Q: A hiker of mass $$50\ \mathrm{kg} is $$trapped on a ledge and is being rescued using a rope that passes over a frictionless corner. The coefficient of static friction between the rock and the ground is $$\mu_s = 0.20. $$What is the maximum mass of a friend (in kg) that the rock can support before it starts sliding, if the rock has a mass of $$m_r$$?

$$m_{\text{friend}} = 0.20 m_r$$

Question 1

Which of the following best describes the direction of the frictional force acting on an object sliding across a horizontal surface?

Accepted Answer

It acts opposite to the direction of motion or attempted motion.

Question 2

A 10 kg box rests on a horizontal floor. The coefficient of static friction between the box and the floor is $$\mu_s = 0.5. $$What is the maximum horizontal force that can be applied to the box before it starts to move? (Take $$g = 9.8\ \mathrm{m/s^2$})$$

Accepted Answer

$$49\ \mathrm{N}$$

Question 3

Which statement correctly distinguishes between static and kinetic friction?

Accepted Answer

Static friction acts when there is no relative motion, and its maximum value is usually greater than kinetic friction for the same surfaces.

Question 4

A 32.0 kg penguin slides down a snowy incline at an angle of $$10.0^\circ to $$the horizontal. The coefficient of kinetic friction between the penguin and the snow is $$\mu_k = 0.350. $$What is the magnitude of the frictional force acting on the penguin?

Accepted Answer

$$109.8\ \mathrm{N}$$

Question 5

If you press harder on a book resting on a horizontal table, what happens to the force of friction between the book and the table?

Accepted Answer

It increases because the normal force increases.

Question 6

A child pulls a toy chest (weight $$180\ \mathrm{N}) $$across a floor with a coefficient of static friction $$\mu_s = 0.42. $$What is the minimum force required to just start moving the chest?

Accepted Answer

$$75.6\ \mathrm{N}$$

Question 7

Which of the following equations correctly gives the magnitude of the kinetic friction force $$f_k$$ acting on an object?

Accepted Answer

$$f_k = \mu_k N$$

Question 8

A block is held against a vertical wall by a horizontal force. The block weighs $$33.0\ \mathrm{N}$$, and the coefficient of static friction between the block and the wall is $$\mu_s = 0.420. $$What is the minimum horizontal force required to keep the block from sliding down?

Accepted Answer

$$78.6\ \mathrm{N}$$

Question 9

A car is moving at a constant speed of $$10\ \mathrm{m/s} on a $$straight, dry road. What can you conclude about the net force acting on the car?

Accepted Answer

The net force is zero.

Question 10

Which of the following statements about friction is correct?

Accepted Answer

Static friction can have a range of values up to a maximum.

Question 11

A hiker of mass $$50\ \mathrm{kg} is $$trapped on a ledge and is being rescued using a rope that passes over a frictionless corner. The coefficient of static friction between the rock and the ground is $$\mu_s = 0.20. $$What is the maximum mass of a friend (in kg) that the rock can support before it starts sliding, if the rock has a mass of $$m_r$$?

Accepted Answer

$$m_{	ext{friend}} = 0.20 m_r$$

Question 12

A sled of mass $$30.0\ \mathrm{kg} is $$pushed by a person of mass $$35.0\ \mathrm{kg} on a $$flat icy surface. The coefficient of static friction between the person and the sled is $$\mu_s = 0.300. $$What is the maximum force the person can exert on the sled without slipping?

Accepted Answer

$$103\ \mathrm{N}$$

Question 13

Which of the following best explains why kinetic friction is usually less than static friction for the same pair of surfaces?

Accepted Answer

Because moving surfaces have less time to form strong microscopic bonds.

Question 14

A 20 kg box is being pulled up a $$15^\circ$$ incline with a coefficient of kinetic friction $$\mu_k = 0.25. $$What is the magnitude of the frictional force acting on the box?

Accepted Answer

$$47.4\ \mathrm{N}$$

Question 15

If the normal force between two surfaces is doubled, what happens to the maximum static friction force?

Accepted Answer

It doubles.

Question 16

A block is at rest on a rough surface. Which of the following statements is true about the static friction force acting on the block?

Accepted Answer

It can have any value up to $$\mu_s N$$ depending on the applied force.

Friction: Force and Motion II – Applications

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