Question

A ball loses 15% of its kinetic energy when it bounces back from a concrete wall. with what speed you must throw it vertically down from a height of 12.4 m to have it bounce back to the same height​

Answer 1

Answer:

Ball must be  thrown with a velocity  of 6.6  m/s

Explanation:

Let say Ball is thrown with a velocity  = A  m/s

height = 12.4 m

Now using V² - U² = 2aS

=> V² - A² = 2(g)(12.4)

=> V² = A² + 2(g)(12.4)

Now KE loss of 15 %  so Velocity = B after bounce back

=> B ² = 0.85 (A² + 2(g)(12.4))

Now again using V² - U² = 2aS

=> 0 - B² = 2(-g)(12.4)

=>-  0.85 (A² + 2(g)(12.4)) = - 2g(12.4)

=>  0.85 (A² + 2(g)(12.4)) =  2g(12.4)

=> 0.85 A² = 0.15 * 2g(12.4)

=> A² =  15 * 2g(12.4) / 85

=> A = 6.6 m/s

Ball must be  thrown with a velocity  of 6.6  m/s