Work

Page 168

What is work?

A little different from what we’re use to in our everyday lives

When you hear the word work, what do you think of?

Scientific Definition of Work

A force that causes a displacement on an object does work on the objectRemember displacement … change in

position (∆X = xfinal – xinitial) Work = force x displacement

W = Fd Work is not done on an object unless

the object is moved because of the action of the force

An example

Is work done in the following scenario?1. A student holds a heavy chair at

arm’s length for several minutes.• No … why?

Parallel Components

Work is done only when components of a force are parallel to a displacement When the force on an object and the object’s

displacement are in different directions, only the component of the force that is in the direction of the object’s displacement does the work

Components of the force perpendicular to a displacement does not do work

For example …

Imagine pushing a crate along the ground

If the force you exert is horizontal, all of your effort moves the crate

If your force is other than horizontal, only the horizontal component of your applied force causes a displacement and does work

See Figure 5.2 page 169

Angles …

Look at Figure 5.2 page 169 W = Fd (cos Θ) If Θ = 0, the cos(0) = 1

Therefore, W = Fd If Θ = 90, cos (90) = 0, so work =

0Remember, work is only done if

force is parallel

Another example

Is work done in the following scenario?

1. A student carries a bucket of water along a horizontal path while walking at constant velocity.

• No … why?

What if many forces are acting on the object?

Then, Wnet = Fnetd (cos Θ)

Net work = net force x displacement x cosine of angle between them

One last thing …

UNITS!! Work has dimensions of force x length In the SI system, newtons x meters = joules (J)

Unit for work is the joule

Let’s check your assignment from yesterday (#1-4 page 171)

1. For each of the following statements, identify whether the everyday or the scientific meaning of work is intended. A. Jack had to work against time as the

deadline neared. B. Jill had to work on her homework before

she went to bed. C. Jack did work carrying the pail of water up

the hill.

Answers

1. A. Everyday senseB. Everyday senseC. Scientific sense

#2

If a neighbor pushes a lawnmower four times as far as you do but exerts only half the force, which one of you does more work and by how much?

Answer

2. The neighbor; twice as much

#3

For each of the following cases, indicate whether the work done on the second object in each example will have a positive or negative value. A. the road exerts a friction force on a

speeding car skidding to a stop. B. A rope exerts a force on a bucket as the

bucket is raised up a well. C. Air exerts a force on a parachute as the

parachutist slowly falls to Earth.

Answer

3. A. NegativeB. PositiveC. Negative

#4

Determine whether work is being done in each of the following examples: A. a train engine pulling a loaded

boxcar initially at rest.B. a tug of war that is evenly matched.C. a crane lifting a car

Answer

4. A. YesB. NoC. Yes