Question

mission we1: work
which of the following are examples of work being done (either positive or negative) upon an object? include any that apply.
a. an upward force is applied to an object to suspend it motionless in mid - air.
b. an upward force is applied to a downward moving object to slow it down as it moves.
c. an upward force is applied to an object to move it upward at a constant speed.
d. an upward force is applied to an object to accelerate it upward.
e. a rightward force is applied to an object to move it to the right at a constant speed.
f. a force is applied to an object to hold it at rest on an inclined plane.
g. a rightward force is applied to an object to accelerate it to the right.

Explanation:

Work is defined as $W = F \cdot d \cdot \cos\theta$, where $F$ is force, $d$ is displacement, and $\theta$ is the angle between them. Work is done only if there is displacement.

• a: No displacement → no work.
• b: Upward force, downward displacement ($\theta=180^\circ$, $\cos\theta=-1$) → negative work done.
• c: Upward force, upward displacement ($\theta=0^\circ$, $\cos\theta=1$) → positive work done.
• d: Upward force, upward displacement (acceleration implies motion) → work done.
• e: Rightward force, rightward displacement → positive work done.
• f: No displacement → no work.
• g: Rightward force, rightward displacement (acceleration implies motion) → work done.

Answer:

b. An upward force is applied to a downward moving object to slow it down as it moves., c. An upward force is applied to an object to move it upward at a constant speed., d. An upward force is applied to an object to accelerate it upward., e. A rightward force is applied to an object to move it to the right at a constant speed., g. A rightward force is applied to an object to accelerate it to the right.