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Worksheet D: Free fall and projectile motion.
v
0
v
e
y
y
= 0
1.
A person standing on a 3 mtall elevator at rest tosses a coin vertically in the air. The
coin leaves the person’s hand at 1.5 m from the elevator’s floor, with an initial speed
of 4 m/s. Right at the same time, the elevator starts moving down at a constant speed
of 1.5 m/s.
a.
Will the coin hit the ceiling or the floor of the elevator? How long after the coin
was thrown will either happen?
To see if the coin hits the ceiling, we will consider the motion of the coin and of the
ceiling of the elevator. Let us take y = o at the position of the bottom of the elevator
at t = 0. The positions as a function of time are:
()
22
Ceiling:
3 m
1.5 m/s
1
Coin:
1.5 m
4 m/s
9.8 m/s
2
yt
t
=+
−
−
The coin hits the ceiling when they both have the same
value of y.
2
3 1.5
1.5 4
4.9
0.48 s (the earliest time of the two solutions)
tt
t
t
−=
+
−
=
This corresponds to y = 2.3 m
So, yes, the coin hits the ceiling, because we were able to find the solution.
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This homework help was uploaded on 03/27/2008 for the course PHYS 221 taught by Professor Herrerasiklody during the Fall '08 term at Iowa State.
 Fall '08
 HerreraSiklody
 Projectile Motion, Work

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