The Kinetic Energy of Objects

When a force causes an object to move through a distance, work is done on the object and energy is transferred. Both work done (W) and energy transferred (E) are measured in Joules (J).

The amount of work done, force and distance are related by the equation:

W = f x d

Work done against frictional force mainly transfers energy to the surroundings and heat them. Power is the work done in a given time and is given by the equation:

P = E/t

Gravitational potential energy is the energy that an object has due to it's vertical position in the gravitational field. Work is done against the gravitational force and the object gains gravitational potential energy resulting from an increase in its height.

Gravitational potential energy can be calculated using the equation: E = m x g x h

The kinetic energy of an object is the energy it has due t its motion. Knietic energy depends on ..

• the mass of the object
• the speed of the object.

Kinetic energy can be calculated using the equation:

E = 1/2 x m x v^2

A moving car has kinetic energy because it has both mass and speed. If it moves at greater speed it has more kinetic energy despite the mass being the same. A lorry moving at the same speed as a car will have greater kinetic energy due to it's much greater mass.

During collisions the kinetic energy can be dissipated by using energy-abosorbing devices such as air bags, crumple zones , seat belts and side impact bars in cars.

Unlike friction brakes, regenerative braking transfers unused energy back into useful electrical energy to recharge batteries and increase the overall efficiency of a vehicle.

Momentum is a fundamental property of moving objects. It depends on...

• the mass of the object
• the velocity of the object

Momentum can be calculated using the formula:

p = m x v

A moving car has momuntum as it has both mass and velocity (speed in a certain direction). If the car moves with greater velocity, then it has more momentum providing it's mass is the same.

For example, a car of mass 1200kg is moving with a velocity of 20m/s. It's momentum is 1200kg x 20m/s = 24000kg m/s. If the car moves with a new velocity of 30m/s then its new momentum is 1200kg x 30m/s = 36000kg m/s.

Momentum (like verlocity) has ...

• size (magnitude)
• direction

The direction of movement is important when undertaking calculation involving momentum.

For Examplpe:

• Car A has a mass of 1400kg and has a velocity of 20m/s to the right and consequently, a momentum of 28000kg m/s to the right .
• Car B of mass 1400kg and a velocity of 20m/s to the left and a momentum of 28000kg to the left or -28000kg m/s with respect to car A beacuse it is moving in the opposite direction to car A. It's momentum is -28000kg.

A fundamental principle of momentum is that in a closed system, i.e. where no other external forces act, the total momentum beofre an event is equal to the total momentum after the event this is called the conservation of momentum.