Question 1

A 10 kg box rests on a horizontal surface. The coefficient of static friction between the box and the surface is 0.5. What is the minimum horizontal force required to start moving the box?

Accepted Answer

$$F = 50 \ \mathrm{N}$$

Question 2

A skier of mass 65 kg slides down a hill inclined at $$22^\circ to $$the horizontal. The coefficient of kinetic friction is $$\mu_k = 0.10. $$What is the acceleration of the skier down the slope?

Accepted Answer

$$a = 3.0 \ \mathrm{m/s^2$}$$

Question 3

A 0.5 kg block is held at rest on a frictionless table and connected to a wall by a spring with spring constant $$k = 200 \ \mathrm{N/m}. If $$the block is pulled 0.1 m from equilibrium and released, what is the magnitude of the force exerted by the spring at maximum displacement?

Accepted Answer

$$F = 20 \ \mathrm{N}$$

Question 4

A 5.0 kg pot of flowers hangs from two wires, each making a $$40^\circ$$ angle with the horizontal. What is the tension in each wire?

Accepted Answer

$$T = 38.2 \ \mathrm{N}$$

Question 5

A 2.0 kg sled is pulled at constant velocity across snow with a frictional force of 2.0 N. The rope makes an angle of $$35^\circ$$ with the horizontal. What is the magnitude of the applied force?

Accepted Answer

$$F = 2.44 \ \mathrm{N}$$

Question 6

A crate of mass 95 kg begins to slide down a truck bed when the tilt angle reaches $$25^\circ. $$What is the coefficient of static friction between the crate and the truck?

Accepted Answer

$$\mu_s = 0.47$$

Question 7

A 3.0 kg mass is suspended by a rope and is lifted with an acceleration of $$0.1g. $$What is the tension in the rope during the lift?

Accepted Answer

$$T = 32.3 \ \mathrm{N}$$

Question 8

A car travels around a curve of radius 45 m. If the coefficient of static friction between the tires and the road is $$\mu_s = 0.82$$, what is the maximum speed the car can have without skidding?

Accepted Answer

$$v_{max} = 18.9 \ \mathrm{m/s}$$

Question 9

Two blocks, $$m_1 = 20.0$$ \ \mathrm{kg}$$ and m_2$ = 10.0 \ \mathrm{kg}$$, are pulled across a frictionless surface by a $$30.0 \ \mathrm{N}$$ force. What is the acceleration of the system?

Accepted Answer

$$a = 2.0 \ \mathrm{m/s^2$}$$

Question 10

A 1.0 kg mass is attached to a string and whirled in a horizontal circle of radius 2.0 m at a constant speed of 4.0 m/s. What is the tension in the string?

Accepted Answer

$$T = 16.0 \ \mathrm{N}$$

Question 11

If you push a wooden crate across the floor at constant speed, then turn the crate on end, reducing by half the surface area in contact with the floor, what happens to the force required to push the crate at the same speed?

Accepted Answer

It is equally great.

Question 12

An object is held in place by friction on an inclined surface. The angle of inclination is increased until the object starts moving. If the surface is kept at this angle, the object:

Accepted Answer

Speeds up.

Question 13

A 0.27 kg block is held at rest on an inclined plane by a spring with $$k = 120 \ \mathrm{N/m}. $$The coefficient of static friction is $$\mu_s = 0.46. If $$the weight of the block is doubled, what happens to the amount of spring compression required to keep it at rest?

Accepted Answer

It doubles.

Question 14

A ball is whirled in a horizontal circle on a string. If the string breaks, what path does the ball follow immediately after breaking?

Accepted Answer

It moves tangentially to the circle at the point of release.

Question 15

Which of the following statements about kinetic and static friction is correct?

Accepted Answer

Kinetic friction is always less than or equal to static friction.

Question 16

A 1.0 kg mass is held stationary by a string. What is the tension in the string?

Accepted Answer

$$T = 9.8 \ \mathrm{N}$$

Question 17

Which of the following is NOT a fundamental force of nature?

Accepted Answer

Frictional force

Question 18

A 25 kg and a 30 kg mass are suspended by pulleys and connected by a string. If the system is released from rest, what is the acceleration of the system? (Assume frictionless pulleys and massless string.)

Accepted Answer

$$a = 0.49 \ \mathrm{m/s^2$}$$

Question 19

A 0.84 kg bag hangs in the middle of a clothesline causing it to sag by an angle of $$3.5^\circ. $$What is the tension in the rope?

Accepted Answer

$$T = 6.7 \ \mathrm{N}$$

Applications of Newton's Laws of Motion: Frictional Forces and Normal Forces

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