Q: A 70 kg person stands on a scale in an elevator accelerating upward at $$2 \, \mathrm{m/s^2$}. $$What does the scale read?

$$W_{app} = 70 \times (9.8 + 2) = 826 \, \mathrm{N}$$

Q: A 5.0 kg block A is pushed with a 3.0 N force against a 10 kg block B on a frictionless surface. What force does block A exert on block B?

$$F = \frac{10}{15} \times 3.0 = 2.0 \, \mathrm{N}$$

Q: A 100 N box is at rest on a horizontal surface. The coefficient of static friction is $$\mu_s = 0.4. $$What is the maximum static friction force before the box starts to move?

$$f_{s, max} = 0.4 \times 100 = 40 \, \mathrm{N}$$

Q: A car of mass 1500 kg accelerates from rest to 12 m/s in 10 s. What is the net force required for this acceleration?

$$F = m a = 1500 \times \frac{12}{10} = 1800 \, \mathrm{N}$$

Q: A 10 kg box is pulled by a rope on a horizontal surface with kinetic friction coefficient $$\mu_k = 0.2. $$What is the friction force opposing the motion?

$$f_k = \mu_k n = 0.2 \times 10 \times 9.8 = 19.6 \, \mathrm{N}$$

Question 1

A 1500 kg car is being towed at a steady speed by a rope held at a 20° angle from the horizontal. If a friction force of 320 N opposes the car's motion, what is the tension in the rope?

Accepted Answer

$$T = \frac{320}{\cos 20^\circ} \, \mathrm{N}$$

Question 2

Which of the following best describes dynamic equilibrium?

Accepted Answer

An object moving in a straight line at constant speed with no net force acting on it.

Question 3

A 70 kg person stands on a scale in an elevator accelerating upward at $$2 \, \mathrm{m/s^2$}. $$What does the scale read?

Accepted Answer

$$W_{app} = 70 	imes (9.8 + 2) = 826 \, \mathrm{N}$$

Question 4

A 5.0 kg block A is pushed with a 3.0 N force against a 10 kg block B on a frictionless surface. What force does block A exert on block B?

Accepted Answer

$$F = \frac{10}{15} 	imes 3.0 = 2.0 \, \mathrm{N}$$

Question 5

Which equation correctly gives the apparent weight of a person in an elevator accelerating downward at $$a$$?

Accepted Answer

$$W_{app} = mg - ma$$

Question 6

A 100 N box is at rest on a horizontal surface. The coefficient of static friction is $$\mu_s = 0.4. $$What is the maximum static friction force before the box starts to move?

Accepted Answer

$$f_{s, max} = 0.4 	imes 100 = 40 \, \mathrm{N}$$

Question 7

A 0.020 kg mouse and a 70 kg skydiver both fall from a plane. Which statement about their terminal speeds is correct?

Accepted Answer

The mouse has a much lower terminal speed than the skydiver due to its smaller mass and cross-sectional area.

Question 8

Which of the following is true about the normal force on an object resting on an inclined plane?

Accepted Answer

$$n = mg \cos 	heta$$

Question 9

A box is pulled to the right at steady speed by a rope that angles upward. Which statement is true about the normal force $$n$$ compared to the box's weight $$mg$$?

Accepted Answer

$$n \u003c mg$$

Question 10

A car of mass 1500 kg accelerates from rest to 12 m/s in 10 s. What is the net force required for this acceleration?

Accepted Answer

$$F = m a = 1500 	imes \frac{12}{10} = 1800 \, \mathrm{N}$$

Question 11

Which formula gives the drag force on a sphere at low Reynolds number (laminar flow)?

Accepted Answer

$$D = 6 \pi \eta r v$$

Question 12

A 10 kg box is pulled by a rope on a horizontal surface with kinetic friction coefficient $$\mu_k = 0.2. $$What is the friction force opposing the motion?

Accepted Answer

$$f_k = \mu_k n = 0.2 	imes 10 	imes 9.8 = 19.6 \, \mathrm{N}$$

Question 13

Which of the following is NOT a step in the equilibrium problem-solving approach?

Accepted Answer

Use kinematics equations to find acceleration.

Question 14

A 5 kg and a 10 kg box are in contact and pushed together on a frictionless surface. If a 15 N force is applied to the 5 kg box, what is the acceleration of the system?

Accepted Answer

$$a = \frac{15}{5+10} = 1.0 \, \mathrm{m/s^2$}$$

Question 15

Which of the following statements about tension in a massless string passing over a frictionless pulley is correct?

Accepted Answer

The tension is the same on both sides of the pulley.

Question 16

A 100 N box is pulled with a horizontal force of 30 N. The coefficient of static friction is $$\mu_s = 0.4. $$Will the box move?

Accepted Answer

No, because $$f_{s, max} = 40 \, \mathrm{N} \u003e 30 \, \mathrm{N}$$

Question 17

Which of the following best describes the condition for terminal speed during free fall with air resistance?

Accepted Answer

The drag force equals the gravitational force, so net force is zero.

Question 18

A block is sliding at constant speed on a horizontal surface. Which statement is true about the forces acting on it?

Accepted Answer

The applied force equals the kinetic friction force.

Question 19

Which of the following is the correct expression for the Reynolds number?

Accepted Answer

$$Re = \frac{\rho v L}{\eta}$$

Applying Newton’s Laws: Equilibrium, Dynamics, and Friction

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