Energy and Work
When a force moves an object, energy is transferred and work is done.
If an object moves a force has to be applied to it. This force needs a supply of energy from somewhere, such as electricity or fuel.
When work is done moving the object, the supplied energy is transferred to the object so the work done is equal to the energy transferred.
Work and energy have the unit joule(J).
When work is done against frictional forces, the energy supplied is mainly transformed into heat.
Work done (J) = force(N) x distance moved in the direction of the force(m)
An elastic object is something that goes back to its original shape after it has been stretched or squashed.
When work is done on an elastic object , the energy transferred is stored as elastic potential energy. The energy is released once the object has gone back to its original shape.
Kinetic energy is the energy of movement.
The kinetic energy of a body depends on its speed and mass. The greater the mass and the faster the speed, the more kinetic energy it has.
Kinetic energy (J) = 1/2 x mass (k) x speed (m/s)²
All moving objects have momentum.
momentum (kg m/s) = mass (kg) x velocity (m/s)
Momentum is conserved whenever objects interact, as long as no external forces act on them.
The interaction could be a collision or an explosion. After a collision the objects may move off together, or they may move apart.
Collisions and explosions
Momentum has both size and direction.
In calculations one direction must be defined as positive, so momentum in the opposite direction is negative.
When two objects are at rest their momentum is zero. In an explosion the objects move apart with equal and opposite momentum. One momentum is positive and the other negative, so the total momentum after the explosion is zero.
When two objects push each other apart, they move apart with equal and opposite momentum.
Changing momentum and safety features
When a force acts on an object that is moving, or able to move, its momentum changes. The equation that describes this is:
force = change in momentum / time taken for the change
Crumple zones in cars are designed to fold in a collision. this increases the impact time and so reduces the force on the car and the people in it.
Air bags work in a similar way. The drivers head changes momentum slowly when it hits an air bag. So the force on the head is less than if it changes momentum quickly by hitting the steering wheel.