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Work, Power, & Energy
Chapter 12 and 13
What is Work?
A: Definition of Work
Work: The product of force and distance when a force is used to move an object.
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.
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.
Is it work?
#1. Pulling books out of your backpack.
#2. Carrying a bag of groceries.
Force applied: lifting (pulling) books
Distance moved: distance from bag to table.
Force applied: lifting bags
Distance: bags move, but different direction than the force
Lift
Motion
LiftMotion
D. Formula for Work
Work = Force X Distance
E. Units for Work
Work = Force X Distance
Force (F) = Newton's (N)
Distance (D) = Meters (m)
Work (W) = Joules (J)
One joule equals 0.2388 calories or 0.0009481 Btu.
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!
F. Formula Triangle
W
F D
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
Work = 20 N X 10 m
Work = 200 J
W
F D
#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
#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 NW
F D
What does Power mean?
Strength
Control
Influence
Machine
Electricity
A. Definition of Power
• The rate at which work is done or the amount of work done in a unit of time.
B. Formula for Power
Power = Work
Time
C. Units for Power
Work – measured in Joules (J)
Time – measured in seconds (s)
Power- measured in J/s or Watts (W)
In Other Words…
1 J/s = 1 Watt
1 Kilowatt (kW) = 1,000 watts
D. Formula Triangle
W
P T
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?
W
F D
Power = Work
Time
Work Triangle
Work = Force X Distance
Work = 8,000 N X 75 m
Work = 600,000 J
Power = 600,000 J
30 s
Power = 20,000 W
P
W
T
Power Triangle
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?
W
F D
Power = Work
Time
Work Triangle
Work = Force X Distance
Work = 10,000 N X 6 m
Work = 60,000 J
Power = 60,000 J
5 s
Power = 12,000 W
P
W
T
Power Triangle
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?
W
F D
Power = Work
Time
Work Triangle
Work = Force X Distance
Work = 9,000 N X 6 m
Work = 54,000 J
Power = 54,000 J
25 s
Power = 2,160 W
P
W
T
Power Triangle
Energy
A. Energy Definition
The ability to do work.
What is Work?
B. Two Types of Energy
a. Kinetic Energy- the energy an object has due to its motion.
b. Potential Energy- energy that is stored in an object due to its position.
Example: a rolling ball.
Example: a bow and arrow.
Example: a battery.
Identify one form of potential energy and one form of kinetic energy.
C. Calculating Energy
a. Kinetic Energy =
Mass X Velocity2
2
K.E. = mv2
2
b. Units –mass= kilograms (kg)velocity = meters per second (m/s)
K.E. = Joules
c. Practice Problemsi. 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
2K.E. = 80 kg X 16 m/s 2
K.E. = 640 J
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
2K.E. = 40 kg X 16 m/s 2
K.E. = 320 J
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
2K.E. = 40 kg X 4 m/s 2
K.E. = 80 J
