NCERT Solutions
Class 11 Physics
Motion In A Straight Line

Q. 2.8
A ball is dropped from a height of 90 m on a floor. At each collision with the floor, the ball loses one tenth of its speed. Plot the speed-time graph of its motion between t = 0 to 12 s.
Given:-
Ball is dropped from a height, s =90m
Initial velocity of the ball, u = 0
Acceleration, a = g = 9.8 m/s
Final velocity of the ball = v
From second equation of motion,
s = ut+ (1/2) at2
90=0 + (1/2) x 9.8(t2)
t = √18.38 = 4.29s
From first equation of motion, final velocity is given
v = u + at
= 0 + (9.8 × 4.29) = 42.04 m/s
Rebound velocity of the ball, ur = (9/10) v= (9/10) x 42.04 =37.84m/s
Time (t) taken by the ball to reach maximum height can be obtained by using First equation of motion as:
v = ur + at′
0 = 37.84 + (– 9.8) t′
t’ = (-37.84)/(-9.8) = 3.86s
Total time taken by the ball = (t + t’) = (4.29+3.86) =8.15s.
As the time of ascent is equal to the time of descent, the ball takes 3.86 s to strike back on the floor for the second time.
The velocity with which the ball rebounds from the floor = ((9/10) x 37.84) =34.05m/s
Total time taken by the ball for second rebound = 8.15 + 3.86 = 12.01 s
The speed-time graph of the ball is represented in the given figure as: