Question

A body is projected vertically upwards with a velocity u. After $1$ and $7$s,it crosses a reference point at a height h. What is the value of u?

• A. $40$m/s
• B. $50$m/s
• C. $30$m/s
• D. $20$m/s

Answer 1

The correct option is A $40$m/s

A body is projected vertically upwards with a velocty $=u$

After $1$ and $7s$

It crosses a reference point at a height $=h$

The value of $=u$

Solution

As the body is projected upwards we can write the equation as

$h=ut+\frac{1}{2}g{t}^{2}g{t}^2-2ut+2h=0$

$t_1+t_2$ are roots of equation

therefore $t_1+t_2-2u/gu=g/2(t_1+t_2)=\frac{10}{2\times 3}$

Therefore $u=40m/s$