Chapter-6
Energy and Oscillations
Outline
1 Simple Machines, Work, and Power
2 Kinetic Energy
3 Potential Energy
4 Conservation of Energy
5 Springs and Simple Harmonic Motion
Everyday Phenomenon: Energy and the
Pole Vault
Motion of a Pendulum
Motion of a Pendulum
Pendulum motion intrigued Galileo.
Things to consider:
•Velocity
•Acceleration
•Energy
Simple Machines
A simple machine is any mechanical device
that multiplies the effect of an applied force.
Ex: Lever
Mechanical Advantage is the ratio of the
output force to the input force.
Work
Work
Work may mean different things in everyday life. In physics, we
define the work done on an object by a force as,
Work = Force  Distance.
W = F  d.
Here the force acts along the distance.
Units
Distance
Force
Work
CGS1
cm
dyne
dyne·cm = erg
SI2
m
newton, N
N·m = J
BE3/USC4
foot, ft
pound, lb
ft·lb
1CGS2SI-
centimeter, gram, second.
International System.
3BE-
British Engineering.
4USC-
Unites States Customary.
Does any force do work?
1. Push on a wall
2. Carry a weight and walk horizontally
Does any force do work?
E2
Power
The rate at which work is done is called the power.
Power
The rate at which work is done is called the power.
Work
Power 
.
Time
Power
The rate at which work is done is called the power.
Work
Power 
.
Time
Power is a scalar quantity. The SI unit for power is Watt, W.
1 W = 1 J/s.
Before the arrival of machines horses were used to do work.
With this originated the unit horsepower, hp.
1 hp = 746 W = 550 ft/lb/s.
Problem
A person weighs 110 lbs, climbs 10 steps of
stairs, in 5 second. If the height of a step is
6 inches. Calculate the work and power.
6.2 Kinetic Energy
Kinetic energy is the energy of motion. The word “kinetic”
originated from the Greek word kinetikos, meaning “motion”.
6.2 Kinetic Energy
Kinetic energy is the energy of motion. The word “kinetic”
originated from the Greek word kinetikos, meaning “motion”.
If an object of mass, m moves with a velocity v, then the kinetic
energy, KE is given by the following equation,
1
2
KE  mv .
2
6.2 Kinetic Energy
Kinetic energy is the energy of motion. The word “kinetic”
originated from the Greek word kinetikos, meaning “motion”.
If an object of mass, m moves with a velocity v, then the kinetic
energy, KE is given by the following equation,
1
2
KE  mv .
2
Kinetic energy is a scalar quantity. It also has the same unit as
work, joule (J).
1 J = 1 kg.m2/s2.
W = F  d = KEf  KEi.
What is negative work?
6.3 Gravitational Potential Energy
Gravitational Potential Energy
Gravitational potential energy, GPE is the energy stored in an
object as a result of its height. It can be calculated using weight,
which is mass times gravity, and height. It is given by the
following equation,
GPE  mgh.
Gravitational potential energy is a scalar quantity.
The SI unit for it is also joule, J.
The gravitational potential energy of
raised crate can be converted to
kinetic energy
Elastic Potential Energy, EPE
Elastic Potential Energy, EPE
Elastic Potential Energy, EPE
1 2
EPE  kx .
2
Conservative Forces
A force is conservative if the work it does on moving an object is
independent of the path.
Gravitational force is a conservative force. The work required
against gravity to move an object between two points depends on
the height between the objects. It does not depend on the path taken
to move the object between the two points.
Other conservative forces are elastic spring force, magnetic force,
and electric force.
6.4 Conservation of Energy
Work done on a pendulum
Conservation of Mechanical Energy
Mechanical energy = KE + PE
In the presence only conservative forces, the
total mechanical energy of a system remains
a constant.
Try this box 6.4
How is energy analysis like
accounting?
Pole Vault
6.5 Springs and Simple Harmonic
Motion
Simple Harmonic Motion
Simple Harmonic Motion
Amplitude is the maximum distance from equilibrium.
Period, T is the time taken for one complete cycle.
Frequency: Number of cycles per unit time.
1
f  .
T