1. Chapter 5 Definition of Work 5.1 Work
Section 1 Work
Chapter 5
5.1 Work
Definition of Work
Work is done on an object when a force causes a displacement of the object.
Work is done only when components of a force are parallel to a displacement.

2. Section 1 Work
Chapter 5
5.1 Work
Definition of Work Cos (0)= 1 therefore, W= F x d in the direction of the force

3. Section 1 Work
Chapter 5
5.1 Work
Work = F x d = KE

4. Example of Work?
5.1 Work
1. A teacher applies a force to a wall and becomes exhausted. Is work done?
This is not an example of work. The wall is not displaced. A force must cause a displacement in order for work to be done
2. A book falls off a table and free falls to the ground. Is work done?
This is an example of work. There is a force (gravity) which acts on the book which causes it to be displaced in a downward direction
3. A waiter carries a tray full of meals above his head by one arm across the room. Is work done?
This is not an example of work. There is a force (the waiter pushes up on the tray) and there is a displacement (the tray is moved horizontally across the room). Yet the force does not cause the displacement. To cause a displacement, there must be a component of force in the direction of the displacement.
4. A rocket accelerates through space. Is work done?
This is an example of work. There is a force (the expelled gases push on the rocket) which causes the rocket to be displaced through space.

5. 5.1 Work Critical Thinking…
1. For each of the following cases, indicate whether the work done on the second object in each example will have a positive or a negative value.
A) The road exerts a friction force on a speeding car skidding to a stop.
The Car-Negative (friction always acts opposite of motion)
B) A rope exerts a force on a bucket as the bucket is raised up a well.
The bucket-Positive (rope and bucket moving up)
C) Air exerts a force on a parachute as the parachutist falls to Earth.
Parachute-Negative (air resistance is a friction,
So acting opposite of the motion)

6. Conservative Forces 5.1 Work
When work done against a force is independent of the path taken, the force is said to be a conservative force
Gravitation is an example of this type of a force
Notice no friction is involved

7. Nonconservative Forces
5.1 Work
Nonconservative Forces
Air resistance and friction are examples of nonconservative forces
The work done against a nonconservative force is dependent upon the path taken
1.0m A
10 Kg B
10 Kg

8. Ff = ukFN ( uk= cos angle) Ff = force of friction
Nonconservative Example
5.1 Work
Wf = Ffd
Ff = ukFN ( uk= cos angle)
Ff = force of friction
FN gets larger
as the angle gets
smaller, so…
A requires more
work against friction than B
W = 98 J
Just to lift it
1.0m A
10 Kg B
10 Kg

9. Chapter 5 Objectives Relate the concepts of energy, time, and power.
Section 5.4 Power
Chapter 5
Objectives
Relate the concepts of energy, time, and power.
Calculate power in two different ways.
Explain the effect of machines on work and power.

10. Rate of Energy Transfer
Section 5.4 Power
Chapter 5
Rate of Energy Transfer
Power is a quantity that measures the rate at which work is done or energy is transformed.
P = W/∆t
power = work ÷ time interval
An alternate equation for power in terms of force and speed is
P = Fv
power = force  speed

11. Section 5.4 Power
Chapter 5
Power

12. Chapter 5 Objectives Identify several forms of energy.
Section 5.2 Energy
Objectives
Identify several forms of energy.
Calculate kinetic energy for an object.
Apply the work–kinetic energy theorem to solve problems.
Distinguish between kinetic and potential energy.
Classify different types of potential energy.
Calculate the potential energy associated with an object’s position.

13. Chapter 5 Kinetic Energy Kinetic Energy
Section 5.2 Energy
Chapter 5
Kinetic Energy
Kinetic Energy
The energy of an object that is due to the object’s motion is called kinetic energy.
Kinetic energy depends on speed and mass.

14. Section 5.2 Energy
Chapter 5
Kinetic Energy

15. Kinetic Energy, continued
Section 5.2 Energy
Chapter 5
Kinetic Energy, continued
Work-Kinetic Energy Theorem
The net work done by all the forces acting on an object is equal to the change in the object’s kinetic energy.
The net work done on a body equals its change in kinetic energy.
Wnet = ∆KE
net work = change in kinetic energy

16. Work-Kinetic Energy Theorem
Section 5.2 Energy
Chapter 5
Work-Kinetic Energy Theorem

17. gravitational PE = mass  free-fall acceleration  height
Section 5.2 Energy
Chapter 5
Potential Energy
Potential Energy is the energy associated with an object because of the position, shape, or condition of the object.
Gravitational potential energy is the potential energy stored in the gravitational fields of interacting bodies.
Gravitational potential energy depends on height from a zero level.
PEg = mgh
gravitational PE = mass  free-fall acceleration  height

18. Section 5.2 Energy
Chapter 5
Potential Energy

19. Section 5.3 Conservation of Energy
Chapter 5
Objectives
Identify situations in which conservation of mechanical energy is valid.
Recognize the forms that conserved energy can take.
Solve problems using conservation of mechanical energy.

20. Chapter 5 Conserved Quantities
Section 5.3 Conservation of Energy
Chapter 5
Conserved Quantities
When we say that something is conserved, we mean that it remains constant.

21. Chapter 5 Mechanical Energy
Section 5.3 Conservation of Energy
Chapter 5
Mechanical Energy
Mechanical energy is the sum of kinetic energy and all forms of potential energy associated with an object or group of objects.
ME = KE + ∑PE
Mechanical energy is often conserved.
MEi = MEf
initial mechanical energy = final mechanical energy (in the absence of friction)

22. Conservation of Mechanical Energy
Section 5.3 Conservation of Energy
Chapter 5
Conservation of Mechanical Energy

23. Roller Coasters
Although not perfectly energy efficient, they are a fun way to view how work, gravitational potential and kinetic energy are exchanged

24. The Downhill skier
What’s this?