KE, GPE, EPE

• Kinetic Energy (KE):


  The energy that a MOVING object has.
• Kinetic energy (J) = 0.5 x Mass (Kg) x Speed^2 (m/s)
• • Ek = 0.5mv^2
  • m = Ek / 0.5v^2
  • v^2 = Ek / 0.5m
• Gravitational Potential Energy (GPE):

  The energy held by an object because it is in a gravitational field & above ground level.

  • The HIGHER up the object is, the GREATER the GPE.
• Gravitational Potential energy (J) = Mass (Kg) x Gravitational field strength (N/kg) x Change in Height (m)
• Gravitational Field Strength (g):

  The strength of gravity, which on Earth is 9.8 Newtons per Kilogram (N/kg).
• • Ep = mgh
  • m = Ep / gh
  • g = Ep / mh
  • h = Ep / mg
• 1.2Kg x 9.8 N/kg x 23m = 270.48 J

  
• When you lift an object up, work done against gravity is stored as gravitational potential energy of the object.
• Factors affecting kinetic energy:

  • mass (kg)
  • speed (m/s)
• • The faster an object is moving, the more kinetic energy it will have.
  • (If the speed is the same) The more mass an object has, the more kinetic energy it will have
• A child riding a bike at a speed of 0.6m/s with a total kinetic energy of 12.4J. If the mass of the child is 30Kg, what's the mass of the bike?

  • Ek = 0.5mv^2
  • m = Ek / 0.5v^2
  • mass in total = 12.4J / (0.5 x 0.6^2 m/s) = 68.89Kg
  • 68.89 - 30Kg = 38.89Kg = the mass of the bike
• Factors affecting Gravitational potential energy:

  • Mass (Kg)
  • Change in height above the ground (m)
  • Gravitational Field Strength
• Gravity:


  A force of attraction between two objects.

  • The size of that force, depends on the mass of those two objects & how far apart those objects are.
• Small objects like an apple or buildings, the gravitational force of attraction is so tiny, like it doesn’t even exist.

  • As they both have relatively low masses.

  The same goes for very large objects that are really far away like Jupiter.

  • As the distance is so far.
• • Large objects that are close by like the earth or the moon, gravity is felt, far more strongly & have a big influence over nearby objects. 
  • For the earth, the gravitational field strength is 9.8 newtons per Kg
  • While for the moon, its 1.6 N/Kg, since its a lot smaller than the earth

  
• • The gravitational field is the field of influence around an object.
  • The gravitational field strength is the strength of this field.

  
• Whenever an object comes into a gravitational field, it experiences a force of attraction.

  • in physics, it’s this force that we refer to as an object’s weight.

  
• Weight (Newtons) = mass (Kg) x gravitational field strength (N/Kg)
• If a person with a mass of 60kg was on earth’s surface, then they would experience a force downwards towards the centre of the earth of:

  • 60 x 9.8 = 588N

  So their weight is 588N
• To lift up an object & overcome the force of gravity is going to require energy or work.

  • This energy will be transferred to the object’s gravitational potential energy store.
• An object's mass:


  A property of that object.

  • The "amount of stuff" in an object (or the amount of 'matter')

  Measured in kilograms (kg)
• An object's weight:


  The force acting on that object due to gravity.

  • It can change depending on the strength of gravity (gravitational field strength)
  •  Measured in newtons (N)

  
• The weight of an object depends on:

  • The strength of the gravitational field acting upon it
  • the object's mass
• The mass of an object is the same anywhere in the universe.
• Tony has a mass of 65 kg on Earth. What is Tony's mass on the moon?


  65Kg
• Every object in the universe exerts a force of gravitational attraction on every other object in the universe
• Factors that determine the strength of the gravitational force of attraction:

  • The distance between the objects
  • The mass of the objects
• • As an object rises, KE is converted to GPE
  • As an object falls, GPE is converted to KE 
  • KE lost = GPE gained
  • KE gained = GPE lost
• Energy transfers of a roller coaster car:

  • At point A, the GPE of the car is at it's highest value & the KE is zero. 
  • As the car descends towards point B, the GPE store decreases as energy is transferred mechanically to the KE store of the car. 
  • The GPE lost is equal to the KE gained. 

  
• On a roller coaster ride, a small amount of energy will be lost due to:

  • Friction between the car & the track.
  • Air resistance acting on the car.

  These losses are not taken into account in calculations, unless stated otherwise.

  
• • As a ball falls through the air, energy is transferred from the ball's gravitational potential energy store to its kinetic energy store.
  • The energy is mechanically transferred between the two energy stores.
• A roller coaster car of mass 500 kg is initially stationary. It then descends through a distance of 20 m.

  Calculate the kinetic energy of the roller coaster car after descending 20 m.

  100000J
• A ball of mass 0.2 kg is thrown upwards with a speed of 2 m/s. 

  Calculate the maximum height reached by the ball. 

  Calculate initial kinetic energy of ball:

  • KE = 0.5 x m x v2
  • kinetic energy = 20.5 x 0.2 x 2^2 = 0.4 J

  Calculate maximum height reached:

  • maximum gravitational potential energy = initial kinetic energy
  • GPE = mgh
  • 0.4 = 0.2 x 10 x h
  • h = 0.4 / (0.2 x 10) = 0.2m
• • Flying drones may damage aircraft
  • falling drones may injure people
  • damage buildings & vehicles

  
• Calculate the change in gravitational potential energy from the position where the student jumps to the point 20.0m below:

  g.p.e. = mass × gravitational field strength × height

  • Ep = 50 × 9.8 × 20 = 9800J

  
• The speed of the miner at the bottom of the slide is much less than the calculated maximum possible speed:

  • Because work is done against friction, between the miner & slide, due to the slide not being perfectly smooth.
  • Or causing kinetic energy to be transferred as heat energy of surroundings.

  
• At the bottom of the slide, all riders & their sleds have approximately the same speed:

  • Because where there's a decrease in gravitational potential energy, there is an increase in kinetic energy.
  • Variation is due to the air resistance / friction.