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Physics E1a
Solutions Homework 4a10
Fall 2010
MC 1 the answer is a) Draw a free body diagram for the problem and find the net force on the stone due to
the tension and gravity. With Newton's second law we have
F
=
T
+
mg
=
ma
=
m
v
2
r
!
if
T
= 3mg then
F
!
=
4
mg
=
m
v
2
r
"
r
4
g
( ) =
v
2
"
v
=
2
gr
MC 2 The answer is c) Twice as big. For the first case
a
1
=
v
2
R
and for the second case,
a
2
=
2
v
( )
2
2
R
=
2
v
2
R
MC 3 The answer is d) g/4
from
ma
=
G
mM
r
2
!
g
=
G
M
r
2
.
Now if we double r (go to an orbit around
the earth at a distance of one earth radius from the surface) then the acceleration or g decreases by a factor
of four.
g at
2
r
( )
=
G
M
2
r
( )
2
=
G
M
4
r
2
MC 4
The correct answer is b)
There is no force that pushes you against the door. The centripetal
force (center directed force) is the friction force acting on the tires and causing the car to turn in a
circle. Your body wants to continue straight ahead so the door pushes on you. So answer a) is
false and b) is true, c) is false, remember the door pushes on you, d) is false, the centripetal force
acts on you from the tire friction through the car frame to the seat, e) is false, after all, that’s what
we have been doing for the last three weeks.
12.(a)
Strategy and Solution
The force of static friction between the inside wall of the rotor and the
people’s backs keeps them from falling.
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 Spring '11
 WolfgangRueckner
 mechanics, Force, Gravity, Work

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