AS Daily A Level Physics question
A small delivery cart moves in a straight warehouse aisle. Its displacement s from the start is recorded against time t. From t = 0 to 20 s, the s–t graph is a smooth curve whose gradient increases with time; over this interval the cart travels 60 m. From t = 20 s to 30 s, the s–t graph is a straight line and s increases by a further 45 m. A second cart moves at constant speed for the whole 30 s and covers a total of 90 m. Which statement must be true?
Answer
The correct answer is B.
Correct: B — Over the 30 s, the first cart's average speed is about 17% greater than the second's. A is wrong because the first cart travels 60 m + 45 m = 105 m in 30 s (3.5 m/s) while the second travels 90 m in 30 s (3.0 m/s), so their average speeds are not equal. B is correct because 3.5 m/s versus 3.0 m/s gives a relative increase (3.5/3.0 − 1) × 100% = 16.7% ≈ 17%. C is wrong because at t = 25 s the second cart is at 75 m, while the first is at 60 m + 5 s × 4.5 m/s = 82.5 m. D is wrong because 'steadily increasing speed' implies positive acceleration but not necessarily a constant acceleration; the increasing gradient of the s–t curve need not be linear.