Work
 Provides
a link between force and energy
 Impulse:
change in momentum:: work:
change in ___________



These ‘analogy’ questions used to be a big
part of the SAT!
Force applied for a particular time causes a
change in momentum
Force applied for a particular distance causes
a change in energy
Transferring Energy

By doing work


By applying a force
Produces a
displacement of the
system
 Work
is a way to
transfer energy by
mechanical means
Work, cont.
W  ( F co s q )  x

We are studying only
cases in which F and
x are parallel, q =zero



F is the magnitude of
the force
Δ x is the magnitude of
the object’s
displacement
q is the angle between
F and x
Work, cont.
 This


gives no information about
the time it took for the displacement to occur
the velocity or acceleration of the object
 Different
way of looking at things
compared to impulse-momentum


Impulse-momemtum: vectors
work-energy: scalars
Work can cause and increase or
decrease in energy

Add energy to the object/system


Ex: force of thrust
Reduce the energy of the object/system
 Ex: friction, air drag
A
variety of external forces can act on an
object/system at the same time and result
in net work done on the object!

Result can be +, - or even zero work
Units of Work and Energy
 SI

Newton • meter = Joule
• N•m=J
• J = kg • m2 / s2
 US

Customary
foot • pound
• ft • lb

no special name
Be careful
 Let
go of any definition of ‘work’ that you
already believe….
 Physics
definition of work is WEIRD!!!!
You try to figure it out…net work
or not?
 Pushing
on a wall?
 Ball rolls at constant velocity?
 Carrying something at constant velocity?
 Pushing a box along a floor at constant
velocity?
 Lifting something?
No net work, really? Still exerting a force..!

Pushing


on wall..
Ball rolls

constant velocity


Yes,

You exert a force
Yes, gravity acts on
ball
 BUT


BUT


no displacement in
direction of force
no change in energy of
wall
No displacement in
direction of force
• Gravity =vertical
• Ball moving horizontal

No change in energy
of ball
Carrying box at constant
velocity? No net work

Exerting a force to
hold it up….

BUT




Earth also exerts a
force downward
Forces are balanced
No displacement in
direction of these
forces
No change in energy

You feel that you are
exerting a force to
move it…

BUT

No change in energy
of the box
Can you explain this one?

Pushing a box across the floor at constant
speed?
•
•
•
•
Positive work = Pushing force
Negative work = friction
Net work on box = ZERO
No change in energy of box
Yes , net work is done on the dirt

The system is the dirt; it is being lifted up

Work is being done on the rocks by the crane

Force upward, displacement upward

Increase in gravitational potential energy
Work or no work?
Object
in free fall?
Ignoring air resistance: Earth
does work on the object
(downward direction). KE
increases by amount of
W = F x D = mgh=1/2 mv2
Summary: To do work you
must…

Apply a force

Cause a displacement of the object/system

Cause a change in energy of the
object/system
2/5/12
Work-Kinetic Energy Theorem

When work is done by a net force on an object
and the only change in the object is its speed,
the work done is equal to the change in the
object’s kinetic energy

W n et  K E f  K E i   K E


Speed will increase if work is positive
Speed will decrease if work is negative
Practice Problem
 When
Albert hits a 46-g golf ball with a
club, the ball picks up 43 J of kinetic
energy. A constant force of 2300 N is
applied to the ball while the ball and the
club are in contact. Over what distance is
the club in contact with the ball?
W = F x d, so d = W/F =
43J/2300N = 0.019 m