Question Constellation Sign up for free AQA · GCSE · Physics · Higher
Braking Distance from Force and Mass
Another train travels at a speed of 60 m/s. A constant braking force of 270 000 N causes the train to decelerate and stop. mass of train = 240 000 kg. Use the Physics Equations Sheet.
Calculate the distance travelled while the braking force is applied.
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Marking points
- 1 Completes one valid six-stage official route: force plus constant-acceleration equations, energy plus work done, or momentum plus stopping time and mean speed.
- 2 On the force route, obtains the deceleration magnitude 1.125 m/s² from F = ma.
- 3 On the force route, uses v² − u² = 2as and obtains 1600 m.
- 4 Alternatively follows the complete energy or momentum route with its stated equation dependency.
Full-mark answer
Using F = ma, a = −270 000 ÷ 240 000 = −1.125 m/s². Then v² = u² + 2as gives 0 = 60² + 2(−1.125)s, so s = 3600 ÷ 2.25 = 1600 m.
Why this answer loses marks
Uses the braking force as though it were the deceleration.
Force must first be divided by mass to obtain deceleration before a constant-acceleration equation can give distance.