Physics

Problem

A stone is projected vertically upward with speed

15\,\text{m/s}

. How long will it take for its velocity to reduce to

5\,\text{m/s}

while still moving upward? Take upward as positive and

g = 10\,\text{m/s}^2

Intuitive explanation

The key is recognizing that velocity changes linearly with time under constant acceleration, so you can use the fundamental kinematic equation $v = v_0 + at$ directly. Since gravity acts downward (negative), it steadily reduces the upward velocity—you just need to find when that velocity drops from 15 m/s to exactly 5 m/s. Plug in the numbers and solve for time; no need to overthink the direction.

Intuitive explanation

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