AQA · GCSE · Physics · Higher

Maximum Speed from Energy Transfer

Q04.3 5 marks

When the drone reached a height of 840 m the motor was switched off. The motor was switched back on after a short time, when the drone had fallen to a lower height above the ground. The change in gravitational potential energy of the drone during the fall to this lower height was 3920 J. The mass of the drone is 2.5 kg.

Calculate the maximum possible speed of the drone when the motor was switched back on.

Use the Physics Equations Sheet.

Give your answer in km/s.

Write your answer first. You can study the marking guidance whenever you need it.

Answer this question

Study the marking See what earns credit and compare it with a full-mark answer.

Marking points

1 Adds the lost gravitational potential energy to the initial kinetic energy to obtain 4070 J.
2 Uses Ek = ½mv² with the drone’s mass and calculated kinetic energy.
3 Correctly rearranges the kinetic energy equation for speed.
4 Obtains approximately 57.06 m/s.
5 Converts the result to approximately 0.057 km/s.

Full-mark answer

The maximum kinetic energy is $3920 + 150 = 4070\ \text{J}$ . Using $E_k = \tfrac12 mv^2$ : $4070 = 0.5 \times 2.5 \times v^2$ , so $v = sqrt{4070/(0.5 × 2.5)} = 57.06ldots\ text{m/s} = 0.05706ldots\ text{km/s}$.

Why this answer loses marks

Stops at a speed in the equation’s base unit.

The task asks for a different speed unit, so convert only after calculating the speed.

Answer this question