1
© Manhattan Press (H.K.) Ltd.
2.4 Motion under gravity2.4 Motion under gravity
2
© Manhattan Press (H.K.) Ltd.
Free fall
2.4 Motion under gravity (SB p. 93)
Free fall – objects fall towards earth withsame acceleration (acceleration of free fallor acceleration due to gravity (g)) withoutresistance
g = 9.81 m s-2
Acceleration is constant –equations of motion under uniformacceleration can be used
3
© Manhattan Press (H.K.) Ltd.
Free fall
2.4 Motion under gravity (SB p. 94)
(a) upward motion,
- positive velocity (upwards)
- positive displacement (above A)
(b) downward motion,
- negative velocity (downwards)
- positive displacement (above A) for H, G, F, E
- negative displacement (below A) for J
4
5
© Manhattan Press (H.K.) Ltd.
Free fall
We can experience the free fall motion byplaying on the free-fall ride in many amusementparks. On the way to the top, we are pressedagainst our seats of the ride, which provide asense of weight. But as we free fall, we fall atthe same rate as our seats. Without the forceof the seats pressing on us, we are just beingacted upon by the force of gravity.
2.4 Motion under gravity (SB p. 93)
6
© Manhattan Press (H.K.) Ltd.
Q:Q:A ball is thrown vertically upwards from the groundwith an initial velocity u. The time taken by the ballto reach a height h is t1. The ball then takes afurther time of t2 to return to the ground. Find, interms of t1, t2 and g,
(a) the initial velocity (u),
(b) the height (h), and
(c) the maximum height (H) reached by the ball.
2.4 Motion under gravity (SB p. 94)
7
© Manhattan Press (H.K.) Ltd.
Solution:Solution:(a) Acceleration (a) = −g
Total time taken for the ball to return to the ground (t) = ( t1 + t2)
When the ball returns to the ground, displacement (s) = 0.
Using
(b) When the ball is at a height h,
.
2.4 Motion under gravity (SB p. 94)
