First slide

Vertical projection from ground

Question

A stone is projected vertically up from the ground with velocity 40 ms^–1. The interval of time between the two instants at which the stone is at a height of 60 m above the ground is (g = 10 ms^–2).

Easy

A

4s
B

6s
C

8s
D

12s

Solution

at h = 60 m velocity v² = u²+2as
v²=40² - 2 x 10 x 60 = 1600 -1200
V² = 400 ⇒ v = 20
$\large \therefore t = \frac{{2v}}{g} = \frac{{2 \times 20}}{{10}} = 4$

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