Q: Carol wants to slide her 32 kg sofa across the carpet using 'sofa sliders' with $\mu_k = 0.080$. What force must she apply to move the sofa at constant speed?

$F = 0.080 \times 32\ \mathrm{kg} \times 9.8\ \mathrm{m/s^2}$

Q: A box with a weight of 100 N is at rest. It is then pulled by a 30 N horizontal force. Will the box move?

No, because $30\ \mathrm{N}$ is less than the maximum static friction

Q: A box is being pulled to the right over a rough surface with a force greater than kinetic friction. Suddenly the rope breaks. What happens to the box?

It slows steadily until it stops.

Q: A car traveling at $20\ \mathrm{m/s}$ stops in a distance of $50\ \mathrm{m}$. If the coefficient of friction between a passenger and the seat is $\mu_k = 0.03$, will a $70\ \mathrm{kg}$ passenger slide off the seat if not wearing a seat belt?

Yes, because the required friction force exceeds the maximum possible friction.

Q: Which of the following statements about kinetic friction is correct?

Kinetic friction has a nearly constant magnitude and does not depend on speed.

Q: On an inclined plane, the weight force $\vec{w}$ can be decomposed into which components?

$w_x = w \sin \theta$,\ w_y = w \cos \theta$

Q: A box is at rest on a horizontal surface. A woman pushes harder and harder, but the box does not move. What happens to the static friction force as she increases her push?

It increases up to a maximum value.

Q: Which of the following is a common mistake when analyzing forces on an incline?

Assuming the normal force is not perpendicular to the surface

Q: Which of the following best describes the adjustment of the static friction force?

It adjusts itself to match the applied force up to a maximum value

Question 1

A 1.2 kg book lies on a table and is pressed down from above with a force of 15 N. What is the normal force exerted by the table on the book?

Accepted Answer

$n = 27\ \mathrm{N}$

Question 2

A box is sitting on the floor of an elevator. The elevator is accelerating upward. Which of the following is true about the normal force $n$ acting on the box?

Accepted Answer

$n > mg$

Question 3

A skier slides down a frictionless slope inclined at $27^\circ$. What is the skier’s acceleration?

Accepted Answer

$a = g \sin 27^\circ$

Question 4

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

Accepted Answer

$n < mg$

Question 5

A box on a rough surface is pulled by a horizontal rope with tension $T$. The box is not moving. What is the relationship between the static friction force $f_s$ and $T$?

Accepted Answer

$f_s = T$

Question 6

Which of the following correctly describes the direction of the normal force for an object on an inclined plane?

Accepted Answer

Perpendicular to the surface

Question 7

Carol wants to slide her 32 kg sofa across the carpet using 'sofa sliders' with $\mu_k = 0.080$. What force must she apply to move the sofa at constant speed?

Accepted Answer

$F = 0.080 	imes 32\ \mathrm{kg} 	imes 9.8\ \mathrm{m/s^2}$

Question 8

A box with a weight of 100 N is at rest. It is then pulled by a 30 N horizontal force. Will the box move?

Accepted Answer

No, because $30\ \mathrm{N}$ is less than the maximum static friction

Question 9

Which equation gives the maximum static friction force for an object of mass $m$ on a surface with coefficient of static friction $\mu_s$?

Accepted Answer

$f_{s,\mathrm{max}} = \mu_s n$

Question 10

A box is being pulled to the right over a rough surface with a force greater than kinetic friction. Suddenly the rope breaks. What happens to the box?

Accepted Answer

It slows steadily until it stops.

Question 11

A car traveling at $20\ \mathrm{m/s}$ stops in a distance of $50\ \mathrm{m}$. If the coefficient of friction between a passenger and the seat is $\mu_k = 0.03$, will a $70\ \mathrm{kg}$ passenger slide off the seat if not wearing a seat belt?

Accepted Answer

Yes, because the required friction force exceeds the maximum possible friction.

Question 12

Which of the following statements about kinetic friction is correct?

Accepted Answer

Kinetic friction has a nearly constant magnitude and does not depend on speed.

Question 13

A wheel rolling on a surface experiences which type of friction?

Accepted Answer

Rolling friction

Question 14

On an inclined plane, the weight force $\vec{w}$ can be decomposed into which components?

Accepted Answer

$w_x = w \sin 	heta$,\ w_y = w \cos 	heta$

Question 15

If the normal force on an object increases, what happens to the maximum possible static friction force?

Accepted Answer

It increases.

Question 16

A box is at rest on a horizontal surface. A woman pushes harder and harder, but the box does not move. What happens to the static friction force as she increases her push?

Accepted Answer

It increases up to a maximum value.

Question 17

Which of the following is a common mistake when analyzing forces on an incline?

Accepted Answer

Assuming the normal force is not perpendicular to the surface

Question 18

What is the formula for the kinetic friction force $f_k$?

Accepted Answer

$f_k = \mu_k n$

Question 19

A box is pulled by a rope at an angle upward and moves at constant speed. Which force balances the horizontal component of the tension?

Accepted Answer

Friction force

Question 20

Which of the following best describes the adjustment of the static friction force?

Accepted Answer

It adjusts itself to match the applied force up to a maximum value

L16-17: Normal Force and Forces on Inclined Planes

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