Download 013 Energy, Work and Power

By using Hooke’s
Law and the
graph shown,
work out the
spring’s Spring
Constant.
Force
(N)
Compression (cm)
A seagull (mass = 20 kg) is flying in a small circle
9 m above the ground.
a) Calculate its gravitational potential energy.
An angry fisherman shoots at the seagull. He
misses, but the seagull, petrified and unable to
move, falls straight onto the ground. The air
friction is negligible. The fisherman is sent to jail.
b) What is the kinetic energy of the seagull the
moment before it hits the ground?
c) Calculate the speed of the seagull the
moment before it hits the ground.
Question
How many different forms of
energy do you know?
Energy
•
Energy is what makes things
‘happen’.
•
Examples - turning the lights on,
driving a car, using a Bunsen, etc.
•
Energy is measured in _____.
Question
What is the ‘Law of
Conservation of Energy’?
Law of Conservation of Energy
•
Energy can neither be created nor
destroyed.
•
Energy can only be transferred
(from one place to another) or;
•
be transformed (from one form to
another)
Energy Transformations
The three forms of Energy
in Physics
1. Kinetic Energy
2. Gravitational Potential Energy
3. Elastic Potential Energy
Kinetic Energy
m = mass of the moving object
v = speed of the moving object
Gravitational Potential Energy
Definition - energy stored in an object which is
raised against the gravitational field
m = mass of the moving object
g = acceleration due to gravity (9.8 ms-2)
h = height of the object
KE & GPE Transformation
A 24 kg rock is dropped on someone’s
head. The speed of the rock the moment
before it comes into contact with the
person’s skull is 19.6 ms-1.
a) Calculate the kinetic energy of the rock
the moment before it hits the person
b) Assuming that no energy is lost due to
friction, calculate the height (above the
head) that the rock was dropped from.
Hooke’s Law
F=kx
F = force applied on the stretchy object
x = length of stretch (or compression)
k = spring constant (measured in ____ )
Example
a) A mass of 0.5 kg hung from the
end of a spring extends the spring
by 25 cm. Calculate the spring
constant.
b) Another mass of 0.5 kg is added to
the first mass. What is the new
extension?
Elastic Potential Energy
k = spring constant
x = length of stretch (or compression)
Example
A 80 kg child stands on a trampoline and
causes the trampoline to sag by 1 m.
a) What is the child’s weight?
b) What is the trampoline’s spring
constant?
c) How much elastic potential energy is
stored in the trampoline?
Gravitational Potential Energy
Kinetic Energy
Elastic Potential Energy
Work
W = Fd
•
Work is the process that transfers
energy from one form to another.
•
The amount of work depends on the
forces involved and the distance
through which those forces act.
•
Work is measured in _________.
Examples
•
If a man pushes his car (mass = 1600 kg)
with a force of 500 N and causes it to
roll 2 m, how much work has he done?
•
If the man now lifts the car above his
head, which is 2 m high above the
ground, how much work has he done?
Chocofish Question
If someone holds a 10 kg mass on his
hands and walk 5 m across the floor,
how much work has he done?
CAUTION!!!
If someone holds a 10 kg mass on his hands and
walk 5 m across the floor, how much work has
he done? – NONE!!!
Why? - The direction of the force applied on the
mass (downward, vertical) and the direction of
its movement (forward, horizontal) are
perpendicular. Since vertical force cannot affect
horizontal movement, the force applied on the
mass (holding) is not accountable for its
movement (across the floor) at all.
Example
A man is pushing his lawnmower with a force of
100 N, at an angle of 30o from the ground. While
he is pushing, the lawnmower moves 10 m across
the lawn.
a) Calculate the vertical
component of the force.
b) Calculate the horizontal
component of the force.
c) Calculate the amount of
work done by the man on his lawnmower.
Power
•
Power is how much work is done “per
second”.
•
It’s not just the amount of work being
done, but HOW FAST it’s being done!
•
Power is measured in _______
Example
Two men are lifting weights. Each man lifts a
weight (mass = 120 kg) 1.4 m above the ground.
a) Calculate the work done by each man.
One of the men, named ‘Andy’, takes 2 seconds
to lift the weight. The other man, named ‘Billy’,
takes 2.5 seconds to lift the weight.
b) Calculate Andy’s power output.
c) Calculate Billy’s power output.