Work and Energy in Physics

Learning Intentions and Success Criteria

• To understand and calculate work done by a force.
• Success criteria include explaining work in science, linking work and energy, and calculating work done by a force.

Key Words

• Work
• Energy
• Distance
• Force

Equations

• Kinetic Energy (KE): KE = 1/2 * m * v² (where m = mass and v = velocity)
• Gravitational Potential Energy (GPE): E = m * g * h (where m = mass, g = gravitational field strength, and h = height)
• Elastic Potential Energy: 0.5 * spring constant * (extension)²
• Work Done: W = F * s (where W = work done, F = force, and s = distance moved in the direction of the force)

Energy Transfers

• Work is done to overcome friction, transferring energy as heat.
• Energy transfers from kinetic energy to thermal energy when brakes are applied.
• Work is fundamental to energy transfer.

Units

• Work done is measured in Joules (J).
• Force is measured in Newtons (N).
• Distance is measured in metres (m).

Example Calculation

• A cyclist uses a constant braking force of 140 N to stop his bicycle in a distance of 24 m. Calculate the work done.
  • Work done = 140 N * 24 m = 3360 J

Further Points

• Work is done when a force moves an object.
  • Energy is transferred to the object.
• The energy transferred is equal to the work done.
• The base units for measurements include:
  • Temperature (°C)
  • Mass (kg)
  • Pressure (Pa)
  • Force (N)
  • Volume (m³)
  • Distance (m)

Knowledge Check and Questions

• How are 'work done' and 'energy transferred' linked? They are equal.
• What are the units for work done? Joules (J).
  • Convert 250 cm to m: 0.25 m
• Which is the correct equation for force? Force = work done / distance.
• What is the work done when a weightlifter uses a force of 20 N to lift a weight of 50 kg?
  • The question is incomplete. You need to know the distance moved by this item. This is required in the equation W=F.d
• A cyclist accelerates from a set of lights. The driving force of the back tyre on the ground is 250 N. How much work is done when the cyclist travels 5 m? 1250 J
• A worker uses a constant force of 60 N to push a box across the floor. They transfer 1680 J of energy to the box. How far have they pushed the box? 28 metres
• A worker uses a constant force of 645N to push a box across the floor. They transfer 20 kJ of energy to the box. How far have they pushed the box? 31 metres