# PHYS 2211 Module 7.1

## Work ### Learning Objectives

By the end of this section, you will be able to:

• Represent the work done by any force
• Evaluate the work done for various forces

### Work Done by a Force

In physics, work is done on an object when energy is transferred to the object. In other words, work is done when a force acts on something that undergoes a displacement from one position to another.

### Work Done by a Constant Force

The simplest work to evaluate is that done by a force that is constant in magnitude and direction. Discuss!

Consider how you would answer these questions. Then bring this to class for a group discussion. For each pair of vectors and , is the sign of positive, negative, or zero?

For each of the following situations:

• Draw a before-and-after pictorial diagram
• Draw and label the displacement vector ∆r on your diagram
• Draw a FBD showing all the forces acting on the object
• For each force acting on the object, is the work done by that force +, -, or zero?
• Is the net work done by all the forces +, -, or zero?

(a) An elevator moves upward at a constant speed.

(b) A descending elevator brakes to a halt.

(c) A box slides down a frictionless slope.

(d) A box slides up a frictionless slope.

(e) A ball is thrown straight up. Consider the ball from one microsecond after it leaves your hand until the highest point of its trajectory.

(f) You toss a ball straight up. Consider the ball from the instant you begin moving your hand until you release the ball. Practice!

### Work Done by a Variable Force Practice!

### Work Done by the Force of a Spring Discuss!

Consider how you would answer these questions. Then bring this to class for a group discussion.

A spring with spring constant k = 350 N/m is initially compressed x = 1.90 cm. It is now allowed to relax back to its equilibrium length and then it is stretched an additional 2.10 cm. How much work is done by the spring during this process?