INTRODUCING 5 - days-a-week problem solving session for Math Olympiad and ISI Entrance. Learn More 

July 2, 2017

Speed of ball thrown down from a height

A boy is standing on top of a tower of height 85m and throws a ball in the vertically upward direction with a certain speed. If 5.25 secs later he hears the ball hitting the ground, then the speed with which the boy threw the ball is ( take g=910m/s2, speed of sound in air=340m/s)

  • 6m/s
  • 8m/s
  • 10m/s
  • 12m/s

Solution:

Time taken by sound= 85/340=0.25secs

Time taken by the ball= 5.25-0.25=5 sec

Now,

we write the equation $$ s=ut+1/2ft^2 $$

Here s=-85 and time t =5 secs

Hence,

$$ -85=5u-1/21025$$

$$\Rightarrow u=8m/s$$

Leave a Reply

This site uses Akismet to reduce spam. Learn how your comment data is processed.

Cheenta. Passion for Mathematics

Advanced Mathematical Science. Taught by olympians, researchers and true masters of the subject.
JOIN TRIAL
support@cheenta.com