Transcript Kinetic energy

KS4 Kinetic and Potential energy
What is energy?
Energy lets you do things.
Another name for energy is work
____
Joules
The units of energy are ______.
There are many different types of energy
you need to know about.
How many joules in a kilojoule? _______
1 000 J
Different types of energy
Light
1. ___________
Sound
2. ___________
Kinetic
3. ___________
Nuclear
4. ___________
Electrical
5. ___________
Chemical
6. ___________
Elastic
7. ___________
8. Gravitational
___________
Heat
9. ___________
Gravitational energy
Any object in a gravitational field has gravitational
potential energy due to its position in that field.
The _____
Moon has gravitational potential energy
due to the gravitational field of the Earth.
The Earth
____ has gravitational potential energy
due to the gravitational field of the Sun.
Changes in gravitational potential energy
For an object in Earth’s gravitational field:
If an object falls will it gain
or lose gravitational
potential energy?
If an object moves up will it
gain or lose gravitational
potential energy?
GPE=Wh
We can calculate the change in gravitational potential energy (GPE) for
a moving object using the formula below:
Change in GPE = Weight x Change in height
GPE=Wh
GPE measured in Joules (J)
Weight measured in Newtons (N)
Height measured in Metres (m)
A parachutist
A parachutist of weight 600N
jumps from a plane, 2000m
above the ground.
How much gravitational
potential energy will she have
lost when she reaches the
ground?
Change in GPE = Weight x change in height
=Wxh
= 600N x 2 000m
= 1 200 000 J
Kinetic energy
This is the energy possessed by an object
due to its movement.
What factors will increase the amount of
kinetic energy a moving object has?
The shape of the object
The mass of the object
The height of the object

x

x
The velocity of the object
Mass and speed
If you increase the mass of a moving object you
increase the kinetic energy.
If you double the mass, you double the kinetic energy.
If you increase the speed of a moving object you
increase the kinetic energy.
BUT… If you double the speed, you quadruple the
kinetic energy.
This is why even if you are slightly above the speed
limit, you increase the kinetic energy of a moving car
a lot, this means it is harder to stop the car and there
is more chance of an accident.
KE=½m(v)²
We can calculate the kinetic energy (KE) for a moving object
using the formula below:
Kinetic Energy = ½x mass x(velocity)2
KE=½mv2
KE measured in Joules (J)
Mass measured in Kilograms (Kg)
Velocity measured in Metres per second (ms-1)
Kinetic energy calculation
A car has a mass of 400kg and a velocity of 10 ms-1,
what is the kinetic energy of the car?
Kinetic energy
= ½ mass(velocity)2
= ½ x 400 x(10)2
= 20 000 J
Kinetic energy
Any object that moves will have kinetic energy.
The amount of kinetic energy an object has can be found using the
formula:
Kinetic energy = ½ x mass x velocity squared
in J
in kg
KE =
½
in m/s
mv2
1) A 70kg boy is running at about 10m/s. What is his kinetic energy?
KE =
KE =
½
½
mv2
x 70 x 102
KE = 3500 J
Some questions…
2) What is the kinetic energy of a 100g tennis ball being thrown
at a speed of 5m/s?
KE =
½
mv2
KE =
½
x 0.1 x 52
KE = 1.25 J
3) A braking force of 1000N is applied by a driver to stop his
car. The car covered 50m before it stopped. How much
work did the brakes do?
(This equals the kinetic energy of the car)
Work done = Force x distance
= 1000N x 50m
= 50,000 J
4) A crane is lifting a 50kg load up into the air with a
constant speed. If the load is raised by 200m how much
work has the crane done?
(The answer isn’t 10,000J
* remember 1Kg = 10 N )
Work done = Force x distance
= 500N x 200m
= 100,000 J
5) What is the kinetic energy of a 60 kg student running at
5 m/s ?
KE =
½
mv2
KE =
½
x 60 x 52
KE = 750 J
Gravitational Potential Energy
(GPE)
Gravitational potential energy is the energy
possessed by an object which has been
raised above the ground.
Gravitational potential energy = Mass x gravity x height ( J )
Find the GPE of a 5kg mass which has been raised
through a height of 1.2 m.
GPE =
mg h
GPE =
5 x 10 x 1.2
GPE =
60 J
a) Work done = F x d
= 10,000 N x ..…
= …………..…
J
b) Work done = GPE
!!
= ……… J
d) KE to h…..…..
and s…..….. energy.
c) GPE becomes KE:
a) Work done = F x d
= 10,000 N x ..…
= …………..…
J
b) Work done = GPE
!!
= ……… J
d) KE to h…..…..
and s…..….. energy.
c) GPE becomes KE:
GPE = KE = ½ mv2
50,000 = ½ x 1000 x V2
50,000 =
100 =
500 x V2
V2
so V = 10 m/s
A person is travelling at 20 m/s ( about 45 mph) and is wearing
a seat belt. The car unfortunately crashes into a tree.
After the accident the belt is found to have stretched 22cm and
saved the person’s life.
a) Calculate the kinetic energy of
the person if their mass is 50 Kg.
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b) If all of the person’s kinetic energy becomes the work done by the
belt stretching, calculate the restraining force of the belt on the person.
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a)
b)
If the car had been travelling at 40m/s and the belt stretches by 44 cm:
a) Calculate the kinetic energy of
the person if their mass is 50 Kg.
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b) If all of the person’s kinetic energy becomes the work done by the
belt stretching, calculate the restraining force of the belt on the person.
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Q4
Answers to past paper questions
Q5
Q14