Download NASC 1110

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
Lecture 5
Work and Energy
Chapter 3.1  3.7
Outline
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Work and Power
Kinetic Energy
Potential Energy
Conservation of Energy
Work
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=Fd
[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
Power
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 = 0.746 kW
An average manpower is ~75 W
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.
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
PE is relative!
multilevel parking lot is:
mgh= (1000 kg)(9.8 m/s2)(50 m)=490 kJ
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.
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
Summary
• Any object, which has mass, contains energy.
• Energy enables objects to do work
• Power is the rate of doing work