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Physics I H
Mr. Tiesler
Solutions to Chapter 4 Homework Problems 6-10
6.) A net force of 265 N accelerates a bike and rider at 2.30 m s 2 . What is the mass of the bike
and rider together?
Use Newton’s second law to calculate the mass.
 F  ma
 m
F 
a
265 N
2.30 m s 2
 115 kg
7.) How much tension must a rope withstand if it is used to accelerate a 960-kg car horizontally
along a frictionless surface at 1.20 m s 2 ?
Use Newton’s second law to calculate the tension.
F  F
T


 ma   960 kg  1.20 m s 2  1.15 103 N
8.) What is the weight of a 76-kg astronaut (a) on Earth, (b) on the Moon  g  1.7 m s2  , (c) on
Mars


g  3.7 m s , (d) in outer space traveling with constant velocity?
2
In all cases, W  mg , where g changes with location.


(a)
WEarth  mg Earth   76 kg  9.8 m s 2  7.4 102 N
(b)
WMoon  mg Moon   76 kg  1.7 m s 2  1.3 102 N
(c)
WMars  mg Mars   76 kg  3.7 m s 2  2.8 102 N
(d)
WSpace  mgSpace   76 kg  0 m s 2  0 N






9.) What average force is needed to accelerate a 7.00-gram pellet from rest to 125 m s over a
distance of 0.800 m along the barrel of a rifle?
The average force on the pellet is its mass times its average acceleration. The average acceleration is
found from Eq. 2-11c. For the pellet, v0  0 , v  125 m s , and x  x0  0.800 m .
aavg
125 m s   0


 9770
2  x  x0 
2  0.800 m 
2
v 2  v02


m s2

Favg  maavg  7.00  103 kg 9770 m s 2  68.4 N
10.) A fisherman yanks a fish vertically out of the water with an acceleration of 2.5 m s 2 using
very light fishing line that has a breaking strength of 22 N. The fisherman unfortunately loses the
fish as the line snaps. What can you say about the mass of the fish?
We assume that the fish line is pulling vertically on the fish, and that the fish is not jerking
the line. A free-body diagram for the fish is shown. Write Newton’s 2nd law for the fish in
the vertical direction, assuming that up is positive. The tension is at its maximum.
F  F
T
FT
 mg  ma  FT  m  g  a  
mg
m
FT
ga

22 N
9.8 m s 2  2.5 m s 2
 1.8 kg