Q: A 10 kg box is pulled up a $$15^\circ$$ incline at constant speed. The coefficient of kinetic friction is $$0.1. $$What is the magnitude of the frictional force?

$$f_k = 0.1 \times 10 \times 9.8 \times \cos 15^\circ$$

Question 1

A car is parked on a hill inclined at $$30^\circ to $$the horizontal. If the car remains at rest, which of the following statements about the forces acting on the car is correct?

Accepted Answer

The net force on the car is zero because it is in static equilibrium.

Question 2

A 5 kg box is pulled along a horizontal surface with a constant velocity by a horizontal force of 20 N. What is the coefficient of kinetic friction between the box and the surface?

Accepted Answer

$$\mu_k = 0.4$$

Question 3

Which of the following best describes the difference between static and kinetic friction?

Accepted Answer

Static friction prevents motion from starting, while kinetic friction acts during motion.

Question 4

A skydiver of mass 80 kg jumps from a plane. As she falls, the drag force increases until it equals her weight. What happens at this point?

Accepted Answer

She reaches terminal speed and continues to fall at constant velocity.

Question 5

A 10 kg object is suspended by a rope and remains at rest. What is the tension in the rope? (Assume $$g = 9.8\ \mathrm{m/s^2$})$$

Accepted Answer

$$98\ \mathrm{N}$$

Question 6

A 2 kg block is pushed along a frictionless surface with a force of 6 N. What is its acceleration?

Accepted Answer

$$3\ \mathrm{m/s^2$}$$

Question 7

Which equation correctly expresses Newton's second law for a particle of mass $$m$$ under a net force $$F$$?

Accepted Answer

$$F = m a$$

Question 8

A 50 kg crate rests on a horizontal floor. The coefficient of static friction is $$0.3. $$What is the minimum horizontal force required to start moving the crate? (Assume $$g = 9.8\ \mathrm{m/s^2$})$$

Accepted Answer

$$147\ \mathrm{N}$$

Question 9

A 70 kg astronaut weighs less on the Moon than on Earth. Which statement best explains why?

Accepted Answer

The acceleration due to gravity on the Moon is less than on Earth.

Question 10

A cyclist coasts down a hill and eventually reaches a constant speed. Which of the following best explains this observation?

Accepted Answer

The drag force equals the component of gravity down the hill.

Question 11

A 3 kg block slides down a rough incline making an angle of $$25^\circ$$ with the horizontal. The coefficient of kinetic friction is $$0.2. $$What is the acceleration of the block? (Assume $$g = 9.8\ \mathrm{m/s^2$})$$

Accepted Answer

$$a = 9.8 (\sin 25^\circ - 0.2 \cos 25^\circ)\ \mathrm{m/s^2$}$$

Question 12

A 60 kg person stands in an elevator accelerating upward at $$2\ \mathrm{m/s^2$}. $$What is the normal force exerted by the floor on the person? (Assume $$g = 9.8\ \mathrm{m/s^2$})$$

Accepted Answer

$$708\ \mathrm{N}$$

Question 13

A 10 kg box is pulled up a $$15^\circ$$ incline at constant speed. The coefficient of kinetic friction is $$0.1. $$What is the magnitude of the frictional force?

Accepted Answer

$$f_k = 0.1 	imes 10 	imes 9.8 	imes \cos 15^\circ$$

Question 14

A 2 kg object falls through air and experiences a drag force proportional to its velocity, $$F_{	ext{drag}} = bv. If$$ $$b = 4\ \mathrm{kg/s}$$, what is the terminal speed?

Accepted Answer

$$v_t = \frac{mg}{b}$$

Question 15

A 5 kg block is at rest on a horizontal surface. A horizontal force of 15 N is applied, but the block does not move. What can you conclude about the static friction force?

Accepted Answer

The static friction force is 15 N, balancing the applied force.

Question 16

A 1.5 kg object is dropped from rest and falls under gravity and air resistance. Which of the following statements is true as it falls?

Accepted Answer

Its acceleration decreases as speed increases due to increasing drag force.

Question 17

A 20 kg box is pulled with a force of 100 N at an angle of $$30^\circ$$ above the horizontal. The coefficient of kinetic friction is $$0.25. $$What is the normal force on the box? (Assume $$g = 9.8\ \mathrm{m/s^2$})$$

Accepted Answer

$$N = mg - F \sin 30^\circ$$

Chapter 6: Dynamics I – Motion Along a Line (Study Notes)

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