Quiz 7
11/12/10
Assume g=10m/s
2
for all problems.
Problem 1.
You want to find how fast you can throw a ball. Here is the experiment you do.
You get to a park and throw a ball at some angle (that you do not know) from a height of 1m
above the ground. You time that the ball takes 4s to hit the ground. You measure the distance
the ball travels to be 80m. Use this information to find out how fast you can throw a ball.
Solution.
Because the ball went 80m we can neglect that it was thrown from 1m
above the ground (correct to 2 sig figs – if you don’t believe me, check it). So since
we have two unknowns (v,
θ
) we need two equations.
x

x
o
=
v
x
t
We know that at t=4s the ball has gone 80m so,
80 = 4
v
cos(
θ
)
(1)
We can also find an equation for velocity from the ycomponent at t=4s
y

u
o
= 0 = 4
v
sin(
θ
) +
1
2
gt
2
80 = 4
v
sin(
θ
)
(2)
To solve this we divide (2) by (1) to get,
tan(
θ
) = 1
=
⇒
θ
= 45
◦
So,
v
=
20
cos(45)
= 20
√
2 = 28
.
3
m/s
Problem 2.
A kid throws a ball from a building 40m high at an angle of 30
◦
below the hori
zontal with a speed of 10m/s. In which direction is the ball moving after 1s.
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 Fall '10
 ManojKaplinghat
 Physics, Want, θ, 16m, 15m/s, 8.66m/s, 28.3m/s

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