1. Work, Power, & Energy
Chapter 12 and 13

2. What is Work?

3. A: Definition of Work
Work: The product of force and distance when a force is used to move an object.

4. B: No Work Without Motion!
In order for you to do work on an object, the object must move some distance as a result of your force.

5. C. Only Force in the Same Direction
In order to do work on an object, the force you exert must be in the same direction as the object’s motion.

6. Is it work? #1. Pulling books out of your backpack.
#2. Carrying a bag of groceries.
Motion
Lift
Motion
Lift
Force applied: lifting bags
Force applied: lifting (pulling) books
Distance: bags move, but different direction than the force
Distance moved: distance from bag to table.

7. D. Formula for Work
Work = Force X Distance

8. E. Units for Work Work = Force X Distance Force (F) = Newton's (N)
Distance (D) = Meters (m)
Work (W) = Joules (J)
One joule equals calories or Btu.

9. What was a Newton?? Newton = kilograms X acceleration of gravity
Newton = kilograms X 10 m/s2
Oh Yeah! Force (in Newtons) is equal to mass times acceleration!

10. F. Formula Triangle W F D

11. G. Practice Problems
#1. To help rearrange the furniture in your classroom, you exert a force of 20 N to push a desk 10 m. How much work did you do?
Work = Force X Distance W
Work = 20 N X 10 m F D
Work = 200 J

12. #2. A hydraulic lift exerts a force of 12,000N to lift a car 2 m
#2. A hydraulic lift exerts a force of 12,000N to lift a car 2 m. How much work is done on the car?
Work = Force X Distance
Work = 12,000 N X 2 m
Work = 24,000 J W F D

13. #3. It took 500 Joules to push a chair 10 meters across the floor
#3. It took 500 Joules to push a chair 10 meters across the floor. With what force was the chair pushed?
Force = Work
Distance
Force = 500 J
10 m
Force = 50 N W F D

15. What does Power mean?
Electricity
Control
Strength
Influence
Machine

16. A. Definition of Power
The rate at which work is done or the amount of work done in a unit of time.

17. B. Formula for Power
Power = Work
Time

18. C. Units for Power Work – measured in Joules (J)
Time – measured in seconds (s)
Power- measured in J/s or Watts (W)

19. In Other Words…
1 J/s = 1 Watt
1 Kilowatt (kW) = 1,000 watts

20. D. Formula Triangle W P T

21. Practice Problems
#1. A crane lifts an 8,000 N beam 75 m to the top of a building in 30 seconds. How much power does the crane use?
Power = Work
Time
Work = Force X Distance
Work = 8,000 N X 75 m
Power = 600,000 J
30 s
Work = 600,000 J
Power = 20,000 W W F D W P T
Work Triangle
Power Triangle

22. Practice Problems
#2. A motor exerts a force of 10,000 N to lift an elevator 6 m in 5 seconds. What is the power produced by the motor?
Power = Work
Time
Work = Force X Distance
Work = 10,000 N X 6 m
Power = 60,000 J
5 s
Work = 60,000 J
Power = 12,000 W W F D W P T
Work Triangle
Power Triangle

23. Practice Problems
#3. A tow truck exerts a force of 9,000 N to pull a car out of a ditch. It moves the car a distance of 6 m in 25 s. What is the power of the tow truck?
Power = Work
Time
Work = Force X Distance
Work = 9,000 N X 6 m
Power = 54,000 J
25 s
Work = 54,000 J
Power = 2,160 W W F D W P T
Work Triangle
Power Triangle

25. Energy

26. A. Energy Definition
The ability to do work.

27. What is Work?

28. B. Two Types of Energy
a. Kinetic Energy- the energy an object has due to its motion.
Example: a rolling ball.
b. Potential Energy- energy that is stored in an object due to its position.
Example: a bow and arrow.
Example: a battery.

29. Identify one form of potential energy and one form of kinetic energy.

30. C. Calculating Energy Kinetic Energy = b. Units – mass= kilograms (kg)
Mass X Velocity2 2
K.E. = mv2 2
b. Units –
mass= kilograms (kg)
velocity = meters per second (m/s)
K.E. = Joules

31. c. Practice Problems i. An 80 kg man is jogging at a velocity of
4 m/s. What is his kinetic energy?
K.E. = mv2 2
K.E. = 80 kg X (4 m/s)2 2
K.E. = 80 kg X 16 m/s 2
K.E. = 640 J

32. ii. A 40 kg boy is jogging at a velocity of
4 m/s. What is his kinetic energy?
K.E. = mv2 2
K.E. = 40 kg X (4 m/s)2 2
K.E. = 40 kg X 16 m/s 2
K.E. = 320 J

33. iii. A 40 kg boy is jogging at a velocity of
2 m/s. What is his kinetic
energy?
K.E. = mv2 2
K.E. = 40 kg X (2 m/s)2 2
K.E. = 40 kg X 4 m/s 2
K.E. = 80 J