Chapter 6, Problem 1CQ

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.

To determine

The objects given are in equilibrium or not.

Explanation of Solution

Given Info:

a. A person is holding a 200 pound barbell.

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

c. A girder is lowered into a place. It is slowing down.

d. A jet plane has reached its cruising speed and is at a high attitude.

e. A box in the back of a truck doesn't slide as the truck stops.

a. Static Equilibrium

The barbell is static, and the velocity is also zero as it is at rest.

b. Dynamic Equilibrium

There is no acceleration, but the girder has a constant velocity.

c. Not in equilibrium

As it is slowing down, it means the acceleration has a direction opposite to the direction of motion.

d. Dynamic equilibrium

There is no acceleration, but the plane has a constant velocity.

e. Not in equilibrium

The box slows down as it has negative acceleration or retardation.

Conclusion:

From the above explanation, a, b and d are in equilibrium while c and e are not.