A car of mass 1500 kg is travelling at 20 m/s. The driver brakes and the car stops.
The car has four steel brake discs. Each disc has a mass of 6.0 kg.
The specific heat capacity of steel is 450 J/kg ∘C450\ \text{J/kg}\ ^\circ\text{C}450 J/kg ∘C.
change in thermal energy=mass×specific heat capacity×change in temperature\text{change in thermal energy} = \text{mass} \times \text{specific heat capacity} \times \text{change in temperature}change in thermal energy=mass×specific heat capacity×change in temperature
Calculate the kinetic energy of the car before it brakes.
Assume that all of the kinetic energy is transferred equally to the thermal stores of the four brake discs.
Calculate the increase in temperature of each disc.
The car stops in a distance of 40 m.
Calculate the average braking force on the car.
Suggest why the actual increase in temperature of each disc is less than your answer to (b).
44 exam-style questions on OCR GCSE Physics 7.1 Work done, covering 7.1.1 Conservation of energy in a closed system, 7.1.2 Energy stores and transfers, 7.1.3 Energy changes by heating, forces and current, 7.1.4 Quantifying energy transfers in a system, and 7.1.5 Kinetic, elastic and gravitational potential energy. Each one has a worked solution and a mark scheme showing where the marks go.