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 #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 #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? Electricity Control Strength 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 = 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

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