1. A Look at Work and Kinetic Energy
Created for CVCA Physics By
Dick Heckathorn
17 February 2K + 4

2. A car at rest possesses:
Amount identified by:

3. A moving car possesses:

4. A moving car also possesses:

5. Car at rest has mass
Moving car has mass
velocity = 0
and velocity

6. What happens when the car starts from rest and achieves some velocity?
The car’s kinetic energy:
Increases

7. What causes the increase in kinetic energy?
How big is the force?

8. Note that work is a scalar quantity. We write the equation as:
The Concept of Work
Note that work is a scalar quantity.
Whenever a vector quantity is multiplied by a vector quantity, and the result is a scalar quantity, we call the product a dot product.
We write the equation as:
Where
is the angle between
and

9. And Now For Some Algebraic Manipulation

10. The amount of work done equals the change in Kinetic Energy
And Now For Some More Algebraic Manipulation
The amount of work done equals the change in Kinetic Energy

11. Let’s examine Work - Energy Relationship
What causes a change in energy?
How big is the force? If
Then If
Then

12. Unit Analysis
What are the units?
How do these units compare?
Same - Same

13. Unit Analysis
is called a
is also called a

14. Summary
If a car changes velocity from zero to some velocity, the car increases in kinetic energy.
It is likewise true for a moving object that comes to a stop, the car decreases in kinetic energy.

15. Gravitational Potential Energy (Ug )
If work is done to raise an object, the work done is equal to:
If the object increases in potential energy, the work done is positive,
if the object decreases in potential energy the work done is negative.

16. 1. How much work is done if one exerts a force of 5-N on an object and moves it 7-m?
35 Joules
W = F d = 5N x 7m

17. 2. How much work is done if a force of 14-N causes an object to raise 3-m?
42 Joules
W = F d = 14N x 3m

18. The distance is down and the force is ...
3. How much work is needed to lower the 14-N object the 3-m back to where it began?
-42 Joules
W = F d = 14N x 3m x cos 180o
The distance is down and the force is ... Up

19. The object has the same energy after as before.
4. How much is done if one picks up a 30-N suitcase from the floor, carries it across the room 9-m and deposits it on the floor?
None
The object has the same energy after as before.
The force was up and the distance the suitcase was moved was horizontal.
30-N x 9-m x cos 90o =
30-N x 9-m x 0 =

20. 5. Calculate the work done by Mr
5. Calculate the work done by Mr. Kamp who pulls a sled a distance of 20-m while exerting a 100-N force on a rope that makes an angle of 45o relative to the horizontal.
1414 Joules

21. 6. How much work do you do when you hold a 60-kg mass above your head for 15-sec?
No work

22. 7. How much work is required to carry a 15-kg suitcase a horizontal distance of 15-m?
No work