Teaching Modern Physics - QuarkNet - Fermilab

Sample Problem:
Free Fall and Projectiles
KEY
Billy stands on the Tacoma Narrows Bridge kicking stones into the water below. If Billy kicks a
stone with a horizontal velocity of 3.5 m/s, and it lands in the water a horizontal distance of
5.4 m from where Billy is standing,
A. What is the height of the bridge?
Horizontal Vertical
v = 3.5 m/s vi = 0 m/s
d = 5.4 m d = ?
t = 1.54 s t = 1.54s
a = -9.8 m/s 2
d = vit + 1/2at 2
d = 0 + 1/2(-9.8m/s 2 )(1.54s) 2
d = 11.7 m
B. If the stone had been kicked harder, how would this affect the time it would take to fall?
Homework:
It would not affect the time. It will be the same time since it’s the same drop, just a
greater horizontal distance.
1. A car drives straight off of a cliff that is 50 m high. The police at the scene of this accident
note that the point of impact is 90 m from the base of the cliff. How fast was the car traveling
when it left the cliff?
d = vit + 1/2at 2
-90 m = 0 + 1/2(-9.8 m/s 2 )t 2
t = 4.28 s
v = d/t = 90 m / 4.28 s = 21 m/s
2. In physics lab, Dimo rolls a 10-g marble down a ramp and off the table with a horizontal
velocity of 3 m/s. The marble falls in a cup placed 2 m from the table's edge. How high is the
table?
v = d/t 3 m/s = 2 m / t t = 0.67 s
d = 0 + 1/2(-9.8 m/s 2 )(0.67s) 2 = 2.2 m
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