Question 1

A 100 kg block hangs from a rope. What is the tension in the rope if the block is accelerating upward at $$5\ \mathrm{m/s^2$}$$? (Take $$g = 9.8\ \mathrm{m/s^2$})$$

Accepted Answer

$$T = 1480\ \mathrm{N}$$

Question 2

A 200 kg Martian lander is descending and slowing down with a deceleration of $$2\ \mathrm{m/s^2$}. If$$ $$g_{Mars} = 3.8\ \mathrm{m/s^2$}$$, what is the thrust of the rocket?

Accepted Answer

$$T = 1160\ \mathrm{N}$$

Question 3

A 1500 kg car is pulled by a tow truck with a tension of $$2500\ \mathrm{N}$$, opposed by a $$200\ \mathrm{N}$$ friction force. If the car starts from rest, what is its speed after $$5.0\ \mathrm{s}$$?

Accepted Answer

$$v = 7.7\ \mathrm{m/s}$$

Question 4

A box with a weight of $$100\ \mathrm{N} is at $$rest. It is then pulled by a $$30\ \mathrm{N}$$ horizontal force. If $$\mu_s = 0.4$$, does the box move and what is the magnitude of the frictional force?

Accepted Answer

No, $$f = 30\ \mathrm{N}$$

Question 5

A 100 kg box sits in the back of a flatbed truck. The coefficients of friction are $$\mu_s = 0.4$$ and $$\mu_k = 0.2. $$What is the maximum acceleration the truck can have without the box slipping?

Accepted Answer

$$a_{max} = 3.92\ \mathrm{m/s^2$}$$

Question 6

A ring is pulled by three forces as shown: two $$10\ \mathrm{N}$$ forces at an angle $$	heta$$ above the horizontal, and a downward force $$F. If $$the ring is in equilibrium, what is the magnitude of $$F$$?

Accepted Answer

$$F = 20\cos	heta$$

Question 7

An astronaut takes her bathroom scale to the Moon, where $$g = 1.6\ \mathrm{m/s^2$}. $$Compared to Earth, what happens to her weight and mass?

Accepted Answer

Her weight is less and her mass is the same.

Question 8

A box is sitting on the floor of an elevator that is accelerating upward. The magnitude of the normal force on the box is:

Accepted Answer

$$n \u003e mg$$

Question 9

Which of the following best describes the condition for equilibrium for a particle?

Accepted Answer

$$\sum F_x = 0$$ and $$\sum F_y = 0$$

Question 10

A car with a weight of $$15,000\ \mathrm{N} is $$being towed up a $$20^\circ$$ slope at constant velocity. The coefficient of rolling friction is $$\mu_r = 0.062. $$What is the minimum tension required in the tow rope?

Accepted Answer

$$T = 5,700\ \mathrm{N}$$

Question 11

A box is being pulled to the right over a rough surface. The tension $$T$$ exceeds $$f_{s,max}$$, so the box is speeding up. Suddenly the rope breaks. What happens to the box?

Accepted Answer

It slows steadily until it stops.

Question 12

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

Accepted Answer

$$\mu_k \u003c \mu_s$$

Question 13

A block just begins to slip on an inclined plane at angle $$	heta_s. $$What is the coefficient of static friction $$\mu_s$$?

Accepted Answer

$$\mu_s = 	an	heta_s$$

Question 14

Which of the following is true about drag force?

Accepted Answer

It increases in magnitude as the object's speed increases.

Question 15

Terminal speed is reached when:

Accepted Answer

The drag force equals the weight of the object.

Question 16

A 100 kg crate is placed on a $$15^\circ$$ incline. What is the acceleration of the crate if $$\mu_k = 0$$?

Accepted Answer

$$a = g\sin 15^\circ$$

Question 17

When you weigh yourself on a spring scale in an elevator accelerating downward, your apparent weight is:

Accepted Answer

Less than your true weight.

Question 18

Which of the following best explains why it is harder to start moving a heavy box than to keep it moving?

Accepted Answer

Static friction is greater than kinetic friction.

Question 19

A 100 lb box with $$\mu_s = 0.4 is $$pulled by a force $$P. $$Which orientation of $$P$$ requires the least force to begin sliding?

Accepted Answer

Parallel to the surface.

Dynamics I: Motion Along a Line, Equilibrium, and Friction

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