Question 4
An object is thrown vertically upwards with a velocity of 20 m s-1. How long does it take to reach its maximum height? (For ease of calculation take g = 10 m s-2.)
The right answer is the one marked above.
2 s. When an object travels vertically upwards its velocity decreases by 10 m s-1 every second.
20 → 10 → 0 : 2 seconds
At the maximum height the object's velocity is zero — from there it begins to fall back down. So it is enough to find how long the velocity takes to fall from 20 to 0: 20 ÷ 10 = 2 s.
For an object travelling vertically upwards the value taken for the acceleration due to gravity is −10 m s-2; the negative sign says the velocity is decreasing.