AQA · GCSE · Combined Science · Higher

Deceleration of the car calculation

Q07.1 3 marks

A car contains a device called a black box. The black box records the velocity and acceleration of the car. The car was travelling at a constant velocity. The driver then reacted to a hazard. Figure 11 shows the velocity–time graph for the car.

Velocity-time graph for a car. Velocity, in metres per second, is on the vertical axis from 0 to about 35. Time, in seconds, is on the horizontal axis from 0 to just over 6. The plotted line is horizontal at about 26 m/s from 0 s to 2.0 s, then slopes down uniformly to 0 m/s at about 5.25 s, then remains at 0 m/s. — Figure 11

Determine the deceleration of the car.

Give the unit.

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Marking points

1 Read two suitable points from the sloping section of the velocity–time graph and use change in velocity divided by change in time.
2 Calculate the deceleration as $8$ or $8.0$ .
3 Give the unit as $\mathrm{m/s^2}$ .

Full-mark answer

From the graph, the velocity changes from about $26\ \mathrm{m/s}$ to $0\ \mathrm{m/s}$ between $2.0\ \mathrm{s}$ and $5.25\ \mathrm{s}$ . Therefore $a=\frac{26-0}{5.25-2.0}=8\ \mathrm{m/s^2}$ , so the deceleration is $8\ \mathrm{m/s^2}$ .

Why this answer loses marks

$8\ \mathrm{m/s}$

The numerical value is correct, but the unit is velocity rather than acceleration.

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