Question 5
An object starting from rest accelerates to 15 m s-1 in 6 s, travels at that velocity for a further 6 s, and finally comes to rest in 3 s. Its velocity–time graph is shown below.
| t (s) | 0 | 6 | 12 | 15 |
|---|---|---|---|---|
| v (m s⁻¹) | 0 | 15 | 15 | 0 |
What is the object's acceleration during the first 6 seconds?
The right answer is the one marked above.
2.5 m s-2. The first 6 s is the rising part of the graph, and its gradient is the acceleration:
acceleration = (15 − 0) m s-1 ÷ 6 s = 2.5 m s-2
So the velocity climbs by 2.5 m s-1 every second. 5 m s-2 is the retardation over the last 3 s — not the value for this section.