1. Chapter 6 Work and Energy
Advanced Physics
Chapter 6
Work and Energy

2. Work and Energy 6-1 Work done by a Constant Force
6-2 Work done by a Varying Force
6-3 Kinetic Energy, and the Work-Energy Principle
6-4 Potential Energy
6-5 Conservative and Nonconservative Forces
6-6 Mechanical Energy and Its Conservation
6-7 Problems Solving Using Conservation of Mechanical Energy
6-8 Other Forms of Energy
6-9 Energy Conservation with Dissipative Forces: Solving Problems
6-10 Power

3. 6-1 Work done by a Constant Force
Describes what is accomplished by the action of a force when it acts on an object as the object moves through a distance
The transfer of energy by mechanical means
The product of displacement times the component of the force parallel to the displacement
Both work and energy are scalar quantities

4. 6-1 Work done by a Constant Force
W = Fd Or
W = Fd cos
where  is the angle between the direction of the applied force and the direction of displacement

5. 6-1 Work done by a Constant Force
W = Fd cos 
Force 
Displacement

6. 6-1 Work done by a Constant Force
Units: joule (N•m)
1 joule = ft•lb

7. 6-1 Work done by a Constant Force
Negative work?
What about friction?
Work done on Moon by Earth?
Work done by gravity depends only on height of hill not incline angle.

8. 6-2 Work done by a Varying Force
Work done by a variable force in moving an object between 2 points is equal to the area under the curve of a Force (parallel) vs. displacement graph between the two points.
Why?
Or we will have to do some calculus on it!

9. 6-2 Work done by a Varying Force

10. 6-3 Kinetic Energy, and the Work-Energy Principle
The ability to do work (and work is?)
Kinetic Energy
Energy of motion; a moving object has the ability to do work
Translational Kinetic Energy (TKE)
Energy of an object moving with translational motion (?)

11. 6-3 Kinetic Energy, and the Work-Energy Principle
Translational Kinetic Energy (KE)
KE = ½ mv2

12. 6-3 Kinetic Energy, and the Work-Energy Principle
The net work done on an object is equal to the change in its kinetic energy
Wnet = Kef – Kei = KE
TKE  m and v2
But…what about potential energy????

13. 6-4 Potential Energy Potential Energy
Energy associated with forces that depend on the position or configuration of a body (or bodies) and the surroundings
Gravitational Potential Energy
Potential energy due to the position of an object relative to another object (gravity)

14. PEgrav = mgy 6-4 Potential Energy Gravitational Potential Energy
Potential energy due to the position of an object relative to another object (gravity)
PEgrav = mgy

15. W = -PE 6-4 Potential Energy Potential Energy
In general the change in potential energy associated with a particular force is equal to the negative of the work done by the force if the object is moved from one point to another.
W = -PE

16. Elastic PE = ½ kx2 6-4 Potential Energy Elastic Potential Energy
Potential energy stored in an object that is released as kinetic energy when the object undergoes a change in form or shape
For a spring:
Elastic PE = ½ kx2
Where k is the spring constant

17. 6-4 Potential Energy Fs = -kx Elastic Potential Energy For a spring:
The force that the spring exerts when it is pushed or pulled is called the restoring force (Fs) and is related to the stiffness of the spring (spring constant-k) and the distance it is compressed or expanded
Fs = -kx

18. 6-4 Potential Energy Fs = -kx Elastic Potential Energy For a spring:
This equation is called the spring equation or Hooke’s Law

19. 6-5 Conservative and Nonconservative Forces
Forces for which the work done by the force does not depend on the path taken, only upon the initial and final positions.
Examples:
Gravitational
Elastic
Electric

20. 6-5 Conservative and Nonconservative Forces
Forces for which the work done depends on the path taken
Examples:
Friction
Air resistance
Tension in a cord
Motor or rocket propulsion
Push or pull by a person

21. 6-5 Conservative and Nonconservative Forces
Work-Energy Principle (final)
The work done by the nonconservative forces acting on a object is equal to the total change in kinetic and potential energy.
Wnc = KE + PE

22. 6-6 Mechanical Energy and Its Conservation
Total Mechanical Energy (E)
E = KE + PE

23. 6-6 Mechanical Energy and Its Conservation
Principle of Conservation of Mechanical Energy
If only conservative forces are acting, the total mechanical energy of a system neither increase nor decreases in any process. It stays constant—it is conserved
KE1 + PE1 = KE2 + PE2
KE = -PE

24. 6-7 Problems Solving Using Conservation of Mechanical Energy
E = KE + PE = 1/2mv2 + mgy
KE = -PE
1/2mv21 + mgy1 = 1/2mv22 + mgy2
Sample problems: P.160 – 165

25. 6-8 Other Forms of Energy Other Forms of Energy:
According to atomic theory, all types of energy is a form of kinetic or potential energy.
Electric energy
Energy stored in particles due to their charge
KE or PE?
Nuclear energy
Energy that holds the nucleus of an atom together

26. 6-8 Other Forms of Energy Other Forms of Energy: Thermal energy
Energy of moving (atomic/molecular) particles
KE or PE?
Chemical energy
Energy stored in the bonds between atoms in a compound (ionic or covalent)

27. 6-8 Law of Conservation of Energy
The total energy is neither increased nor decreased in any process.
Energy can be transformed from one form to another, and transferred from one body to another, but the total remains constant
This is one of the most important principles in physics! 

28. 6-9 Energy Conservation with Dissipative Forces: Solving Problems
Forces that reduce the total mechanical energy
Examples:
Friction
Thermal energy

29. 6-9 Energy Conservation with Dissipative Forces: Solving Problems
Problem Solving (Conservation of Energy)
Draw a diagram
Label knows (before/after) and knowns (before/after)
If no friction (nonconservative forces) then… KE1 + PE1 = KE2 + PE2
If there’s friction (nonconservative forces) then add into equation
Solve for the unknown

30. 6-10 Power Power The rate at which work is done
The rate at which energy is transferred
Units (what?)
1W = 1J/s
746 W = 1 hp

31. 6-10 Power
Power
P = W/t = Fd/t
P = F v (since v =d/t)