Physics P2.3

Physics P2.3

1. Energy and Work

   • Work is done on an object when a force makes the object move
   • Energy transferred = work done
   • W = F x d
   • Work done to overcome friction is transferred into energy by heating
   • Brakes applied - friction between break pads and wheel disc oppose motion of wheel

2. Gravitational Potential Energy

   • GPE - depends on the weight of an object and how far it moves vertically
   • When object moves upwards the GPE gains to equal work done by lifting force
   • Ep = m x g x h
   • Ep = change in GPE, m = mass in kg, g = gravitational field strength N/kg
   • Power = rate of energy transfer
     • P = E/t

3. Kinetic Energy

   • The greater the mass and the faster the speed of an object the more kinetic energy
   • Ek = 0.5 x m x v^2
     • v = speed in m/s, Ek = kinetic energy in J
   • Elastic Potential Energy - stored in an object that streches and then regains its strength

4. Momentum

   • The greater the mass and velocity the greater its momentum
   • p = m x v
     • p = momentum in kgm/s
   • Total change in momentum = 0

5. Explosions

   • When objects are at rest the momentum = 0
   • In an explosion objects move with equal opposite momentum
     • One direction is positive, one is negative
   • Objects push apart with different speeds if the masses are unequal

6. Car Safety

   • Side impact bars and crumple zones fold during impact
   • Seatbelts and airbags - spread forces on body over a larger area
   • Seatbelt stretches slightly increasing impact time
   • Conservation of momentum can be used to calculate speed of a car before collision

7. Impact Forces

   • When a force acts on an object its momentum changes
   • Smaller impact time = bigger force
   • In a collision momentum becomes 0 during impact
   • Crumple zones in cars - designed to fold - reduce force, increases impact time