Work Done & Energy Transfer (AQA GCSE Physics)

• The formula for work is:

W = F × s

• Multiplying force and distance produces units of newton-metres (N m)
  • Work is measured in joules (J)

• This leads to a simple conversion:

1 J = 1 N m

• Therefore, the number of Joules is equal to the number of newton-metres, making conversions between the units very straightforward, for example:

1000 J = 1000 N m

• One Joule is equal to the work done by a force of one newton acting through one metre

• Whenever any work is done, energy is transferred from one store to another
• Mechanical (or electrical) working is an energy transfer pathway
• The amount of energy transferred (in joules) is equal to the work done (also in joules)

energy transferred (J) = work done (J)

 

• If a force acts in the direction that an object is moving, then the object will gain energy (energy is transferred to its kinetic store)
• If the force acts in the opposite direction to the movement then the object will lose energy (energy is transferred away to the thermal store of the object and the surroundings)

• Take the example of an object which is lifted vertically with a 100 N force for a distance of 10 m
• The work done on the object is equal to:

W = F × s

W = 100 N × 10 m = 1000 N m

• Energy is transferred mechanically to the gravitational potential store of the object 
• During that energy transfer 1000 J of energy was transferred

Part (a)

• • Work is done by the woman as she exerts a force on the rope to pull the bucket up
  • The work done on the bucket is due to overcoming the weight of the bucket for a distance of 15 m
  • As the bucket rises, energy is transferred to the gravitational potential store of the bucket

Part (b)

Step 1: List all of the known values

• • Mass, m = 10 kg
  • Gravitational field strength, g = 9.8 N/kg
  • Height, h = 15 m

Step 2: Write the equation relating work, force and distance

Work = Force × Distance

Step 3: Write out the equation for weight and substitute it into the work equation

Weight = m × g

Work = m × g × h

• • Note: This is the equation for gravitational potential energy

Step 4: Calculate the work done on the bucket

Work = 10 × 9.8 × 15 = 1470 N m

Step 5: Convert the work done into energy transferred

Energy transferred in joules = Work done in newton-metres

Energy transferred = 1470 J

• • The bucket gained 1470 J of gravitational potential energy