Two balls A and B are thrown with same speed u from the top of a tower .Ball A is thrown vertivally upwards and ball B is thrown vertically downwards.If time taken taken by the ball A and ball B to reach the ground are 6 and 2 sec Respectively.
Find the distance travelled by ball in t=3 s.Ans=25 m
Find the height of the tower.Ans=60 m
Let the height of the tower be ‘h’.
Ball A is thrown vertically upwards with speed ‘u’. It takes 6 s to reach the ground.
Ball B is thrown vertically downwards with speed ‘u’. It takes 2 s to reach the ground.
Now for ball A,
-h = ut – ½ gt2 [h and g are downwards so negative signs are added]
=> -h = 6u – ½ × 10 × 62
=> -h = 6u – 180 …………….(1)
And for ball B,
-h = -ut - ½ gt2 [here u, h and g are all downwards]
=> h = 2u + ½ × 10 × 22
=> h = 2u + 20 …………………(2)
From (1) and (2) we have,
h = 60 m and u = 20 m/s
Now, lets find the distance travelled by A in 3 s.
For that lets first find out the maximum height reached by the ball A using,
v2 = u2 + 2ax
=> 0 = 202 – 2 × 10x
=> x = 20 m
Time taken in travelling this 20 m is found using,
v = u + at
=> 0 = 20 – 10t
=> t = 2 s
Now, remaining 1 s it free falls from top, so distance travelled in this 1 s is,
x2 = 0 + ½ gt2
=> x2 = ½ × 10 × 12
=> x2 = 5 m
So, the distance traveled in 3 s by A is = 20 + 5 = 25 m