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Work and Energy
Salud N. Velasco
Ramon Magsaysay (Cubao) High School
Lesson Objectives:
1. Determine the relationship between
the work done and the energy
transferred.
2. Familiarize with the formulas for
work done, kinetic and potential
energies.
3. Understand real life application of
work done, kinetic and potential
energies.
Read the following statements and determine
whether or not they represent work. Click on
the puzzle piece to verify your answer.
1. A teacher applies a force to a wall and
became exhausted.
2. A book fall off a table and free falls to the
ground.
3. A waiter carries a tray full of meals above
his head by one arm straight across the
room at constant speed.
4. A rocket accelerates through space.
Explanation:
Yes. This is an example of work. There is
a force (gravity) which acts on the book
which causes it to displaced in a
downward direction.
Answer:
No. This is not an example of work. There is
force (the waiter pushes up on the tray) and
there is displacement ( the tray is moved
horizontally across the room). Yet the force
does not causes the displacement. To cause a
displacement, there must be a component of
force in the direction of the displacement.
Answer:
Yes. This is an example of work. There
is a force (the expelled gases push on
the rocket) which causes the rocket to be
displaced through space.
Answer:
No. This is not an example of work. The wall
is not displaced.
The Concept of Work
force
distance
For work to be done, three conditions must be meet;
1. There must be a force acting on the object.
2. The object has to move a certain distance.
3. There must be a component of the force in the
direction of the motion.
From the three conditions
given, express work done
mathematically.
W = F cosØ d
Scenario A:
The force and the displacement are in the
same direction.
d
Ø = 180º
F
W = F cosØ d
Scenario B:
The force and the displacement are in
opposite direction.
d
Ø = 0º
F
W = F cosØ d
Scenario C:
The force and the displacement are right
angle to each other.
d
Ø = 90º
F
Sample Problem:
Marie lift a 5 kg mass from the floor and puts
it on a table one meter high. What is the work
done?
Which is the answer?
A. 5 Nm
B. 10 Nm
C. 25 Nm
D. 50 Nm
Congratulations!
It’s good to know that you have
been paying attention.
Solution:
Work Done = Force x distance
(ma) (d)
( 5 kg) (10 m/s2) (1 m)
50 Nm = 50 J
Concept of Energy
 Energy
is the capacity to do work.
 The release of energy does work - and
doing work on something adds energy to
it so - energy and work are actually
equivalent concepts.
E = W = F x d .
Unit for Work and Energy
 In
the English system, the unit of energy is
the Foot.Pound.
 In
the metric system it’s the Newton-Meter,
which is also called the Joule.
 One
joule = The ability to exert a force of
one Newton over a distance of one Meter
Two forms of Energy
Suppose I throw a ball. I do
work getting the ball
moving.
 I exert a force F over a
distance d. The ball then
has acquired some energy,
the energy of motion, or
KINETIC energy.
 K. E. = 1/2 m v2.

Two forms of Energy
A man lift an object to a
height. As he exert a force
W = object’s weight over a
distance h, He do work W x
h = mgh.
 Potential Energy is the
energy with respect to
position.
 P.E. = mgh.

Sample Problem
A 50 kg box falls from a
bridge and lands in the
water 20 m below. Find
its initial PE and
maximum KE.
Which is the answer?
A. PE = 10000 J KE = 10000 J
B. PE = 10000 J KE = 0
C. PE = 0 KE = 10000 J
D. PE = 500 J KE = 10000 J
Congratulations!
It’s good to know that you have been paying attention.
Solution:
a. Initial energy is taken from the top. Therefore,
PEi = mgh
= (50 kg)(10 m/s2)(20 m)
= 10000 J
b. The maximum KE is equal to the total PE at the
top. Kemax = 10000 J
Let’s do more example
A package of 5kg is lifted vertically
through a distance of 10m at a constant
speed. Taking the acceleration due to
gravity to be 10m/s2 What is the
gravitational potential energy gained by
the package?
Congratulations!
It’s good to know that you have been
paying attention.
Solution:
PEi = mgh
= (5 kg)(10 m/s2)(10 m)
= 500 J
Conservation and Conversion of
Energy
 Potential
Energy is
potential because it can
be gotten back as “real”
kinetic energy.
 To get it back, all I have
to do is to let the object
fall.
Conservation and Conversion of
Energy
 As
the object falls faster
and faster, its PE is
gradually converted into
KE.
 At the bottom, just before
impact, its KE is zero.
 And its original PE has
become entirely KE
1/2 m v2 = m g h.
Conservation and Conversion of
Energy
 Conservation
of energy
tells us that the total
energy of the system
does not change but
maybe converted into
other forms
 What happens to the
objects energy when he
hits the floor?
Conservation and Conversion of
energy.
Trace the energy conversion in the given
situation.
POTENTIAL ENERGY
Evaluation
Complete the table.
PE
______
15 J
______
KE
0J
Work
Done
______
______
______
______
15 J
References:
http://www.google.com for the pictures
http://www.conceptualphysics.com
Physics Matters by Dr Chew et al
Sorry.
You need to revise on the topic.