to the object over a displacement of 6.0 m, as shown in the graph. What is the total work done on the object at the end of 6.0 m?

A. 150 J

B. 170 J

C. 190 J

D. 200 J

Explanation: When you are given a graph of F vs. x and asked for the total work done, you need to solve for the area of the triangle. For this graph, the area is W = Fd = 1/2(50*6) = 150 J.

📄 Study Unit 4.2: Work and Mechanical Energy

6. Rockets are launched from an airplane in the forward direction of motion. The kinetic energy of the airplane will be

A. decreased

B. unchanged

C. increased

D. increased then decreased

Explanation: Since the plane loses mass and K = 1/2mv^2, then the plane will LOSE kinetic energy!

📄 Read Unit 4.1: Open and Closed Systems: Energy

7. An arrow in a bow has 70 J of potential energy. Assuming no loss of energy to heat, how much kinetic energy will it have after it has been shot?

A. 0 J

B. 35 J

C. 70 J

D. 140 J

Explanation: The bow initially has spring energy which is then converted to kinetic energy. If there is not loss of energy, then all of the energy is converted from spring to kinetic = 70 J.

📄 Read Unit 4.1: Open and Closed Systems: Energy

8. As a pendulum swings back and forth

A. at the end points of its swing, its energy is all potential.

B. at the lowest part of its swing, its energy is all kinetic.

C. kinetic energy is transformed into potential energy.

D. All of the choices are correct!

Explanation: When a pendulum is swinging, the top of each path holds all potential energy due to gravity (Ug). At the bottom of the path, all of the energy is then kinetic (K). As the pendulum swings, the Ug is converted to K, then to Ug, then to K, etc.

📄 Read Unit 4.1: Open and Closed Systems: Energy

9. The amount of potential energy possessed by an elevated object is equal to

A. the force needed to lift it.

B. the distance it is lifted.

C. the power used to lift it.

D. the work done in lifting it.

Explanation: Remember, work can simply be thought of as the change in energy!

📄 Study Unit 4.2: Work and Mechanical Energy

10. When a car’s speed triples, its kinetic energy

A. remains the same.

B. triples.

C. increases four times.

D. increases nine times.

Explanation: Since K = 1/2mv^2, if the speed triples and the mass stays the same, K ~ v^2 and K ~ 3^2 ~ 9 times! Don't forget to SQUARE the velocity to see the relationships!

📄 Study Unit 4.2: Work and Mechanical Energy

11. A job is done slowly, and an identical job is done quickly. Both jobs require the same amount of work but different amounts of

A. Power

B. Kinetic energy

C. Potential energy

D. Spring energy

Explanation: Remember, work is the change in energy but power is the change in energy divided by the time it takes to do the work.

📄 Study Unit 4.2: Work and Mechanical Energy

12. A ball is thrown into the air with 200 J of kinetic energy, which is transformed to gravitational potential energy at the top of its trajectory. When it returns to its original level after encountering air resistance, its kinetic energy is

A. less than 200 J.

B. 200 J.

C. more than 200 J.

D. Not enough information is given.

Explanation: Since there is an "outside force" acting on the system (air resistance), there is a loss of energy. If the final energy is less than the original energy, then the kinetic energy has to be less than the original 200 J.

📄 Study Unit 4.2: Work and Mechanical Energy

13. If an object has kinetic energy, then it also must have

A. acceleration.

B. force.

C. impulse.

D. momentum.

Explanation: The equation for kinetic energy is K = 1/2mv^2. Since momentum is p = mv, then it is the only answer that makes sense in this situation.

📄 Read Unit 4.1: Open and Closed Systems: Energy

14. A constant force, F, is applied to a block sitting on a bench. In which of the following cases is no work done on the block by F?

A. When the force is applied at a 25 degree angle to the motion.

B. When the force is applied perpendicular to the block's motion.

C. When the force is applied parallel to the blocks motion.

D. When the force is applied anti-parallel to the blocks motion.

Explanation: We know that work = Fdcos(angle). Since cos(90) = 0, then no work is done by a force on an object if the force is perpendicular to the motion of the object.

📄 Read Unit 4.1: Open and Closed Systems: Energy

15. A crane lifts a 300 kg load at a constant speed to the top of a building 60 m high in 15 s. The average power expended by the crane to overcome gravity is:

A. 10,000 W

B. 12,000 W

C. 15,000 W

D. 30,000 W

Explanation: First, you will need to find the change in energy for the object. Ug = mgh = 300(10)60 = 180,000 J. Since W = change in energy, and P = W/t = 180,000/15 = 12,000 W.

📄 Study Unit 4.2: Work and Mechanical Energy

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