Are the objects described here in equilibrium while at rest, in equilibrium while in motion, or not in equilibrium at all? Explain.

1. A 200 pound barbell is held over your head.

2. A girder is lifted at constant speed by a crane.

3. A girder is being lowered into place. It is slowing down.

4. A jet plane has reached its cruising speed and altitude.

5. A box in the back of a truck doesn’t slide as the truck stops.

A ball tossed straight up has $v=0$ at its highest point. Is it in equilibrium? Explain.

Kat, Matt, and Nat are arguing about why a physics book on a table doesn’t fall. According to Kat, “Gravity pulls down on it, but the table is in the way so it can’t fall.” “Nonsense,” says Matt. “An upward force simply overcomes the downward force to prevent it from falling.” “But what about Newton’s first law?” counters Nat. “It’s not moving, so there can’t be any forces acting on it.” None of the statements is exactly correct. Who comes closest, and how would you change his or her statement to make it correct?

If you know all of the forces acting on a moving object, can you tell the direction the object is moving? If yes, explain how. If no, give an example.

An elevator, hanging from a single cable, moves upward at constant speed. Friction and air resistance are negligible. Is the tension in the cable greater than, less than, or equal to the gravitational force on the elevator? Explain. Include a free-body diagram as part of your explanation.

An elevator, hanging from a single cable, moves downward and is slowing. Friction and air resistance are negligible. Is the tension in the cable greater than, less than, or equal to the gravitational force on the elevator? Explain. Include a free-body diagram as part of your explanation.

Are the following statements true or false? Explain.

1. The mass of an object depends on its location.

2. The weight of an object depends on its location.

3. Mass and weight describe the same thing in different units.

An astronaut takes his bathroom scale to the moon and then stands on it. Is the reading of the scale his weight? Explain.

The four balls in FIGURE Q6.9 have been thrown straight up. They have the same size, but different masses. Air resistance is negligible. Rank in order, from largest to smallest, the magnitude of the net force acting on each ball. Some may be equal. Give your answer in the form $\text{A}>\text{B}>\text{C}=\text{D}$ and explain your ranking. Assume that the Reynolds number is high.

FIGURE Q6.9

Suppose you attempt to pour out 100 g of salt, using a pan balance for measurements, while in a rocket accelerating upward. Will the quantity of salt be too much, too little, or the correct amount? Explain.

An astronaut orbiting the earth is handed two balls that have identical outward appearances. However, one is hollow while the other is filled with lead. How can the astronaut determine which is which? Cutting or altering the balls is not allowed.

A hand presses down on the book in FIGURE Q6.12. Is the normal force of the table on the book larger than, smaller than, or equal to mg?

FIGURE Q6.12 

Boxes A and B in FIGURE Q6.13 both remain at rest. Is the friction force on A larger than, smaller than, or equal to the friction force on B? Explain.

FIGURE Q6.13

Suppose you push a hockey puck of mass m across frictionless ice for 1.0 s, starting from rest, giving the puck speed v after traveling distance d. If you repeat the experiment with a puck of mass 2m, pushing with the same force,

1. How long will you have to push for the puck to reach the same speed v?

2. How long will you have to push for the puck to travel the same distance d?

A block pushed along the floor with velocity $v_{0x}$ slides a distance d after the pushing force is removed.

1. If the mass of the block is doubled but its initial velocity is not changed, what distance does the block slide before stopping?

2. If the initial velocity is doubled to $2v_{0x}$ but the mass is not changed, what distance does the block slide before stopping?

A crate of fragile dishes is in the back of a pickup truck. The truck accelerates north from a stop sign, and the crate moves without slipping. Does the friction force on the crate point north or south? Or is the friction force zero? Explain.

Five balls are moving through the air with the instantaneous velocities shown in FIGURE Q6.17. All five have the same size and shape. Air resistance is not negligible. Rank in order, from largest to smallest, the magnitudes of the accelerations $a_{\text{A}}$ to $a_{\text{E}}$. Some may be equal. Give your answer in the form $\text{A}>\text{B}=\text{C}>\text{D}>\text{E}$ and explain your ranking. Assume that the Reynolds number is high.

FIGURE Q6.17