Q: A skydiver with a mass of 80.0 kg is falling and experiences a 200.0 N upward drag force. What is the net acceleration of the skydiver at this moment? (Assume $$g = 9.8\ \mathrm{m/s^2$}$$ downward.)

$$a = \frac{mg - F_{\mathrm{drag}}}{m} = \frac{(80.0 \times 9.8) - 200.0}{80.0} = 7.3\ \mathrm{m/s^2$}$$ downward

Q: A 2.0 kg box is placed on a ramp inclined at $$30^\circ to $$the horizontal. The box just begins to slide. What is the coefficient of static friction between the box and the ramp?

$$\mu_s = \tan(30^\circ) = 0.577$$

Q: If the normal force on a box is $$20\ \mathrm{N}$$ and the coefficient of kinetic friction is $$0.2$$, what is the magnitude of the kinetic friction force?

$$f_k = 0.2 \times 20 = 4.0\ \mathrm{N}$$

Question 1

A skydiver with a mass of 80.0 kg is falling and experiences a 200.0 N upward drag force. What is the net acceleration of the skydiver at this moment? (Assume $$g = 9.8\ \mathrm{m/s^2$}$$ downward.)

Accepted Answer

$$a = \frac{mg - F_{\mathrm{drag}}}{m} = \frac{(80.0 	imes 9.8) - 200.0}{80.0} = 7.3\ \mathrm{m/s^2$}$$ downward

Question 2

Which of the following statements about equilibrium is correct?

Accepted Answer

An object in equilibrium can be at rest or in uniform motion.

Question 3

A 2.0 kg box is placed on a ramp inclined at $$30^\circ to $$the horizontal. The box just begins to slide. What is the coefficient of static friction between the box and the ramp?

Accepted Answer

$$\mu_s = 	an(30^\circ) = 0.577$$

Question 4

A sled is pulled by two players at an angle of $$15.0^\circ$$ between the ropes. The friction force on the sled is $$115\ \mathrm{N}. If $$both players pull with equal force and the sled moves at constant velocity, what is the magnitude of the force each player must exert?

Accepted Answer

$$F = \frac{115}{2 \cos(7.5^\circ)} \approx 58.0\ \mathrm{N}$$

Question 5

Which equation correctly expresses the maximum static friction force?

Accepted Answer

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

Question 6

A car is rolling to the right and accelerating. What is the direction of the static friction force acting on the point of contact between the tire and the road?

Accepted Answer

To the right, in the direction of acceleration.

Question 7

Which of the following material pairs has the highest coefficient of static friction?

Accepted Answer

Rubber on concrete

Question 8

A 2.0 kg box is on a ramp inclined at $$15^\circ. $$The coefficient of static friction is $$0.25. $$What will the box do?

Accepted Answer

The box will remain at rest.

Question 9

When a car slams on the brakes and the tires skid, which type of friction is acting between the tires and the road?

Accepted Answer

Kinetic friction

Question 10

A 2.0 kg box is on a ramp inclined at $$45^\circ. $$The coefficient of static friction is $$0.25. $$What is the net acceleration of the box down the ramp? ($$g = 9.8\ \mathrm{m/s^2$})$$

Accepted Answer

$$a = g \sin(45^\circ) - \mu_s g \cos(45^\circ) \approx 6.93\ \mathrm{m/s^2$}$$

Question 11

Which statement best describes kinetic friction?

Accepted Answer

Kinetic friction acts when an object is sliding and is generally less than static friction.

Question 12

If the normal force on a box is $$20\ \mathrm{N}$$ and the coefficient of kinetic friction is $$0.2$$, what is the magnitude of the kinetic friction force?

Accepted Answer

$$f_k = 0.2 	imes 20 = 4.0\ \mathrm{N}$$

Question 13

A submersible craft is being lowered at a steady rate by a cable. The tension in the cable is $$6000\ \mathrm{N}. $$What does this tell you about the forces acting on the craft?

Accepted Answer

The craft is in equilibrium; the upward tension balances the downward forces.

Question 14

Which formula gives the coefficient of static friction required to keep a box at rest on an inclined plane at angle $$	heta$$?

Accepted Answer

$$\mu_s = 	an(	heta)$$

Question 15

A box is at rest on a horizontal surface. A horizontal force is applied but the box does not move. What can be said about the friction force?

Accepted Answer

The friction force is equal and opposite to the applied force.

Question 16

Which of the following is true about the coefficient of friction?

Accepted Answer

It is a unitless value between zero and one (typically).

Question 17

Under normal driving conditions, which type of friction acts on the portion of a car tire in contact with the road?

Accepted Answer

Static friction

Question 18

A box is sliding down a ramp at constant velocity. Which statement is true about the forces acting on the box?

Accepted Answer

The friction force is equal to the component of gravity down the ramp.

Question 19

If the angle of a ramp is increased, what happens to the normal force acting on a box placed on the ramp?

Accepted Answer

It decreases.

Equilibrium and Friction: Forces in Physics

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