Cycle 3 Grade 9 Advanced Physics
Chapter 8
Work, Energy and Machines
Work
Definition: Work is the transfer of
energy that occurs when a force
(F) is applied through a
displacement (d).
Unit is the Joule (J) or the
Newton-Meter (N.m).
*Note Force x Displacement =
Newton x Meter = N.m
Work at an angle
When dealing with forces
exerted at an angle, work can
then be calculated as follows:
Work at an angle
The work done may be positive, zero, or negative, depending on the angle
between the force and the displacement.
Total Work Done on a System
The total work done on a system is the sum of the work done by each body that exerts a
force on the system.
Work done by a variable force
Force (N)
The area under a force-displacement graph is equal to the work.
Area = workdone
Displacement (m)
Work – Problem Solving Strategies
Practice Questions
Q1. Ahmed uses a force of 25 Newtons to lift his grocery bag while
doing 50 Joules of work. How far did he lift the grocery bags?
Answer = 2 m
Q2. A baseball player does 1234 Joules of work when hitting a baseball
into left field. Assuming the baseball landed 100 meters away from
home plate, how much force did the player use to hit the ball?
Answer = 12.34 N
Q3. A force of 70 N acts on a 12 kg object at 60ᵒ to the horizontal and as
a result, the object is displaced horizontally through a displacement of 23
m. Determine the work done on the object.
Answer = 805 J
Energy
Energy is quite difficult to define!
Definition: the ability to do work.
Unit is the Joule (J)
There are essentially two types of
energy in nature:
1. Potential
2. Kinetic
Kinetic Energy
Kinetic energy is the energy of a
moving object.
The term ‘kinetic’ means movement.
When stored energy is being used up,
it is making things move or happen.
K.E. = ½ mv2
Units = (kg)(m/s)2 = 1 Joule
Potential Energy
Potential energy is the energy stored in an
object.
The term ‘potential’ means the energy has
the ability to do something useful later on.
Gravitational Potential Energy (PEGRAV)
is the potential energy a physical object
with mass has in relation to another
massive object due to gravity.
PEGRAV = -mgh
Units = (kg)(m/s2)(m) = 1 Joule
Work Done by Gravity
Work = Force x Displacement (or height)
W=Fxd
W = -mg x h
W = - mgh
Work-Energy Theorem and Power
Work done on a system is equal to the change in the system’s energy.
W = ΔE
Power is equal to the change in energy divided by the time required
for the change.
P = ΔE/t = W/t
Unit is the Watt (W).
W = J/s
One Watt is equal to 1 J of energy transformed in 1 s.
The Law of Conservation of Energy
The total energy of an isolated system remains constant; it is said to be
conserved over time. This law means that energy can neither be created nor
destroyed; rather, it can only be transformed or transferred from one form to
another.
Practice Questions
A rollercoaster is released from rest
at point A and moves along the track
to the right. The mass of an empty
coaster is 500 kg.
Assuming there is no friction,
answer the questions below.
a. At what location has:
i)
ii)
iii)
iv)
Greatest potential energy? A
Lowest potential energy? C
Greatest kinetic energy? C
Lowest kinetic energy? A
Practice Questions
A rollercoaster is released from rest
at point A and moves along the track
to the right. The mass of an empty
coaster is 500 kg.
Assuming there is no friction,
answer the questions below.
b. Calculate the:
i)
ii)
iii)
iv)
Potential Energy at point A 98000 J
Potential Energy at point E 49000 J
Kinetic Energy at point C 98000 J
Velocity at point C 19.79 m/s