Work, Power, & Energy

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. 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.

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 W Work = 20 N X 10 m F D Work = 200 J

#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 N W F D

What does Power mean? Strength Electricity Control Influence Machine

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? 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

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

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

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. 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.

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

C. Calculating Energy • 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 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

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

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

Work, Power, & Energy