Macroteaching
Topic: Power
Presented By:
Hasyyati binti Abdullah
L20091002826
FLASH BACK
WORK
Work is the product of an applied force(F) and
displacement(s) of a direction of the applied force
Work (W) = Applied force(F) x Displacement(s)
FLASH BACK
There was no work done when:
a) a force is applied but no displacement occurs
b) an object undergoes a displacement with no applied
force acting on it
FLASH BACK
ENERGY
Energy can be defined as the capacity to do work
When work is done, energy is transferred from one
form to another
Energy comes in many shapes.
kinetic energy (KE) where KE = ½ mv2
potential energy (PE) where PE = mgh
LEARNING OUTCOME
At the end of the lesson, student
should be able to:
Define power and state that P = W/t.
Solve problems involving power
TODAY’S ACTIVITIES
- Show interesting picture and
watching video
- Define formula for power by
referring illustration
- Solve problem involving power
- Conclude today’s lesson
Which one has more power?
Which one has more power?
Which one has more power?
Nissan 350Z Nissan
VIDEO Turbo 300ZX
OPEN YOUR MIND
50N
3 metres
30N
3 metres
CONSTANT: Displacement = 3m, Time = 2s
OPEN YOUR MIND
50N
5 metres
50N
3 metres
CONSTANT: Weight = 50N, Time = 2s
OPEN YOUR MIND
50N
3 seconds
50N
5 seconds
CONSTANT: Displacement = 3m, Weight = 50N
POWER
Power
Power (P) is work( W) done in unit time (t).
Power, P = Work Done
Time Taken
Power (P) is also energy (E) consumed or generated
per unit time (t).
Power, P = Energy Transformed
Time Taken
POWER
Power also can be express in terms of force, F and velocity,v
Power, P = Work Done
Time Taken
= Force x Displacement
Time
= Force x Displacement
Time
= Force x Velocity
Power, P = F x v
UNIT OF POWER
FROM THE HP IS
______
EQUATION, I UNIT FOR
CAN GET THE POWER THAT
UNIT FOR COMMONLY
POWER WHICH USED
Js-1
IS ______ ELECTRICAL
APPLIANCE
SI UNIT
FOR
POWER IS
Watt
______
UNIT OF POWER
1W = 1J = 1Js-1
1s
In measuring power, horsepower (hp) is a unit which is in
common use. (in electrical appliances such as air conditioner)
This unit was used by James Watt to express the power of his
steam engine in terms of the number of horses the engine
could replace
However SI unit of power is the watt(W).
So the first thing to do in solving any problem related to
power is to convert horsepower to Watts.
1 horsepower (hp) = 746 Watts
PROBLEMS INVOLVING POWER
Q2: A student of mass 55kg
runs up a flight of 50 steps,
each of 20 cm high as
shown in figure beside.
What is the power
generated by the boy if he
takes 22s to climb to the
top of the stairs? 20cm
(g=10ms ) -2
PROBLEMS INVOLVING POWER
SOLUTION:
m = 55kg
h = 50 x 20cm
= 1000cm
= 10m
t = 22s
P = mgh = 55kg x 10m/s2 x10m = 250W
t 22s
PROBLEMS INVOLVING POWER
Q5: A forklift lifting a crate of mass 100 kg at a constant
velocity to a height of 2 m over a time of 4 s. The forklift
then holds the crate in place for 20 s. Given g=10m/s2.
a.How much power does
the forklift exert in lifting
the crate?
b. How much power does
the forklift exert in holding
the crate in place?
PROBLEMS INVOLVING POWER
SOLUTION:
a. P = Fs = 1000N x 2m = 500W
t 4s
b. Power is measured as work divided by time, and work is
the dot product of force and displacement. While the crate
is being held in the air, it is not displaced, so the
displacement is zero. That means the forklift does no work,
and thus exerts no power.
LEARNING OUTCOME
At the end of the lesson, student
should be able to:
Define power and state that P = W/t.
Solve problems involving power
CONCLUSION
force & velocity
4
2 is the product of 5
time POWER is SI unit for
Watt
is
done in unit is the rate is the rate of transform of
1 of doing 3
work is the capacity
energy
to do
HOMEWORK
Refering to your text book, turn to page 48,
do exercise number 4, 5, 6 and 7
Submit your assignment in the next class
this Monday on 1st June 2009.
Don’t forget to make revision and prepare
for the next class with topic Power Efficiency
