homework 15 – PAPAGEORGE, MATT – Due: Feb 23 2008, 4:00 am
1
Question 1, chap 7, sect 3.
part 1 of 3
10 points
A block of mass
m
is pushed a distance
D
up an inclined plane by a horizontal force
F
.
The plane is inclined at an angle
θ
with
respect to the horizontal.
The block starts
from rest and the coefficient of kinetic friction
is
μ
k
.
m
D
μ
k
F
θ
If
N
is the normal force, what is the work
done by friction?
1.
W
= +
μ
k
N
D
2.
W
= +
μ
k
(
N 
m g
cos
θ
)
D
3.
W
= 0
4.
W
=

μ
k
(
N
+
m g
cos
θ
)
D
5.
W
=

μ
k
N
D
correct
6.
W
= +
μ
k
(
N
+
m g
cos
θ
)
D
7.
W
=

μ
k
(
N 
m g
cos
θ
)
D
Explanation:
The
force
of
friction
has
a
magnitude
F
friction
=
μ
k
N
.
Since it is in the direc
tion opposite to the motion, we get
W
friction
=

F
friction
D
=

μ
k
N
D.
Question 2, chap 7, sect 3.
part 2 of 3
10 points
What is the work done by the normal force
N
?
1.
W
= (
N
+
m g
cos
θ
+
F
sin
θ
)
D
2.
W
=
N
D
3.
W
= 0
correct
4.
W
=
N
D
5.
W
=
N
D
sin
θ
6.
W
= (
N 
m g
cos
θ

F
sin
θ
)
D
7.
W
= (
m g
cos
θ
+
F
sin
θ
 N
)
D
8.
W
=
N
D
cos
θ
Explanation:
The normal force makes an angle of 90
◦
with the displacement, so the work done by it
is zero.
Question 3, chap 7, sect 3.
part 3 of 3
10 points
What is the final speed of the block?
1.
v
=
radicalbigg
2
m
(
F
sin
θ
+
μ
k
N
)
D
2.
v
=
radicalbigg
2
m
(
F
cos
θ

m g
sin
θ
)
D
3.
v
=
radicalbigg
2
m
(
F
cos
θ

m g
sin
θ

μ
k
N
)
D
correct
4.
v
=
radicalbigg
2
m
(
F
cos
θ

m g
sin
θ
+
μ
k
N
)
D
5.
v
=
radicalbigg
2
m
(
F
sin
θ

μ
k
N
)
D
6.
v
=
radicalbigg
2
m
(
F
cos
θ

μ
k
N
)
D
7.
v
=
radicalbigg
2
m
(
F
cos
θ
+
m g
sin
θ

μ
k
N
)
D
8.
v
=
radicalbigg
2
m
(
F
cos
θ
+
m g
sin
θ
)
D
Explanation:
The work done by gravity is
W
grav
=
m g D
cos(90
◦
+
θ
)
=

m g D
sin
θ .
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homework 15 – PAPAGEORGE, MATT – Due: Feb 23 2008, 4:00 am
2
The work done by the force
F
is
W
F
=
F D
cos
θ .
From the workenergy theorem we know that
W
net
= Δ
K ,
W
F
+
W
grav
+
W
friction
=
1
2
m v
2
f
.
Thus
v
f
=
radicalbigg
2
m
(
F
cos
θ

m g
sin
θ

μ
k
N
)
D .
Question 4, chap 7, sect 3.
part 1 of 1
10 points
A rock of mass
m
is thrown horizontally
off a building from a height
h
. The speed of
the rock as it leaves the thrower’s hand at the
edge of the building is
v
0
, as shown.
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 Spring '09
 KLEINMAN
 Physics, Force, Kinetic Energy, Mass, Work, Cos, KF, MATT –

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