GPE and KE How can we calculate energy?
Starter: Multiple-choice quiz
GPE LO: State the GPE depends on mass and height above ground Calculate changes in GPE
When you lift an object up, energy is transferred from your muscles in to gravitational potential energy of the object. Gravitational potential energy (GPE) is energy stored in an object because of its position in the earths gravitational field.
Which books have GPE? Which book has the most GPE? Which book has the least GPE?
What would happen to the GPE of book C, if you moved it to the higher shelf? What would happen to the GPE of book B, if you moved it to the lower shelf?
If both flower pots were to fall out the window, which one is more likely to break? Why?
The higher up an object is the greater its GPE. The larger the distance something falls through the greater the amount of GPE the object loses as it falls. As most of this GPE gets changed into kinetic energy, the higher up the object starts from the faster it will be falling when it hits the ground. So a change in gravitational potential energy depends on the height an object moves through.
To lift an apple 1m would be easier than to life the apple tree 1m. Why?
To lift an apple 1m would be easier than to life the apple tree 1m. Why? Well, the tree has more mass. Therefore, it would need more GPE to reach the same height. So a change in gravitational potential energy also depends on the mass of the object that is changing height.
Put the pictures in order, starting with the one with the most GPE
To move book C, work must be done. (We need to put a force in to move it). Work done depends on: -- how far the object is moved -- and how much it weighs
What do you think GPE would depend on? The amount of gravitational potential energy an object on Earth has depends on its: Mass Height above the ground
So change in GPE = mass x change in height x gravitational field strength What are the units?
So change in GPE (joules) mass (kg) change in height (m) gravitational field strength (m/s 2 )
So change in GPE = mass x change in height x gravitational field strength Calculate change in GPE, when a 1000kg aeroplane, takes off up to a height of 500m.
So change in GPE = mass x change in height x gravitational field strength Calculate change in GPE, when a 1000kg aeroplane, takes off up to a height of 500m. Gravitational field strength on earth= 10N/Kg =1000kg x 500m x 10N/kg =5,000,000j
Gravitational potential energy E p = mgh m J kg N/kg or m/s 2
Woof! (help!) Example A dog of mass 12 kg falls from an aeroplane at a height of 3.4 km. How much gravitational energy does the dog have as it leaves the aeroplane?
On earth g = 10 m/s 2 Example Mass of dog = 12 kg Height = 3.4 km = 3400 m
On earth g = 10 m/s 2 Example Mass of dog = 12 kg GPE of dog = mgh = 12 x 10 x 3400 = 408 000 J Height = 3.4 km = 3400 m
Example GPE of dog = mgh = 12 x 10 x 3400 = 408 000 J Just before the dog hits the ground, what has this GPE turned into?
Kinetic energy
Where is this useful? Many theme park rides use the transfer of gravitational potential energy to kinetic energy and kinetic energy to gravitational potential energy.
GPE increases GPE decreases Kinetic energy increases
Energy Transfer Schiffschaukel - YouTube [720p].mp4
Plenary: GPE calculations
Second Part Lesson 5: KINETIC ENERGY LO: Explain what kinetic energy of an object depends on (C) Calculate kinetic energy (B)
Feeling energetic? Who has the most kinetic energy: Rita or the cat? Write down how you know
Kinetic energy is energy an object has due to motion. It depends on two things: Any ideas???
Kinetic energy tells us how much movement energy something has. It doesn't just depend on how fast something is moving, it also depends on mass. If a car and a lorry are both travelling at the same speed the lorry will do much more damage if it hits something, than if the car does. The lorry has more kinetic energy even though they are both travelling at the same speed.
Kinetic energy is energy an object has due to motion. It depends on two things: The objects mass The objects speed
Kinetic energy = ½ x mass x speed 2 Units of KE= joules Mass= kg Speed= m/s
A car with a mass of 500 kg is moving at a speed of 12 m/s. How much kinetic energy does it have?
A car with a mass of 500 kg is moving at a speed of 12 m/s. How much kinetic energy does it have? ½ x 500 x (12 2 ) = 36,000 J
Rearranging the KE equation Sometimes it is necessary to rearrange the kinetic energy equation in order to calculate the mass or the velocity of a moving object. KE = ½mv 2 What are the rearranged versions of this equation for calculating mass and velocity? m = 2KE v = 2KE v 2 m
Calculating velocity question A lorry has a mass of 20,000 kg. If its kinetic energy is 2.25 mj, at what velocity is it travelling? KE = ½ x mass x velocity 2 velocity = 2KE 2 = = 15 m/s mass x 2,250,000 20,000
Assuming that all the gravitational potential energy of a falling object is transferred to kinetic energy: GPE = KE
GPE = KE Mass x gravitational field strength x height ½ x Mass x speed 2
GPE = KE Mass x gravitational field strength x height ½ x Mass x speed 2
GPE = KE Mass x gravitational field strength x height ½ x Mass x speed 2
GPE = KE gravitational field strength x height ½ x speed 2 G x H = ½ V 2
G x H = ½ V 2 Can be rearranged: h = v2 2g For example, how high must an object be dropped so that is reaches 30 m/s? (g = 10 N/kg) 30 height = 2 2 10 = 900 20 08/01/2016 = 45 m cgrahamphysics.com 2016
KE, mass and velocity calculations
An object is elastic if- it regains its shape after being stretched or squashed. Elastic potential energy is stored in the object that is being stretched or squashed. What can this be transferred in to?
KINETIC ENERGY
Glossary conservation of energy The law that states that energy cannot be created or destroyed, just transferred into different forms. energy transfer A process in which one form of energy is transferred into another form. gravitational field strength The strength of gravity in a particular location. gravitational potential The energy an object has because of its position in a gravitational field. joule The unit of energy. kinetic The energy an object has because it is moving. velocity A measure of the speed and direction of a moving object.
Plenary: Anagrams
Multiple-choice quiz