A stone is projected vertically upward from the edge of a cliff that is above the…
PHYSICS · Subjective · A Level · Physics · Mechanics and materials · Mechanics and materials
Problem
A stone is projected vertically upward from the edge of a cliff that is 45m above the ground with an initial speed of 20ms−1. Neglect air resistance.
(a) Choosing upward as positive and taking g=9.8ms−2,
(i) write the expression for the stone’s velocity as a function of time,
(ii) determine the time taken to reach the highest point,
(iii) obtain the two possible times at which the stone reaches ground level.
(b) Now choose downward as the positive direction and re-derive the expression for velocity as a function of time.
(c) Using the expression in part (b), calculate again the times obtained in part (a)(iii) and comment on the signs that appear in intermediate steps. Show explicitly that the final numerical answers are unchanged by the choice of sign convention.