GPE and Kinetic Energy

GCSE Physics double award notes

1. Gravitational Gravitational
Potential and Potential and
Kinetic EnergyKinetic EnergyGCSE PhysicsGCSE Physics

2. Learning IntentionsLearning Intentions
By the end of the lesson we will be able By the end of the lesson we will be able
to…to…
-State how State how Work DoneWork Done and and GPE GPE are are
linkedlinked
-Recall the Recall the equationsequations for calculating for calculating
PotentialPotential and and KineticKinetic energy energy
-Solve some simple problems related to Solve some simple problems related to
GPE and Kinetic energyGPE and Kinetic energy

3. What is Gravitational What is Gravitational
Potential Energy (GPE) Potential Energy (GPE)
??
An object lifted up An object lifted up
vertically has the vertically has the
ability to fall back ability to fall back
to its original to its original
position, it has the position, it has the
potential to potential to
movemove. We say the . We say the
object has gained object has gained
GPEGPE..
Page 13

4. GPE in action…GPE in action…

8. GPE linked to Work GPE linked to Work
DoneDone
Consider the stone lifted up- what is the Consider the stone lifted up- what is the
work donework done on the stone? on the stone?
Wd = F x dWd = F x d
= (2 x 10) x 5= (2 x 10) x 5
= 100 J= 100 J
This is also equal to This is also equal to potential energy potential energy
gainedgained by the stone. by the stone.
5 m
2 kg

9. Equation for GPEEquation for GPE
GPE = Weight x Height above the groundGPE = Weight x Height above the ground
Joules (J) = Newton (N) x metre (m)Joules (J) = Newton (N) x metre (m)
Important Note- it is the height lifted Important Note- it is the height lifted
against gravity that mattersagainst gravity that matters
5 m
2 kg
Word
Equation
Units

10. Newton’s CradleNewton’s Cradle
Newton’s cradle can help us understand the Newton’s cradle can help us understand the
factors that effect GPEfactors that effect GPE
If we increase the height the ball is released If we increase the height the ball is released
from, what will happen to show that the GPE from, what will happen to show that the GPE
has also increased?has also increased?
When we lift up more than one ball and release When we lift up more than one ball and release
them what happens?them what happens?
Which factor does this show effects GPE?Which factor does this show effects GPE?

11. What is Kinetic What is Kinetic
Energy?Energy?
Any object which is Any object which is in motionin motion has kinetic has kinetic
energy.energy.
Speed around the Sun –
107 000 km/hr or 66 500 mph

12. Fastest man on the planet over
100 metres
10.32 m/s or 23.1 mph

13. What does KE depend What does KE depend
on?on?
Consider a flying pig heading towards you- if Consider a flying pig heading towards you- if
you had to you had to stop the pigstop the pig what two factors (or what two factors (or
variables) would you be concerned about?variables) would you be concerned about?
The The speedspeed it is moving towards you it is moving towards you
The The massmass of the pig of the pig

14. Equation for KEEquation for KE
Kinetic Energy = ½ x Mass x SpeedKinetic Energy = ½ x Mass x Speed
22
KE KE = ½ = ½ xxmmxxvv
22
JoulesJoules = =kg x metre/secondkg x metre/second
Word
Equation
Units
Dimensions

15. KE linked to Work KE linked to Work
DoneDone
Consider a stone of mass m kilograms, Consider a stone of mass m kilograms,
increasing in speed from 0 to v m/s in time t increasing in speed from 0 to v m/s in time t
secondsseconds
Work Done = Work Done = ForceForce x x DistanceDistance
ForceForce = Mass x Acceleration = Mass x Acceleration
= = m x (v / t)m x (v / t)
DistanceDistance = Average Velocity x Time = Average Velocity x Time
= = ½ v x t½ v x t
Work DoneWork Done = = m x (v / t)m x (v / t) x x ½ x v x t½ x v x t
= = ½ x m x v½ x m x v
22
Kinetic Energy = Work Done= ½ x m x vKinetic Energy = Work Done= ½ x m x v
22
t = 0 t = t
v
0

16. Example Question on Example Question on
KEKE
What is the KE of a football of mass 0.4 What is the KE of a football of mass 0.4
kg moving at a speed of 10 m/s?kg moving at a speed of 10 m/s?
KE = ½ x m x vKE = ½ x m x v
22
= 0.5 x 0.4 x 10= 0.5 x 0.4 x 10
22
= 20 J= 20 J

17. Back to Earth…Back to Earth…
Mass is 5.98 x 10Mass is 5.98 x 10
2424
kg kg
Speed is 30 000 m/sSpeed is 30 000 m/s
KE = ½ x m x vKE = ½ x m x v
22
= 0.5 x (5.98x10= 0.5 x (5.98x10
2424
) x 30 000) x 30 000
22
= 26.9x10= 26.9x10
3232
J J
26 900 000 000 000 000 000 000 000 000 000 000 J

18. Your turn…Your turn…
Page 16Page 16
Try question 25Try question 25

19. Learning IntentionsLearning Intentions
By the end of the lesson we will be able By the end of the lesson we will be able
to…to…
-Recall how Recall how GravitationalGravitational PotentialPotential and and
KineticKinetic energy are linked energy are linked
-Solve some simpleSolve some simple problems problems relating relating
GPE and Kinetic energyGPE and Kinetic energy
-State how to calculate the State how to calculate the efficiencyefficiency of a of a
systemsystem

20. Conservation of EnergyConservation of Energy
Energy cannot be made or destroyed but it can Energy cannot be made or destroyed but it can
change form, change form, total energy stays constanttotal energy stays constant
Consider a marble at the top of a ramp with 2.5 Consider a marble at the top of a ramp with 2.5
J of energy…J of energy…
GPEGPE = 2.5 J
KEKE = 0 J
GPEGPE = 0 J
KEKE = 2.5 J
GPEGPE = 1.25J
KEKE = 1.25 J
Total Energy- 2.5 J 2.5 J 2.5 J
a
b
c
cba
Ignoring
heat energy
losses!

21. Conservation of EnergyConservation of Energy
When a falling object ‘loses’ GPE it is When a falling object ‘loses’ GPE it is
transformed to KE ‘gained’transformed to KE ‘gained’
GPEGPE
lostlost = KE = KE
gainedgained

22. Example QuestionExample Question
A stone of mass 3 kg is dropped from a A stone of mass 3 kg is dropped from a
height of 60 cm onto the ground.height of 60 cm onto the ground.
a) Calculate its GPE before it is droppeda) Calculate its GPE before it is dropped
b) When it hits the ground how much GPE b) When it hits the ground how much GPE
does it have? How much KE does it does it have? How much KE does it
have?have?
c) At what c) At what speedspeed will it hit the ground? will it hit the ground?

23. AnswerAnswer
a)a)GPE = Weight x heightGPE = Weight x height
= (3 x 10) x 0.6= (3 x 10) x 0.6
= 18 J= 18 J
b) GPE is zero as its height off the ground is b) GPE is zero as its height off the ground is
zero. KE is 18 J, all the GPE has been zero. KE is 18 J, all the GPE has been
converted into KEconverted into KE
c) KE = ½ x m x vc) KE = ½ x m x v
22
18 = ½ x 3 x v18 = ½ x 3 x v
22
12 = v12 = v
22
3.46 m/s = v3.46 m/s = v

24. Question TimeQuestion Time
Page 16 Question 27Page 16 Question 27
GPE = Weight x heightGPE = Weight x height
KE = ½ x m x vKE = ½ x m x v
22
GPEGPE
lostlost = KE = KE
gainedgained
total energy stays constanttotal energy stays constant