Q: A projectile is launched from the ground with an initial speed $$v_0 at an $$angle $$\theta$$ above the horizontal, as shown in the image. What is the time of flight until it returns to the same vertical level? (Neglect air resistance.)

$$\frac{2 v_0$ \sin \theta}{g}$$

Q: A block of mass $$m is at $$rest on a ramp with angle $$\theta$$ and coefficient of static friction $$\mu_s. $$What is the maximum angle $$\theta_{max}$$ before the block starts to slide?

$$\theta_{max} = \arctan(\mu_s)$$

Question 1

A projectile is launched from the ground with an initial speed $$v_0 at an $$angle $$	heta$$ above the horizontal, as shown in the image. What is the time of flight until it returns to the same vertical level? (Neglect air resistance.)

Accepted Answer

$$\frac{2 v_0$ \sin 	heta}{g}$$

Question 2

Consider a person standing in an elevator that is moving upward at constant speed. The magnitude of the upward normal force, $$N$$, exerted by the elevator floor on the person’s feet is:

Accepted Answer

$$N = W$$

Question 3

Now suppose the elevator is accelerating upward. How does the normal force $$N$$ compare to the weight $$W of $$the person?

Accepted Answer

$$N \u003e W$$

Question 4

A block of mass $$m is on a $$frictionless inclined plane at angle $$	heta. $$What is the component of the gravitational force parallel to the incline?

Accepted Answer

$$mg \sin 	heta$$

Question 5

A block of mass $$m is on a $$frictionless inclined plane at angle $$	heta. $$What is the component of the gravitational force perpendicular to the incline?

Accepted Answer

$$mg \cos 	heta$$

Question 6

A hockey puck is sliding at constant speed on a frictionless air track. What can you say about the net force acting on the puck?

Accepted Answer

The net force is zero.

Question 7

A person twirls a rock on a string in space. If the string breaks at the point shown, which path does the rock follow?

Accepted Answer

The rock moves in a straight line tangent to the circle at the point of release.

Question 8

A book is lying at rest on a table. The book will remain at rest because:

Accepted Answer

There is no net force on the book.

Question 9

A block is pulled across a rough surface at constant speed. Which set of relationships is correct?

Accepted Answer

The applied force equals the kinetic friction force.

Question 10

A block of mass $$m is at $$rest on a ramp with angle $$	heta$$ and coefficient of static friction $$\mu_s. $$What is the maximum angle $$	heta_{max}$$ before the block starts to slide?

Accepted Answer

$$	heta_{max} = \arctan(\mu_s)$$

Question 11

A conical pendulum (tetherball) moves in a horizontal circle at constant speed. What is the direction of the net force on the ball?

Accepted Answer

Toward the center of the circle.

Question 12

A motorcycle is turning in a circle of radius $$r on a $$flat road. If the coefficient of static friction is $$\mu_s$$, what is the maximum speed $$v_{max}$$ before sliding occurs?

Accepted Answer

$$v_{max} = \sqrt{\mu_s g r}$$

Question 13

A child pulls a 5.0 kg crate with a string at a $$30^\circ$$ angle above the horizontal. The tension is 17 N, and the crate accelerates at $$0.6\ \mathrm{m/s^2$}$$ horizontally. What is the correct expression for the coefficient of kinetic friction $$\mu_k$$?

Accepted Answer

$$\mu_k = \frac{T \cos 30^\circ - m a}{m g - T \sin 30^\circ}$$

Question 14

In Atwood's machine, mass $$A is $$twice the mass of $$B$$ ($$m_A = 2 m_B). $$What is the tension $$T in $$the string above mass $$A$$?

Accepted Answer

Neither of these

Question 15

A block slides down a frictionless ramp. If it is then pushed uphill, what is the direction of the net force while it moves uphill?

Accepted Answer

Down the ramp, same as before.

Question 16

A person is riding in a car that suddenly turns left. From the perspective of an outside (inertial) observer, what force acts on the person?

Accepted Answer

The door exerts a leftward force on the person.

Question 17

A block of mass $$m is $$being lowered on a cord at a speed that is decreasing. What is the relationship between the tension $$T in $$the cord and the weight $$mg$$?

Accepted Answer

$$T \u003e mg$$

Question 18

A block of mass $$m is $$being raised on a cord at a speed that is increasing. What is the relationship between the tension $$T in $$the cord and the weight $$mg$$?

Accepted Answer

$$T \u003e mg$$

Question 19

A book sits on a table. The normal force from the table on the book equals the weight of the book. Are these two forces a Newton's Third Law action-reaction pair?

Accepted Answer

No

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