18. A ball thrown up vertically returns to the thrower after 6 s.
Find
(a) the velocity with which it was thrown up.
(b) the maximum height it reaches, and
(c) its position after 4 s.
Answers
Explanation:
The ball takes a total of 6 s for its upward and downward journey. Hence, it has taken 3 s to attain the maximum height. Hence, the ball was thrown upwards with a velocity of 29.4 m s−1
Answer:
Explanation:
The ball returns to the ground after 6 seconds.
Thus the time taken by the ball to reach to the maximum height (h) is 3 seconds i.e t=3 s
Let the velocity with which it is thrown up be u
(a). For upward motion,
v=u+at
∴ 0=u+(−10)×3
⟹u=30 m/s
(b). The maximum height reached by the ball
h=ut+ 1 /2 at2
h=30×3+ 1/2 (−10)×3 2
h=45 m
(c). After 3 second, it starts to fall down.
Let the distance by which it fall in 1 s be d
d=0+ 1/2 at 2 ′
where t =1 s
′
d= 1/2×10×(1) 2
=5 m
∴ Its height above the ground, h
′
=45−5=40 m
Hence after 4 s, the ball is at a height of 40 m above the ground.