A ball is thrown vertically upward from the ground with an initial speed of 20 m/s. How long does it take to reach its highest point? (Ignore air resistance and take g = 10 m/s^2.)
Show the answer and explanation
Correct answer
B. 2.0 s
Principle or equation
For uniformly accelerated motion, the velocity at the highest point is zero. Using v = u + at with v = 0, u = 20 m/s, a = -10 m/s^2 gives t = (v - u)/a = (0 - 20)/(-10) = 2.0 s.
Why this answer is correct
At the highest point, the ball's velocity is 0. The acceleration due to gravity is downward, so a = -10 m/s^2. Using v = u + at, we solve for t: 0 = 20 + (-10)t, so t = 2.0 s.
Example
If a ball is thrown upward at 30 m/s, the time to reach the highest point is (0 - 30)/(-10) = 3.0 s.
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