Work Energy & Power
Name: ________________
Class: _________________
Index: ________________
Learning Objectives
1) show understanding that kinetic energy, elastic potential energy,
gravitational potential energy, chemical potential energy
2) state the principle of the conservation of energy
3) apply the principle of the conservation of energy to new situations
or to solve related problems
4) state that kinetic energy = ½ mv2 and gravitational potential energy
= mgh (for potential energy changes near the Earth’s surface)
5) apply the relationships for kinetic energy and potential energy to
new situations or to solve related problems
6) recall and apply the relationship work done = magnitude of a force
x the distance moved in the direction of the force
7) understand the relation between work done and energy change
8) recall and apply the relationship power = work done/time taken to
new situations or to solve related problems
Work, Energy and Power
• When a force moves something, WORK is done.
• In everyday language, work means almost anything that people do.
• But in physics, work has a more precise meaning.
• Work is done whenever a force produces movement.
• For example, it takes work to push a heavy object such as a stalled car.
• When the displacement of the car is greater , then more work is done.
• When the pushing force is greater, then more work is done.
Work, Energy and Power.
Work is said to be done when it moves under the influence of a force.
Work done by a constant force is given by the product of the force
and the distance moved in the direction of the force.
The SI unit for work done is the Joule (J).
The man is doing work if
the car moves a distance.
Is the man doing any work?
W = Fs
where W = work done by a constant force,
F = constant force (in Newtons),
s = distance moved in the direction of the force.
1 Joule = 1 Newton ×1 metre.
Definition of Joule
One Joule is defined as the work done by a force of one
Newton which moves an object through a distance of
one metre in the direction of the force.
Worked example 1
Force F = 30 N
Distance s = 50 m.
Calculate the work done by the force on the trolley if it is moved
through 50 m.
Solution
W = Fs
= 30 N x 50 m
= 1500 J
Worked example 2
5m
F =10 N
Calculate the work done by the force when the object is put on
top of the printing machine.
Solution
W = Fs
= 10 N x 5 m
= 50 J
ENERGY
Energy is the capacity to do work.
The SI unit for energy is the Joule (J).
Two forms of energy that we will learn:
Potential energy (P.E.),
Kinetic energy (K.E.).
P.E. and K.E. are known as mechanical energy.
Forms of energy
Energy exists in different forms.
Mechanical energy
Kinetic energy--all objects in motion.
Potential energy--- gravitational potential energy , elastic
potential energy.
Chemical energy----fuels such oil,wood, coal,electric cells, food and
explosives.
Nuclear energy.-----atomic bombs, nuclear reactors.
Electromagnetic energy------the electromagnetic spectrum such as
visible light, radiowaves, infra-red, ultra-violet, X-rays and gamma
rays.
Electrical energy----the energy associated with the current in an
electric cell.
Thermal energy------the energy possessed by the atoms or molecules
of matter in the form of kinetic energy and potential energy.
Coal, oil &
natural gases
Fossil fuel
Hydroelectric
Energy
resources
Wind
Geothermal
Solar
Nuclear
Potential energy
Potential energy is the energy
possessed by a body by virtue
of its position or condition.
There are two types of potential
energy:
1) Gravitational Potential Energy
2) Elastic Potential Energy
Elastic potential energy
Gravitational Potential
Energy
Gravitational Potential Energy
• Simplified to P.E.
• Formula for P.E.:
P.E. = mgh
• m = mass (kg),
g = acceleration due to
gravity (m/s2),
h = height of object (m).
Gravitational potential energy.
Worked example
35 m
w
Solution
P.E. = mgh
= 25 x 10 x 35
= 8.75 kJ
A package of mass 25 kg. is
lifted up a distance of 35m. What
is the P.E. gained by the
package? Assume g = 10 m/s2.
Worked example
The mass on the pack = 50 kg. Calculate the
P.E. gained by the pack when the man scaled
up a height of 150 m. Assume g = 10 m/s2.
H = 150 m.
Start here .
Solution
P.E. = mgh
= 50 x 10 x 150
= 75 kJ
Kinetic energy
Kinetic energy (K.E.) is the energy possessed
by a body by virtue of its motion.
A moving object has kinetic energy.
The faster it moves, the more kinetic
energy it has.
The heavier it is, the more kinetic
energy it has.
K.E. = ½ mv2
Kinetic energy is a scalar quantity.
Formula for kinetic energy
• K.E. = ½ mv2
Where m = mass (kg)
v = velocity of object(m/s)
Conversion and conservation of energy
The Principle of Conservation of Energy states that:
‘Energy can neither be created nor destroyed in any
process. It can be converted from one form to another or
transferred from one body to another, but the total amount
remains constant.’
When there is no loss of energy due to friction,
the PE of the ball transforms to KE
Example
A diver on a spring board.
» chemical energy in the diver
» bents the diving board - elastic P.E. in the board
» converts to K.E. for the diver when he is pushed
up.
• Burning of fuels
» chemical energy in the
fuels
» radiant heat energy and
light energy.
• Battery connected to a
filament lamp
» chemical energy in
cell/battery
» electrical energy
» radiant heat energy and
light energy.
POWER
• Power is the rate of work done or energy converted.
P=W=E
t
t
where P = Power,
W= work done (J)
E = energy converted
t = time taken (s).
The SI unit for power is
the Watt (W).
Example
The girl’s weight is 450 N. She takes 10 s to run up the stairs
of height 5 m. What is her body power?
Solution
P = W/t
= (450 x 5) / 10
= 225 W
x
y
X and Y have the same mass and they took 10 and 20 s
respectively to climb to the top of the building? Who has more
power, X or Y?
Solution
X has more power as the time taken was shorter compared to
Y.
Efficiency
Efficiency = useful energy output× 100%
energy input
Example
A heater produces 500 J of heat energy every
second. It is rated at 800 W. Calculate the
efficiency of this heater.
Solution
Efficiency = (500/800) x 100%
= 62.5%
The roller-coaster
• The forms of energy are;
1) P.E.
2) K.E.
3) Heat and sound energy.
• If the roller-coaster is frictionless and there is no air
resistance, then the energy converts between P.E. and
K.E.
• (provided no extra work is done on the roller coaster)
Further Example
The following diagram shows several positions
of a pendulum during one period.
(a) Assuming the point X is 20 cm higher than the point Y and the mass
of the bob is 200 g, calculate the difference in gravitational potential
energy between the two points.
P.E. = mgh
= (0.2) (10) (0.2)
= 0.4 J
(e)
What is the speed of the bob;
i. at X,
ii. at Y?
K.E. = 0 J,
Gain in K.E. = Loss in P.E.
v = 0 ms-1
½ mv2 = mgh
½ (0.2) v2 = 0.4
v = 2 ms-1
Reference
http://www.clipart.com
http://www.joyriders.com
http://www.ngdir.ir
http://www.heatingcentral.com
http://www.inspiration.com/Kidspiration-science-examples