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Work
Physics 11
Be careful…….
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
In everyday life, work can mean a variety of
different things, however when we describe work
from a scientific standpoint, work has a very
precise definition
Work carries a specific meaning in physics!
Work

is a transfer of mechanical of energy

Energy is the capacity to do work

Two main categories of energy
 Kinetic
Energy: Energy of motion
 Potential Energy: Stored (latent) capacity to do work
 Energy can be converted between types
Work:

Symbol: W
 
W  F  d
W  Fd cos 



Product of the vectors force (N) and displacement (m)
But work is a scalar quantity
Units = N·m or J (joule)
A bit of an aside….vector products:

When multiplying vectors together, there are two
common products that are used:

Dot product



Cross product



Scalar product
Involves cosine
Vector product
Involves sine
Work is a dot product!
Example of work:

If you push a box with a force of one Newton
for a distance of one meter, you have done
exactly one joule of work.
Work (force is parallel to distance):
Force (N)
Work (joules)
W=Fxd
Distance (m)
Work (force at angle to distance):
Force (N)
Work (joules)
W = Fd cos ()
Angle
Distance (m)
When work done is zero:
W  Fd cos

How can work equal zero?

We are will look at 3 specific cases
Zero Work Conditions:
1.
Force applied is equal to zero
Ex: a hockey puck
sliding across a
frictionless surface
at a constant
velocity
Zero Work Conditions:
 Displacement is
equal to zero
Zero Work Conditions:
3. The angle between
the force and the
displacement is equal
to ninety degrees
Example #1: A 58.9kg log rolled off a cliff and fell to the
ground below. If the force of gravity did 4559 J of work on
the log, how far did the log fall?
Fg = mg
= (58.9)(9.81)
= 578 N
W = F Δd(cosϴ)
(4559) = (578)Δd(cos 0)
4559 = Δd
(578)(1)
7.8 m = Δd
Example #2:

A mother is pushing her child in a wagon in the
park. How much work does she do if she applies a
force of 285.9N at an angle of 52º with the
horizontal for 125m along the path?
W = F Δd(cosϴ)
= (285.9)(125)(cos52)
= 22 000J
Try it :
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Page 221 # 1, 2, 3
Page 225 # 4-10
Page 235 # 16, 17, 18