Energy and Work
When a force moves an object, energy is transferred and work is done.
Whenever an object starts to move, a force must have been applied to it. This force needs a supply of energy from somewhere, such as electricity of 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.
Both 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 = Force x Distance moved in the direction of the force
Air resistance (Drag)
- Object moves through air, air resistance acts in opposite direction to the motion
- Air resistance depends on the shape of the object and its speed
- When two objects are pushed together, exert equal and opposite forces
- Force that resists movement between two surfaces which are in contact
- Force that pulls objects towards Earth, force of gravity on an object is its weight
- The Earth pulls with a force of about 10 newtons on every kilogram of mass
Elastic Potential Energy
An elastic object is one that will go back to its original shape after it has been stretched or squashed.
Elastic objects such as elastic bands and squash balls can change their shape. They can be stretched or squashed, but energy is needed to change their shape.
When work is done on an elastic object to stretch or squash it, the energy transferred to it is stored as elastic potential energy. When the object to its original shape this energy is released.
Kinetic energy is the energy of movement.
Kinetic Energy = 1/2 Mass x Speed^2
The kinetic energy of a body depends on its mass and its speed. The greater its mass and the faster its speed, the more kinetic energy it has.
All moving objects have momentum. It is difficult to change the direction of movement of an object with a lot of momentum.
Momentum (kg m/s) = Mass (kg) x Velocity (m/s)
Whenever two objects interact, the total momentum before the interaction is equal to the total momentum afterwards, provided no external forces act on them. This is called 'conservation of momentum'. The total change in momentum is zero.
The interaction could be a collision or an explosion. After a collision the objects may move off together, or they may move apart.
More on 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.
Firing a bullet from a gun is an example of an explosion. The bullet moves off with a momentum in one direction and the gun 'recoils' with equal momentum in the opposite direction.
Changing in Momentum
When a force acts on an object that is moving, or able to move, its momentum changes.
Force = Change in momentum / time taken for the change
For a particular change in momentum, the longer the time taken for the change, then smaller the force.
In a collision, the momentum of an object often becomes zero during the impact - object comes to rest. If the impact time is short, the forces on the object are large. As the impact time increases, the forces become less.
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 driver's head changes momentum slowly when it hits an airbag. So the force on the head is less than if it changes momentum quickly by hitting the steering wheel.