Question

Two balls are thrown simultaneously, $\left(A\right)$ vertically upwards with a speed of $20m/s$ from the ground and $\left(B\right)$ vertically downwards from the height of $40$ $m$ with the same speed and along the same line of motion. At which points the balls will collide? (take $g=10m/s^2$)

• A. $15m$ above the ground
• B. $15m$ below from the top to the tower
• C. $20m$ above from the ground
• D. $20m$ below from the top to the tower

Answer 1

Answer: (A)

$15m$ above the ground

$h=20t-\frac{1}{2}\times 10\times t^2$

$40-h=20t+\frac{1}{2}\times 10t^2$

$\Rightarrow 40-20t+5t^2=20t+5t^2$

$\Rightarrow t=1s$

$\Rightarrow h=20\times 1-5(1{)}^2=20-5=15m$

Therefore $15m$ above from the ground