Q: A block is pushed downward with a force of 10 N. If the coefficient of static friction is 0.5 and the block's mass is 4 kg, what is the normal force?

$$N = mg + F_{\text{down}} = 4 \times 9.8 + 10 = 49.2\ \mathrm{N}$$

Question 1

An elevator is moving downward and slowing down. Which free-body diagram (FBD) best represents the forces acting on the elevator?

Accepted Answer

The FBD where the upward force (tension) is greater than the downward force (weight).

Question 2

A 40 kg person is inside an elevator and the scale reads a steady 450 N. Which statement is correct?

Accepted Answer

The elevator is moving downward and slowing.

Question 3

A force of 70.0 N is acting at an angle of $$20^\circ$$ from the x-axis. What are the components of the force?

Accepted Answer

$$F_x = 65.8\ \mathrm{N},\ F_y = 23.9\ \mathrm{N}$$

Question 4

The coefficients of static and kinetic friction between a 6.0 kg box and a floor are $$\mu_s = 0.30$$ and $$\mu_k = 0.20$$ respectively. What is the force of friction on the box when pushed horizontally with a force of 25 N?

Accepted Answer

$$8.8\ \mathrm{N}$$

Question 5

A 6.0 kg object is sliding up a ramp inclined at $$40^\circ. $$The coefficient of kinetic friction is $$0.2. $$What is the numerical value of the frictional force?

Accepted Answer

$$9.2\ \mathrm{N}$$

Question 6

Consider vectors $$\vec{P}$$ and $$\vec{Q} as $$shown in the diagram. Which vector represents the sum $$\vec{P} + \vec{Q}$$?

Accepted Answer

$$\vec{C}$$

Question 7

Assume friction is negligible and both the string and pulley are ideal. The mass of the hanging body is 1.0 kg and the other block is 2.0 kg. What will the acceleration be after the rope is released?

Accepted Answer

$$3.33\ \mathrm{m/s^2$}$$

Question 8

Two trucks are pushing against each other and accelerating in the $$+x$$ direction. Which statement is correct?

Accepted Answer

The magnitude of force applied by the larger truck on the smaller truck is equal to the magnitude of force by the smaller truck on the larger truck.

Question 9

A 4.0 kg box is pushed vertically downward with a force of 10.0 N. If the coefficient of static friction between the box and floor is 0.50, how much horizontal force is necessary to overcome static friction and get the box to start sliding?

Accepted Answer

$$7.0\ \mathrm{N}$$

Question 10

A boy pulls a 20 kg sled across snow with a rope at $$20^\circ$$ above the horizontal. The tension is 80 N. What is the horizontal component of the tension force?

Accepted Answer

$$T_x = 80 \cos 20^\circ = 75.2\ \mathrm{N}$$

Question 11

A 70 kg person is in a 1200 kg elevator. The elevator starts from rest and reaches 2.0 m/s in 0.5 s downward. What is the acceleration during this stage?

Accepted Answer

$$a = -4.0\ \mathrm{m/s^2$}$$

Question 12

A wrecking ball (8.0 kg) is suspended by two ropes. Rope 1 is at $$50^\circ$$ and Rope 2 at $$56.9^\circ$$ with the vertical. If the tension in Rope 1 is 70 N, what is the vertical component of this tension?

Accepted Answer

$$T_{1y} = 70 \cos 50^\circ = 45.0\ \mathrm{N}$$

Question 13

Which of the following is a necessary condition for an object to be in static equilibrium?

Accepted Answer

$$\sum \vec{F}_x = 0$$ and $$\sum \vec{F}_y = 0$$

Question 14

A block is sliding up a ramp with velocity $$v as $$shown. Which force diagram best represents the forces acting on the block?

Accepted Answer

The diagram with three forces: gravity downward, normal perpendicular to the ramp, and friction parallel to the ramp, opposite to motion.

Question 15

A student records three numbers (60, 540, 600) during an elevator ride. Which is most likely the mass in kg?

Accepted Answer

60

Question 16

A wrecking ball is in static equilibrium, suspended by two ropes. If the vertical component of tension in Rope 2 is $$T_{2y}$$, what is the value of $$T_{2y}$$?

Accepted Answer

$$T_{2y} = mg - T_{1y}$$

Question 17

A 20 kg sled is pulled with a rope at $$20^\circ$$ above the horizontal with a tension of 80 N. The coefficient of static friction is $$0.20. $$What is the maximum static friction force?

Accepted Answer

$$f_{s,\max} = \mu_s N$$

Question 18

A block is pushed downward with a force of 10 N. If the coefficient of static friction is 0.5 and the block's mass is 4 kg, what is the normal force?

Accepted Answer

$$N = mg + F_{	ext{down}} = 4 	imes 9.8 + 10 = 49.2\ \mathrm{N}$$

Physics Unit III Test Study Guidance: Forces, Friction, Vectors, and Equilibrium

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