1. 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

2. 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

3. Work, cont.
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

4. 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

5. 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

6. Units of Work and EnergySI
Newton • meter = Joule
N • m = J
J = kg • m2 / s2
US Customary
foot • pound
ft • lb
no special name

7. Be careful
Let go of any definition of ‘work’ that you already believe….
Physics definition of work is WEIRD!!!!

8. 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?

9. No net work, really? Still exerting a force..!
Pushing
on wall..
Yes,
You exert a force
BUT
no displacement in direction of force
no change in energy of wall
Ball rolls
constant velocity
Yes, gravity acts on ball
BUT
No displacement in direction of force
Gravity =vertical
Ball moving horizontal
No change in energy of ball

10. 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

11. 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

12. 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

13. 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

15. 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

16. 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
Speed will increase if work is positive
Speed will decrease if work is negative

17. 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 = m