Question

A man standing on the edge of the roof of a $20m$ tall building projects a ball of mass $100gm$ vertically up with a speed of $10m{s}^{-1}$. The kinetic energy of the ball when it reaches the ground will be $[g=10m{s}^{-2}]$:

• A. $5J$
• B. $20J$
• C. $25J$
• D. $Zero$

Answer 1

The correct option is C $25J$

KE at the ground$=$ PE at the top.

PE at the top $=mgh$

Now, the ball will go say $xm$ up with initial velocity $10m/s$

at the highest point $v=0$ using kinematics third equation:

So, $2gx=$$v^2-u^2$$=0-100$

Or, $x=5$

So, the ball will be $25m$ up the ground when it comes to rest.

PE at that point $=$$m\times g\times h$ =$10\times {10}^{-1}\times 25$ = 25 J

So, KE at the ground will be $25J$