In the first video Walter Lewin demonstrates a device that launches
a golf ball with a certain initial velocity v0 by means of the release of a compressed
spring.
First, in order to determine v0, he launches the ball vertically upward
and measures the maximum height h
of its trajectory.
At that position, which the ball reaches at the instant, say τ, the velocity of the ball vanishes,
hence v(τ)=0.
From page 15 we learn
vy(t)
= v0y
- gt
Here, for vertical movement,
v0y=v0.
Consequently, at the instant τ
we have
0 =
vy(τ)
= v0
- gτ
→
τ = v0
/ g
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