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Energy is defined as the ability to do work. Energy can take many forms. In mechanics the two most common forms of energy are Kinetic Energy and Potential Energy.
Potential Energy is the energy stored in an object when it has been lifted off the ground.
An object with mass m (in kilograms), lifted up at a height of h meters above ground, against gravity, will have stored gravitational potential energy P.E.
P.E. = m x g x h
The units of Potential Energy, are the same as the unit of work.
Work and energy are both calculated in
JOULES.
Example 1:
In the picture above, the ball on the cliff has a mass of 300 g and the
cliff is 20 meters above the ground. What is the potential energy of the
ball?
Solution:
Given: m = 300 g = 0.300 Kg Find:
P.E. P.E. = m x g x h
h = 20 m
= (0.300 Kg)((9.8 N/Kg)(20 m)
g = 9.8 K/Kg
= 58.8 Nm = 58.8 J
Kinetic Energy is the energy of a moving object.
The kinetic energy of an object depends on the mass (m)
of the object and on the speed (v) of the object.
Example 2:
A 200 g soccer ball is moving with a speed of 20.0 m/s. Calculate its kinetic energy.
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Solution:
Given: m = 200 g = 0.200 Kg v = 20 m/s |
Find K.E. (or Ek) |
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