Q: A 20 kg box rests on a horizontal surface. The coefficient of static friction between the box and the surface is $$\mu_s = 0.4. $$What is the maximum horizontal force that can be applied to the box before it starts to move?

$$F_{max} = 0.4 \times 20 \times 9.8 = 78.4\ \mathrm{N}$$

Q: A person stands on a scale in an elevator accelerating upward at $$2\ \mathrm{m/s^2$}. If $$the person's mass is $$70\ \mathrm{kg}$$, what does the scale read (apparent weight)?

$$w_{app} = 70 \times (9.8 + 2) = 826\ \mathrm{N}$$

Q: Which statement about the normal force is correct for an object resting on an inclined plane at angle $$\theta$$?

The normal force is $$n = mg \cos(\theta)$$, perpendicular to the surface.

Q: A 32 kg sofa is pushed at constant speed across a carpet using 'sofa sliders' with $$\mu_k = 0.08. $$What force must be applied to keep it moving?

$$F = \mu_k mg = 0.08 \times 32 \times 9.8 = 25\ \mathrm{N}$$

Q: A 200 kg stage set is suspended by a rope over a pulley. A 100 kg stagehand is accidentally hoisted upward when the set falls. What is the acceleration of the stagehand?

$$a = \frac{(200 - 100) \times 9.8}{200 + 100} = 3.3\ \mathrm{m/s^2$}$$

Question 1

A 20 kg box rests on a horizontal surface. The coefficient of static friction between the box and the surface is $$\mu_s = 0.4. $$What is the maximum horizontal force that can be applied to the box before it starts to move?

Accepted Answer

$$F_{max} = 0.4 	imes 20 	imes 9.8 = 78.4\ \mathrm{N}$$

Question 2

A person stands on a scale in an elevator accelerating upward at $$2\ \mathrm{m/s^2$}. If $$the person's mass is $$70\ \mathrm{kg}$$, what does the scale read (apparent weight)?

Accepted Answer

$$w_{app} = 70 	imes (9.8 + 2) = 826\ \mathrm{N}$$

Question 3

Which statement about the normal force is correct for an object resting on an inclined plane at angle $$	heta$$?

Accepted Answer

The normal force is $$n = mg \cos(	heta)$$, perpendicular to the surface.

Question 4

A 32 kg sofa is pushed at constant speed across a carpet using 'sofa sliders' with $$\mu_k = 0.08. $$What force must be applied to keep it moving?

Accepted Answer

$$F = \mu_k mg = 0.08 	imes 32 	imes 9.8 = 25\ \mathrm{N}$$

Question 5

A box is being pulled to the right over a rough surface and is speeding up. Suddenly the rope breaks. What happens to the box?

Accepted Answer

It slows steadily until it stops.

Question 6

Which of the following best describes the direction and magnitude of kinetic friction?

Accepted Answer

Kinetic friction acts opposite to the direction of motion and has magnitude $$f_k = \mu_k n.$$

Question 7

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

Accepted Answer

$$F_{A\ on\ B} = \frac{3.0\ \mathrm{N}}{1 + \frac{5.0}{10}} = 2.0\ \mathrm{N}$$

Question 8

A 200 kg stage set is suspended by a rope over a pulley. A 100 kg stagehand is accidentally hoisted upward when the set falls. What is the acceleration of the stagehand?

Accepted Answer

$$a = \frac{(200 - 100) 	imes 9.8}{200 + 100} = 3.3\ \mathrm{m/s^2$}$$

Question 9

Which statement about tension in a massless rope passing over a massless, frictionless pulley is correct?

Accepted Answer

The tension is the same at all points in the rope.

Question 10

A car of mass $$m is $$moving at constant speed $$v on a $$flat road. The drag force at high Reynolds number is given by $$D = C_D \rho A v^2$. Which change will increase the drag force the most?

Accepted Answer

Doubling the speed $$v ($$since $$D \propto v^2$)

Question 11

A small sphere of radius $$r$$ falls through honey at low Reynolds number. According to Stokes' law, what is the drag force?

Accepted Answer

$$D = 6 \pi \eta r v$$

Question 12

Which of the following is true about static friction?

Accepted Answer

Static friction adjusts up to a maximum value $$f_{s,max} = \mu_s n to $$prevent motion.

Question 13

A cyclist of mass $$80\ \mathrm{kg}$$ reaches terminal speed while descending a hill. If the drag force equals the weight, what is the drag force?

Accepted Answer

$$D = mg = 80 	imes 9.8 = 784\ \mathrm{N}$$

Question 14

Which of the following is NOT a correct statement about Newton's third law?

Accepted Answer

The two forces in an action/reaction pair act on the same object.

Question 15

A block is sliding to the right on a horizontal surface. Which force opposes its motion?

Accepted Answer

Kinetic friction, acting to the left.

Question 16

Which scenario best describes when an object reaches terminal speed?

Accepted Answer

When the drag force equals the applied force and net acceleration is zero.

Question 17

A rolling wheel experiences which type of friction?

Accepted Answer

Rolling friction, which is generally much less than kinetic friction.

Question 18

Which of the following is true about the Reynolds number?

Accepted Answer

High Reynolds number ($$Re \u003e 1000) $$means inertial forces dominate and drag is proportional to $$v^2.$$

Physics for Life Sciences I: Forces, Friction, Drag, and Interacting Objects (Lecture 9, Sections 5.5-5.8)

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