1. I. Energy and Work (p.124-131) Energy Work Conservation of Energy
Ch. 4 - Energy
I. Energy and Work (p )
Energy
Work
Conservation of Energy

2. W = Fd A. Work Work transfer of energy through motion
force exerted through a distance
W = Fd
W: work (J)
F: force (N)
d: distance (m)
1 J = 1 N·m
Distance must be in direction of force!

3. A. Work
Brett’s backpack weighs 30 N. How much work is done on the backpack when he lifts it 1.5 m from the floor to his back?
GIVEN:
F = 30 N
d = 1.5 m
W = ?
WORK:
W = F·d
W = (30 N)(1.5 m)
W = 45 J

4. A. Work GIVEN: m = 40 kg d = 1.4 m - during d = 2.2 m - after W = ?
A dancer lifts a 40 kg ballerina 1.4 m in the air and walks forward 2.2 m. How much work is done on the ballerina during and after the lift?
GIVEN:
m = 40 kg
d = 1.4 m - during
d = 2.2 m - after
W = ?
WORK:
W = F·d F = m·a
F =(40kg)(9.8m/s2)=392 N
W = (392 N)(1.4 m)
W = 549 J during lift
No work after lift. “d” is not in the direction of the force.

5. Example problems
You push a refrigerator with a horizontal force of 100 N. If you move the refrigerator a distance of 5 m while you are pushing, how much work do you do?
How much work do you do when you lift a 100 N child 0.5 m?
The brakes on a car do 240,000 J of work in stopping the car. If the car travels a distance of 40 m while the brakes are being applied, how large is the force that the brakes exert on the car?
How much work is done in lifting an object that has a mass of 5 kg a vertical distance of 2 m?

6. Machines
A device that changes the force or increases the motion from work.
2 Types of machines:
Simple machine
Compound machine

7. Simple Machine
A machine that does work with only one movement of the machine
6 types: lever, pulley, wheel and axle, include plane, screw, wedge

8. Compound Machine
A machine that is a combination of two or more simple machines
Ex: scissors – 2 wedges and 2 levers
bicycle

9. Efficiency
Machines can increase force OR increase speed (no machine can increase both at the same time)
You always put more work into a machine than you get out of that machine

10. Efficiency Problems
You do 20 J of work in pushing a crate up a ramp. If the output work from the inclined plane is 11 J, then what is the efficiency of the inclined plane?
The input work on a pulley system is 75 J. If the pulley system is 84% efficient, then what is the output work from the pulley system?
Workers do 8000 J of work on a 2,000N crate to push it up a ramp. If the ramp is 2 m high, then what is the efficiency of the ramp?

11. B. Energy ENERGY Without energy nothing would ever change.
The ability to cause change.
THERMAL
internal motion of particles
ENERGY
MECHANICAL
NUCLEAR
motion of objects
changes in the nucleus
ELECTRICAL
joules (J)
CHEMICAL
motion of electric charges
bonding of atoms

13. B. Energy Kinetic Energy (KE) energy due to motion
depends on mass and velocity
80 km/h
50 km/h
Which has the most KE?
Which has the least KE?
80 km/h truck
50 km/h motorcycle

14. Kinetic Energy

15. Kinetic Energy Examples
A jogger with a mass of 60 kg is moving forward at a speed of 3 m/s. What is the jogger’s kinetic energy?
A baseball will a mass of 0.15 kg is moving at a speed of 40 m/s. What is the baseball’s kinetic energy?
A 1500 kg car doubles its speed from 50 km/h to 100 km/h. By how many times does the kinetic energy from the car’s forward motion increase?

16. B. Energy Potential Energy (PE) stored energy
depends on position or configuration of an object
Which boulder has greater gravitational PE?
What other ways can an object store energy?

17. Potential Energy
Elastic potential energy – energy that is stored by compressing or stretching an object (spring, rubber band, etc)
Chemical potential energy – energy that is due to chemical bonds
Gravitational potential energy – energy that is due to the gravitational forces between objects

18. Potential Energy Examples
A 4 kg ceiling fan is placed 2.5 m above the floor. What is the potential energy of the ceiling fan?
An 8 kg history textbook is placed on a 1.25 m high desk. What is the potential energy of the textbook relative to the floor?

19. C. Conservation of Energy
Law of Conservation of Energy
Energy may change forms, but it cannot be created or destroyed under ordinary conditions.
EX:
PE  KE (apple in a tree)
mechanical  thermal (friction, wheels of car)
chemical  thermal (reaction that creates heat)

20. C. Conservation of Energy
PE  KE
View pendulum animation.
View roller coaster animation.

21. C. Conservation of Energy
Mechanical  Thermal
View rolling ball animations.
View skier animation.

22. Power
Rate at which energy is converted
Power = Watts (W) = J/s

23. Power Examples
You transform 950 J of chemical energy into mechanical energy to push a sofa. It took you 5 s to move the sofa, what was your power?
If a runner’s power is 400 W as she runs, how much chemical energy does she convert into other forms in 10 minutes?
One horsepower is a unit of power equal to 746 W. How much energy can a 150 hp engine transform in 10 s?