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PHYSICS 6
SPRING 2006
HOMEWORK 2
SOLUTIONS
1.
A hockey puck is hit and travels at a velocity of 90 Km/hr toward the
goal, 60 m away. How long will it take to reach the goal?
time=distance/speed=60m/(90x10
3
m/hr)=6.67x10
4
hr
=6.67x10
4
hr x 3600 sec/hr = 2.4 sec
2.
A spy satellite travels around the earth in a low orbit every 90 minutes.
What is the speed of the satellite?
Circumference of earth =24,000 miles = 38,000 Km
90 min=1.5 hr
Speed = 24,000 mi /1.5 hr = 16,000 mph
= 26,000 Km/hr
or
4.4 miles/sec
= 7.0 Km/sec
Is it accelerating? Yes, since its direction is changing continually.
3.
A ball is dropped straight down from the top of a building. The ball hits
the ground in 5 sec.
a. How tall is the tower?
Distance fallen = (1/2)gt
2
= (1/2) (9.8 m/sec
2
) (5 sec)
2
= 4.9 x 25 m = 120 m
b. If the ball is thrown from the tower horizontally at 10 m/s, how long
does it take to reach the ground?
Same 5 sec.
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 Spring '06
 GaryR.Goldstein
 Physics, Energy, Work

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