A boy standing on the edge of a cliff throws a stone vertically upwards with an initial velocity of 10 m/s. If the cliff is 20 m high, how long will it take for the stone to reach the ground?
Correct: C
First, we need to find the time it takes for the stone to reach its maximum height. Using the equation v = u + at, we get 0 = 10 - 9.8t, which gives t = 10 / 9.8 = 1.02 s. Then the stone takes the same time to fall back to the boy's hand. Then it falls 20 m, so using the equation s = ut + 0.5at^2, we get 20 = 0 + 0.5 * 9.8 * t^2, which gives t = sqrt(40 / 9.8) = 2.02 s. So the total time is 1.02 + 2.02 = 3.04 s, which is closest to 3 s.