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AQA GCSE · Question 07.2 · Forces
Explain what happens to the braking force as the train decelerates. Use information from Figure 12.
Explain what happens to the braking force as the train decelerates. Use information from Figure 12.
How to approach this question
1. Recall that acceleration/deceleration is the gradient of a velocity-time graph.
2. Recall that braking force is related to deceleration by F=ma. So, the braking force is proportional to the deceleration.
3. Look at the gradient of the graph during the deceleration phase (from t=650 s onwards). How does the steepness of the slope change?
Full Answer
The braking force is initially constant and then it decreases.
From the graph, between 650 s and 800 s, the line has a constant negative gradient, which means the deceleration is constant. Therefore, the braking force is constant (F=ma).
After 800 s, the gradient of the line becomes less steep, meaning the deceleration is decreasing. Therefore, the braking force must also be decreasing.
The braking force is directly proportional to the deceleration of the train (F=ma). The deceleration is represented by the gradient of the velocity-time graph.
1. **Between 650 s and 800 s:** The graph is a straight line with a constant negative gradient. This indicates a constant deceleration. Therefore, the braking force applied during this period is constant.
2. **After 800 s:** The slope of the graph becomes less steep (the gradient gets closer to zero). This indicates that the rate of deceleration is decreasing. Since the braking force is proportional to the deceleration, the braking force must also be decreasing during this time.
Common mistakes
✗ Only describing the velocity (e.g., "the train slows down").\n✗ Only describing one part of the deceleration phase (e.g., only mentioning the constant force).\n✗ Confusing the gradient with the value on the y-axis.
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