Question 1

A 70 kg person stands on a scale in an elevator accelerating upward at $$2.0\ \mathrm{m/s^2$}. $$What does the scale read?

Accepted Answer

$$826\ \mathrm{N}$$

Question 2

A 1500 kg car is towed at a constant speed by a rope at a $$20^\circ$$ angle above the horizontal. If the friction force is $$320\ \mathrm{N}$$, what is the tension in the rope?

Accepted Answer

$$340\ \mathrm{N}$$

Question 3

A 100 kg block hangs from a rope. What is the tension in the rope if the block is accelerating upward at $$2.5\ \mathrm{m/s^2$}$$?

Accepted Answer

$$1230\ \mathrm{N}$$

Question 4

A skier of mass $$m$$ slides down a frictionless $$27^\circ$$ slope. What is her acceleration?

Accepted Answer

$$g\sin 27^\circ$$

Question 5

A 32 kg sofa is pushed at a steady speed across a floor with kinetic friction coefficient $$\mu_k = 0.08. $$What force does Carol apply?

Accepted Answer

$$25\ \mathrm{N}$$

Question 6

A 50 kg student stands on a scale in a falling elevator (cable breaks). What does the scale read?

Accepted Answer

$$0\ \mathrm{N}$$

Question 7

A box is pulled to the right at steady speed by a rope that angles upward. How does the normal force $$n$$ compare to $$mg$$?

Accepted Answer

$$n \u003c mg$$

Question 8

A 100 N box is at rest. It is pulled by a 30 N horizontal force. The maximum static friction is 40 N. Does the box move?

Accepted Answer

No

Question 9

A 4.8 kg penguin glides in water at $$1.60\ \mathrm{m/s}$$ and decelerates at $$0.52\ \mathrm{m/s^2$}. If $$its frontal area is $$0.020\ \mathrm{m^2$}$$, what is its drag coefficient $$C_D$$? (Water density $$= 1000\ \mathrm{kg/m^3$})$$

Accepted Answer

$$0.098$$

Question 10

A 75 kg skydiver and a 0.020 kg mouse fall from a plane. Both are modeled as cylinders with $$C_D = 1.1$$ and air density $$1.2\ \mathrm{kg/m^3$}. $$Which statement is correct about their terminal speeds?

Accepted Answer

The skydiver has a much higher terminal speed than the mouse.

Question 11

A block is at rest on a rough surface. You pull horizontally with tension $$T. $$What is the static friction force $$f_s$$?

Accepted Answer

$$f_s = T$$

Question 12

A 1.2 kg book is pressed down with a 15 N force on a table. What is the normal force from the table?

Accepted Answer

$$27\ \mathrm{N}$$

Question 13

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

Accepted Answer

$$2.0\ \mathrm{N}$$

Question 14

A box is being pulled to the right over a rough surface. The tension $$T is $$greater than kinetic friction $$f_k. $$The rope breaks. What happens?

Accepted Answer

The box slows steadily until it stops.

Question 15

A 50 kg student stands on a scale in an elevator accelerating upward. What does the scale read?

Accepted Answer

$$\u003e 490\ \mathrm{N}$$

Question 16

A block of mass $$m is $$pulled by a rope at an angle $$	heta$$ above the horizontal. The normal force $$n is$$:

Accepted Answer

$$n = mg - T\sin	heta$$

Question 17

A 200 kg stage set is attached to a 100 kg stagehand by a rope over a frictionless pulley. When the rope is released, what is the acceleration of the stagehand?

Accepted Answer

$$3.3\ \mathrm{m/s^2$}$$

Question 18

A block slides down a frictionless incline of angle $$	heta. $$What is the normal force?

Accepted Answer

$$mg\cos	heta$$

Question 19

Which of the following is true about the tension in a massless rope passing over a massless, frictionless pulley?

Accepted Answer

The tension is the same at both ends.

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

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