Work and Power

Whenever an object is made to move, energy is being transferred to that object. Physicists refer to this as ‘work done’ and how fast the energy is transferred is called ‘power’.

 
 

Work done

When a force acts on an object and causes it to move, we describe this as ‘work done’ on the object. The force causes energy to be transferred to the object and the work done is equal to the energy transferred.

To calculate how much work has been done, you multiply the force exerted on the object by the distance moved.

 
 

Worked example:

A girl cycles a distance of 50 meters using a total force on the pedals of 150 N. Calculate the work done on the bicycle.

  • Work done = force x distance

  • Work done = 150 N x 50 m

  • Work done = 7,500 J

Gravitational Potential Energy

Lifting an object from a lower position to a higher one requires a force to be exerted on it, i.e. work is done on the object. Energy is transferred into gravitational potential energy of the object. The amount of gravitational potential energy depends on three things: mass of the object, the height it is raised to and the strength of the gravitational field it is in. These factors are linked in the following equation:

Worked example:

A 55 kg woman is lifted to the top of a 1500 meter mountain using a ski lift. Calculate the gain in gravitational potential energy. The gravitational field strength on Earth is 10 kg/N.

  • Gravitational potential energy = mgh

  • Gravitational potential energy = 55 x 10 x 1500

  • Gravitational potential energy = 825 000 J

When an object falls, gravitational potential energy is transferred into kinetic energy. Remember that the conservation of energy states that energy cannot be created or destroyed but can be converted from one form into another. If you picture a rollercoaster slowly climbing up its tracks, it is gaining gravitational potential energy which is equal to the work done. This energy is swiftly converted into kinetic energy as the rollercoaster speeds down towards the ground again. As always, some of this energy will be dissipated to the surroundings as thermal energy.

Kinetic energy

Anything that is moving has kinetic energy. The amount of kinetic energy a moving object has depends on its speed and its mass. The heavier the object and the greater its speed, the more kinetic energy an object will have. To work out the kinetic energy of a moving object, use the equation:

 
 

Worked example:

A 65 kg woman skis at a speed of 8 m/s. Calculate her kinetic energy.

  • Kinetic energy = 0.5 x mass x speed2
  • Kinetic energy = 0.5 x 65 x 82
  • Kinetic energy = 2080 J

Power

Power is a measure of how quickly work is being done. In other words, it tells us how fast energy is transferred to an object. It can be calculated using the following equation:

 
 

Worked example:

A refrigerator transfers 10 kJ of energy in one hour. Calculate its power.

  • Power = work done (or energy transferred) / time taken

  • First convert the energy transferred from kJ to J

  • Convert the time from hours to seconds

  • Power = 10,000 / 3,600

  • Power = 2.78 W