AP PHYSICS 1 -UNIT 2 REVIEW

Three objects are located in the xy-plane as shown in the figure. If mA = 2.0 kg, mB = 3.0 kg, and mC = 5.0 kg, what is the center of mass of the three-object system?

• (6.6 m, 6.0 m)

• (7.0 m, 7.0 m)

• (7.3 m, 6.7 m)

• (7.8 m, 5.2 m)

Answer explanation

---

Answer D

Correct. The location of a system’s center of mass along a given axis can be calculated using the equation:

x_cm = (sum of m_i * x_i) / (sum of m_i)

For the x-coordinate, this is:

x_cm = (2 kg 8 m) + (3 kg 4 m) + (5 kg * 10 m)
-------------------------------------------
2 kg + 3 kg + 5 kg

x_cm = 7.8 m

For the y-coordinate, this is:

y_cm = (2 kg 12 m) + (3 kg 6 m) + (5 kg * 2 m)
-------------------------------------------
2 kg + 3 kg + 5 kg

y_cm = 5.2 m

A rope is attached to a crate at an angle to the horizontal, as shown on the left. The rope pulls the crate to the right across a rough horizontal surface at a constant speed. The free-body diagram on the right shows the forces exerted on the crate at the instant shown on the left. As the crate moves across the surface, the coefficient of kinetic friction between the crate and the surface increases but the speed of the crate remains constant. Which of the following free-body diagrams could show the forces exerted on the crate a short time later, after the coefficient of friction has increased?

• 
• 
• 
• 

Answer explanation

Answer C

Correct. The coefficient of kinetic friction increases as the crate moves to the right, so the frictional force must increase. To maintain constant speed, the horizontal component of the force exerted by the rope must increase to offset the horizontal friction force.

Tags

NGSS.HS-PS2-1

The figures show the forces exerted on an object moving on a horizontal surface at two different moments in time. Which of the following correctly compares the change in direction, if any, of the velocity of the object in the two figures?

• A. Not changing (Figure 1), Not changing (Figure 2)

• B. Not changing (Figure 1), Changing in the x-direction (Figure 2)

• C. Changing in the y-direction (Figure 1), Not changing (Figure 2)

• 4. D. Changing in the y-direction (Figure 1), Changing in the x-direction (Figure 2)

Answer explanation

Answer B

Correct. In Figure 1, the components of the forces are balanced in both the x- and y-directions, which means the velocity is not changing in either direction. In Figure 2, while the components of the forces are balanced in the y-direction, they are not balanced in the x-direction, so the velocity must be changing in the x-direction.

Tags

NGSS.HS-PS2-1

A person stands on a scale in an elevator. Consider the following four forces:

Fp–s : The force of the person on the scale

Fs–p : The force of the scale on the person

Fp–E : The force of the person on Earth

FE–p : The force of Earth on the person.

Which of the following correctly ranks the magnitudes of these forces while the elevator is moving upward with increasing speed?

• (Fp–s = Fs–p) > FE–p > Fp–E

• Fs–p = Fp–E > Fp–s > FE–p

• (Fp–s = Fs–p) > (Fp–E = FE–p)

• FE–p = Fp–E > (Fp–s = Fs–p)

Answer explanation

Answer C

Correct. The net force on the person must be in the direction of the acceleration. Since the elevator is accelerating upward, the upward force of the scale on the person must be greater than the downward force of the Earth on the person. The equal forces are interacting force pairs described by Newton’s third law.

Tags

NGSS.HS-PS2-1

• 20 N

• 50 N

• 60 N

• 120 N

Answer explanation

Tags

NGSS.HS-PS2-1

Planets A and B, with masses 2m and m, respectively, orbit a distant star of mass M. Planet A is a distance 2R from the center of the star, and Planet B is a distance R from the center of the star. Which of the following is a correct expression for the magnitude of the net force exerted on Planet B?

Answer explanation

Tags

NGSS.HS-PS2-1

NGSS.HS-PS2-4

A block of mass m is held at rest against a vertical wall by an applied force F_A. The wall exerts a normal force F_N on the block. The coefficient of static friction between the block and the wall is μ_s. If the magnitude of F_A is larger than the minimum required magnitude to keep the block in place, which of the following equations correctly describes the force of static friction, F_f, exerted on the block?

• F_f = μ_s F_N

• F_f = μ_s F_A

• F_f = μ_s mg

• F_f = mg

Answer explanation

Answer D

Correct. The net force in the vertical direction is equal to zero, so the magnitude of the force of static friction must be equal to the magnitude of the force of gravity.

A block of mass m is attached to a spring on an inclined plane that makes an angle θ with the horizontal. Initially, the block is held in place with the spring at its unstretched length. The block is then slowly lowered until it comes to rest at equilibrium at a displacement x₀ from its original position. Which of the following expressions is equal to the spring constant of the spring?

• mg/x₀

• (mg cos θ)/x₀

• (mg sin θ)/x₀

• (g sin θ)/x₀

Answer explanation

Tags

NGSS.HS-PS2-1

A ball of mass m is attached to a vertical pole by a rope and travels in a horizontal circle. The rope is attached to the top of the pole, has length L, and makes an angle θ with the pole. The ball travels with constant speed v. Which of the following is a correct expression for the tension force in the rope?

Answer explanation

Tags

NGSS.HS-PS2-1

An applied force is exerted on a textbook, pushing it against a rough wall, as indicated by the arrow in the figure. The applied force keeps the textbook at rest. Which of the following free-body diagrams could represent the forces exerted on the textbook?

• 
• 
• 
• 

Answer explanation

Tags

NGSS.HS-PS2-1