Work energy power 2 reading assignment -revision 2
Basic Energy Model
Energy Transfer by using force
Energy Transfer by using force
work is done ON the system
work is done BY the system
We can calculate the amount of energy removed/added to
the system when using forces by using the following
So… How to determine the amount of Energy removed/ Added to
the system by using forces?
W = F . ∆x
In physics we say work is done when a force acts on
something and makes it move.
No motion, no work. Simple as that.
Again be Reminded of this:
The force should be in line with the displacement . So, we may
need cosine or sine to find the component that is in line with the
displacement first, before we can calculate the amount of energy
Determine the equation to find the work done
on the following object :
• Mechanical Energy is energy that is possessed by an object due to its motion
or its position.
• Mechanical energy talks about objects you can see ‘with the naked eye’
Types of Mechanical Energy:
total mechanical energy (TME) = total kinetic energy (KE) + total potential energy (PE)
‘Kinetic energy of an object is the energy it
possesses by virtue of its motion.’
An object has kinetic energy because it has motion.
If an object of mass m has a speed v then it must
have been accelerated from rest to that speed by
some agent, doing work on
KE = (½ )mv2
The Kinetic Energy is defined as :
• The diagram shows a child’s swing. A child of a mass of 28kg is
released from rest at position A.
Ignore all frictional and dissipative effects,
a) Kinetic energy of the child when at point B.
b) Speed of the swing at point B
GPE = 0
GPE = mgh
what is Gravitational Potential Energy
We define GPE of an object in the
body-Earth as the energy it has by
virtue of its position relative to the
The chart illustrate that the GPE of the object –
earth system is defined as :
• A skydiver of total mass 100kg leaves an aircraft flying at 3000m
and freefalls down to a height of 1200m. By this time, she has
reached a constant vertical velocity of 90 m/s. Ignore all
frictional and dissipative effects,
a) Calculate the loss of gravitational potential energy during the fall.
b) Calculate the gain in kinetic energy of the skydiver
• We Already know that :
• Now imagine, Suppose you throw a ball upward :
1. What does work while it is flying through the air? GRAVITY
2. Is the CHANGE in kinetic energy POSITIVE or NEGATIVE? NEGATIVE
3. Is the CHANGE in potential energy POSITIVE or NEGATIVE? POSITIVE
Thus, we can conclude that :
ENERGY IS CONSERVED
The law of conservation of mechanical energy states:
Energy Cannot Be Created Or Destroyed, Only
TRANSFORMED AND TRANSFERED!
Now, Let’s investigate how does the energy transform
Challenge yourself : try to fill in the blank before you move to the other slide
• The Energy Bar Chart below illustrates how
the transformation energy occurs in the
Position 1 Position 2 Position 3
KE KE KEPE PE PE
• Use the figure in slide 13 as your reference.
if the mass of the boy is 56 kg and the initial velocity of
the skateboard is 5 m/s.
Calculate the energy transformation (KE,PE,and TME)
in each position. Illustrate the transformation in Energy
Power is the amount of transfer/ transformation of energy in certain time taken
to realise this transfer/transformation. Thus, the amount of Power can be
P = ∆E System/ ∆ t.
In the special case that there is no heat involved (you will study ‘heat’ later, but ‘heat’
is transfer of energy by using differences of temperature), this reduces to:
P = W/ ∆ t