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**KS4 Kinetic and Potential energy**

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What is energy? Energy lets you do things. Another name for energy is ____ work The units of energy are ______. Joules There are many different types of energy you need to know about. How many joules in a kilojoule? _______ 1 000 J

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**Different types of energy**

Light ___________ Sound Kinetic Nuclear Energy Electrical Chemical Elastic Gravitational Heat

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Gravitational energy Any object in a gravitational field has gravitational potential energy due to its position in that field. The _____ has gravitational potential energy due to the gravitational field of the Earth. Moon The ____ has gravitational potential energy due to the gravitational field of the Sun. Earth

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**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?

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**GPE=Wh GPE=Wh Change in GPE = Weight x Change in height**

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)

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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 = W x h = 600N x 2 000m = J

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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 velocity of the object The shape of the object The mass of the object The height of the object x x

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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.

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**Kinetic Energy = ½x mass x(velocity)2**

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)

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**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 = J

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**Kinetic energy = ½ x mass x velocity squared**

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 in m/s KE = ½ mv2 A 70kg boy is running at about 10m/s. What is his kinetic energy? KE = ½ mv2 KE = ½ x 70 x 102 KE = 3500 J

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**Some questions… KE = ½ mv2 KE = ½ x 0.1 x 52 KE = 1.25 J**

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

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**KE = ½ mv2 KE = ½ x 60 x 52 KE = 750 J Work done = Force x distance**

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

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**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 )

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**Find the GPE of a 5kg mass which has been raised**

through a height of 1.2 m. GPE = m g h GPE = 5 x 10 x 1.2 GPE = J

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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:

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**GPE = KE = ½ mv2 50,000 = ½ x 1000 x V2 50,000 = 500 x V2 100 = V2**

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 = x V2 100 = V2 so V = 10 m/s

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**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. ……………………………………………………………….. 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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b) a)

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**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. ……………………………………………………………….. 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**

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Q5

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Q14

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Chapter 5 Section 1: What is Energy?

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