Transcript Work

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 object


Remember 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 = 0

Remember, 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 sense
 B. Everyday sense
 C. 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. Negative
 B. Positive
 C. 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. Yes
 B. No
 C. Yes
