1. Summary of the Laws of Motion
First Law: If no net force acts on it, then
an object remains at rest OR
an object in motion remains in motion
Second Law: F = ma
Third Law: Action force is equal to reaction force

2. Lecture 5 Work and Energy
Chapter 3.1  3.7
Outline
Work and Power
Kinetic Energy
Potential Energy
Conservation of Energy

3. Work W = work F=force d=distance
The work done by a force acting on an object is equal to the force magnitude times the distance through which the force acts.
A force is any influence that can change the speed or direction of motion of an object
W = work F=force d=distance
W = F d
[W] = joule, 1 joule (J) = 1 N x 1 m
Raising an apple from a table to your mouth takes ~1 J of work: m=0.1 kg, w=mg=1 N, d ~ 1m  W~1 N x 1 m ~ 1 J

4. An average manpower is ~75 W
Power is defined as a rate of doing the work
P = power W = work t = time W
P = ---- t
[P] = watt, 1 watt = 1 joule/ 1 s (J/s)
1 Horsepower (hp) = 746 W = kW
An average manpower is ~75 W

5. Kinetic Energy Energy is a property that enables something to do work
Energy has various forms (kinetic, potential, rest energy)
Kinetic energy is the energy of a moving object
mv2
KE = 2
It takes ~1000 N (or ~ 225 lbs) to hit a nail and drive it 5 mm into a surface.

6. Potential Energy Potential energy is a capacity of doing some work
Potential energy is the energy of position
Gravitational Potential Energy
W = Fd = mgh = PE (potential energy)
PE of a 1000-kg car at the top of a 50-m multilevel parking lot is:
mgh= (1000 kg)(9.8 m/s2)(50 m)=490 kJ
PE is relative!

7. Energy Transformations
Many mechanical processes involve interchanges between KE, PE, and work.
Energy exists in some other forms: chemical energy, heat energy, radiant energy, etc.

8. Conservation of Energy
Energy cannot be created or destroyed.
It can only be changed from one form to another.
The above statement is called the law of conservation of energy

9. Summary Any object, which has mass, contains energy.
Energy enables objects to do work
Power is the rate of doing work