Question

A body is thrown vertically upward from a point 'A' $125m$ above the ground. It goes up to a maximum height of $250m$ above the ground and passes through 'A' on its downward journey . The velocity of the body when it is at a height of $70m$ above the ground is: ($g=10{ms}^{-2}$)

• A. $60{ms}^{-1}$
• B. $80{ms}^{-1}$
• C. $20{ms}^{-1}$
• D. $50{ms}^{-1}$

Answer 1

The correct option is A $60{ms}^{-1}$

At the maximum height, the velocity of the body is zero and then it starts falling down due to the action of gravity with an acceleration $g=10m/s^2$.

We will consider the motion of body from maximum height of $250m$ to the height $70m$ as a free fall.

Then, $initialvelocity\left(u\right)=0,displacement\left(h\right)=250-70=180m,acceleration\left(g\right)=10m/s^2$

Using the equation of motion:

$v^2-u^2=2gh$

$\Rightarrow v^2-0^2=2\times 10\times 180$

$\Rightarrow v^2=3600$

$\Rightarrow v=\pm 60m/s$

Since the downward direction is taken as positive, $v=60m/s$ is the accepted result.