Grade 11 power, energy and work

1. WORK, ENERGY & POWER
Presented by Vusi

2. DEFINITION OF WORK
• In physics, the definition of WORK is the application of a force
through a displacement
W = F· ∆x = F∆x cosθ
• W is the work done (J) 1 J = 1 kg. m2.s-2 = 1 N.m
• F is the force applied (N)
• ∆x is the displacement through which the force acts (m)
• Cosθ is the angle between force and displacement
• Only the force that acts in the direction of motion counts towards
work

3. LET’S TALK ABOUT THE ANGLE θ
Force and Displacement in the same Direction
θ = 0o
The force does positive work on the object and increases the energy of the
system
Force and Displacement in Opposite Directions
θ = 180o
The force does negative work on the object and decreases the energy of
the system
Force and Displacement Perpendicular to each Other
θ = 90o
There is no Work Done on an Object

4. LET’S TALK ABOUT THE ANGLE θ
Force and Displacement at an acute angle to each other,
The force in the direction of the displacement is the component Fx
θ
F
Fx = F cosθ
Fy = F sinθ
NOTE: If there is more than
one force acting on an object
then the formulae becomes:
Wnet = Fnet ∆x Cosθ

5. EXAMPLE 1
• A toy car is pulled along a rough surface by a piece of string which is
at 30° to the horizontal. Calculate the work done in pulling the toy if
the tension in the string is 10N, and it is pulled along 5m with a
constant frictional force of 5N.
10 N
5m
4 N

7. YOUR TURN
Frank is pushing a block of 50 kg over the floor, with a force
of 600 N downward and forward, making a 20° angle with
the horizontal for a distance of 10m. The coefficient of
sliding friction between the block and the floor is 0.39.
Calculate the total work done on the block.

8. OBJECT ON AN INCLINE

9. EXAMPLE 2

10. ENERGY

11. ENERGY
Energy is the measure of the ability of an object or a system to perform work.
There are many types of energy:
kinetic energy – energy of an object due to its speed
gravitational potential energy – energy of an object due to position in a
gravitational field

12. ENERGY TRANSFER
When work is done, energy is transferred.
• gravitational potential energy – e.g. when an object changes
height within a gravitational field
ΔEp = mgΔh
• kinetic energy – e.g. when an object changes speed
Ek = ½mv2

13. CONSERVATION OF ENERGY
Energy cannot be created, or destroyed;
it can only be changed into another form.
A bungee jumper’s gravitational
potential energy is changed into
kinetic energy as they jump,
and then stored as elastic
potential energy as the bungee
rope stretches.

14. Ek and Ep
• If resistive forces, such as friction and air resistance, are
ignored, Ek and Ep are related as follows:
loss of Ek = gain in Ep
lose of Ep = gain in Ek
• For example, if an object of mass m is released above the
ground at height h, it will gain speed, v, as it falls.
• Due to the conservation of energy, and assuming air resistance
is negligible, after falling a height of Δh:
½mv2 = mgΔh

15. Conservation of energy: example
question
1. A ball of mass 400 g is thrown upwards at a speed of 5 ms-1. (g
= 9.8 m.s-2).
a) What is the ball’s Ek as it is released?
b) What is the ball’s maximum gain of Ep?
c) What is the ball’s maximum height?