Work energy power 2 reading assignment -revision 1
1.
2. In physics we say work is done when a force acts on
something and makes it move.
No motion, no work. Simple as that.
3. The amount of work done by a force on an object
depends on
1. How big that force is, and
2. How far the force moves the object.
In physics, we combine two unlike quantities by making
a product.
The work done W by a force F when the distance
moved in the direction of the force is s is defined
as
W = F . ∆x
4. Determine the equation to find the work done
on the following object :
A)
B)
C)
D)
Question 1
5. Basic Energy Model
SystemEnvironment
Energy Transfer by using force
(pulling/expanding)
Energy Transfer by using force
(pushing/Compressing)
work is done ON the system
SystemEnvironment
work is done BY the system
6. We can calculate the amount of energy removed/added to
the system by using:
So… How to determine the amount of Energy removed/ Added to
the system?
W = F . ∆x
Remember :
The force should be in line with the displacement . So, we may
need cosine or sine to find the component first, before we can
calculate the amount of energy removed/added)
7. Mechanical Energy
• Mechanical Energy is energy that is possessed
by an object due to its motion or its position.
Types of Mechanical Energy:
Kinetic Energy
Potential Energy
8. Kinetic Energy
‘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 :
10. Question 3
• A 5.00 g bullet moving at 600 m/s penetrates
a tree trunk to a depth of4.00 cm.
(a) Use the work-energy theorem, to
determine the average frictional force that
stops the bullet.
(b) Assuming that the frictional force is
constant, determine how much time elapses
between the moment the bullet enters the
tree and the moment it stops moving?
11. GPE = 0
h
m
GPE = mgh
Potential Energy
what is Gravitational Potential Energy?
We define GPE of an object in a the
body-Earth as the energy it has by
virtue of its position relative to the
earth.
The chart illustrate that the GPE of the object –
earth system is defined as :
G(PE) =mgh
F
mg
12. Let’s Brainstorm….
• 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
-∆KE = ∆PE
- (KE-KEo) = PE- PEo
- KE + KEo = PE- PEo
PEo + KEo = PE +KE
Energy Before = Energy After
13. The law of conservation of mechanical energy states:
Energy Cannot Be Created Or Destroyed, Only
Transformed AND TRANSFERED!
KEo
KE
PEo PE
18. continue
• The Energy Bar Chart below ilustrates how the
transformation energy occur in the previous
cases
Position 1 Position 2 Position 3
KE KE KEPE PE PE
19. Question 4
• Use the figure in slide 10 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. Ilustrate the transformation in Energy
Bar chart
20. Power
Power is the amount of transfer/ transformation of energy in certain time taken
to realise the transformation. Thus, the amount of Power can be defined as:
P = ∆E System/ ∆ t.
In the special case that there is no heat involved (i.e. no transfer of energy by using
differences of temperature), this reduces to:
P = W/ ∆ t
