1. Ch 8 Energy Notes
ENERGY

2. What is Energy? Ability to do WORK
Ch 8 Energy Notes
Ability to do WORK
Measured in Joules (calories or kilocalories)
What is Energy?

3. What is Work? Work depends on:
Ch 8 Energy Notes
Work depends on:
The amount of force applied to the object.
The distance that the object moves while the force is applied.
The direction of the force with respect to the direction the object moves.
What is Work?

4. Work:
Work is done on an object whenever a force is exerted on an object through some distance.
If the force is constant and parallel to the direction of movement then:
W = F d
W = 0, if d = 0
No work done if object doesn’t move.
W = 0, if F = 0
No work done, whenever there is no force.

5. Ch 8 Energy Notes
If the force on the object is in the direction the object moves, the work done is:
W = Fx F x

6. Ch 8 Energy Notes
If the direction of the force is opposite the direction the object moves, work is:
W = -Fx F x

7. Ch 8 Energy Notes
If the force is perpendicular to the direction the object moves, the work done is 0.
If the object doesn’t move, the work done is 0. F
W = 0 x
Force is NOT Work

8. How is all energy divided?
Potential
Energy
Kinetic
Energy
Gravitation
Potential
Energy
Elastic
Chemical

9. What is Potential Energy?
Energy that is stored and waiting to be used later

10. What is Gravitational Potential Energy?
Potential energy due to an object’s position
P.E. (gravitational) = mass x
height x gravity
Don’t look down, Rover!
Good boy!

11. What is Elastic Potential Energy?
Potential energy due compression or expansion of an elastic object.
Notice the ball compressing
and expanding

12. Work done by a spring = ½ Kx2

13. What is Chemical Potential Energy?
Potential energy stored within the chemical bonds of an object

14. K.E. = ½ mv2 What is Kinetic Energy?
Ch 8 Energy Notes
An object’s kinetic energy depends on:
the object’s mass.
Kinetic energy is directly proportional to mass.
the object’s speed.
Kinetic energy is directly proportional to the square of the object’s speed.
Energy an object has due to its motion
K.E. = ½ mv2
What is Kinetic Energy?

15. Kinetic energy is a scalar quantity.
Ch 8 Energy Notes
Kinetic energy is a scalar quantity.
When the kinetic energy of an object changes, work has been done on the object.
Work is a scalar quantity.
What is the kinetic energy of an object with mass of 1 kg, moving at 1 m/s?
0.5 Joule.

16. Work and Kinetic Energy
Ch 8 Energy Notes
The work done on an object by the net force equals the object’s change in kinetic energy.
Wnet = DKE
Work and Kinetic Energy

17. Law of Conservation of Energy
If no external forces act on a system, the total energy of the system will remain constant.
Mechanical Energy = PE + KE

18. What are different Forms of Energy/Energy Transformations?

19. Your work = Fx = (10 N)(1 m) = 10 Joules
Suppose you have to exert a force of 10 Newtons to push a book 1 meter across a horizontal table at constant velocity (as shown above). Since your force is in the direction the book moves, the work you do on the book is given by:
Your work = Fx = (10 N)(1 m) = 10 Joules
Work done by Friction

20. Work done by friction = -Fx = -(10 N)(1 m) = -10 Joules
The work-energy equation would lead you to think that doing 10 Joules of work on the book should increase the kinetic energy of the book by 10 Joules - but that doesn't happen (the speed of the book is constant). What's going on?
A friction force opposes the motion of the book. This force must also be 10 Newtons (Since the book moves at constant velocity, the net force on it must be zero.).
The friction force pulls in the opposite direction from the direction the book moves, the work done by friction is given by:
Work done by friction = -Fx = -(10 N)(1 m) = -10 Joules
This means that the friction force removes 10 Joules of energy from the book. So, while you were adding 10 Joules of energy to the book, friction was busy taking the 10 Joules of energy away from the book. This is why the kinetic energy of the book does not change!

21. DWork Power = time W P t Power is the rate work is done.
Ch 8 Energy Notes
Power is the rate work is done.
Units of power: 1 Joule/sec = 1 Watt
1000 Watts = 1 kilowatt
Power is a scalar quantity.
DWork
Power = W
time P t

22. What are (Simple) Machines?
Ch 8 Energy Notes
A machine is a mechanical device used to do work.
Examples of simple machines:
Inclined plane
Lever
pulley
What are (Simple) Machines?

23. Workout = Workin (Simple) Machines
Ch 8 Energy Notes
A machine can never output more work (energy) than is put into it.
At best,
Workout = Workin
Machine
Workin
Workout
(Simple) Machines

24. What is Mechanical Advantage?
Ch 8 Energy Notes
Machines can’t multiply work or energy, but they can multiply force.
Mechanical advantage measures how much a machine multiplies force.
Force machine exerts
MA =
Force you exert
What is Mechanical Advantage?

25. How do we Measure Efficiency?
Ch 8 Energy Notes
The efficiency of a machine tells how much of the energy (work) that goes into the machine actually does useful work.
It is usually expressed as a percent.
Useful work done
Efficiency =
x 100%
Energy input
How do we Measure Efficiency?