Question

A stone thrown vertically upward returns back to the thrower in 4 s. Calculate (i) the initial velocity of the stone (ii) the maximum height attained by the stone. take g=10m/s.
answer: 1) 20m/s
2) 20m​

Answer 1

(i) u = ?

t = 2u/g = 4 seconds

u = 20 m/s

h =

$u {}^{2} \div 2g$

h = 20 metres

Answer 2

the time taken by stone to go up is 2 second.

t=2 second

v=0

v=u+gt

0=u+(-10)×2

u=20m/s

the value of u is negative because stone is moved upward against gravitational force.

h=ut+1/2gt^2

=20×2+1/2(-10)×4

=40-20

=20 m