Create assignment, 00223, Homework 5, Oct 02 at 10:06 am
1
This
printout
should
have
14
questions.
Multiplechoice questions may continue on
the next column or page – find all choices
before answering.
The due time is Central
time.
Conceptual 08 03
07:01, highSchool, numeric,
<
1 min, normal.
001
A car is moving at 60 miles per hour.
The
kinetic energy of that car is 500000 J.
How much energy does the same car have
when it moves at 120 miles per hour?
Correct answer: 2
×
10
6
J.
Explanation:
Let :
v
1
= 60 mi
/
h
,
v
2
= 120 mi
/
h
,
and
K
1
= 500000 J
.
K
=
1
2
m v
2
∝
v
2
,
so
K
2
K
1
=
v
2
2
v
2
1
K
2
=
v
2
2
v
2
1
K
1
=
(120 mi
/
h)
2
(60 mi
/
h)
2
(500000 J)
=
2
×
10
6
J
.
keywords:
Cart Full of Bricks
07:03, trigonometry, numeric,
>
1 min, nor
mal.
002
A cart loaded with bricks has a total mass of
18
.
2 kg and is pulled at constant speed by a
rope.
The rope is inclined at 20
◦
above the
horizontal and the cart moves 20 m on a hori
zontal floor. The coefficient of kinetic friction
between ground and cart is 0
.
5
.
The acceleration of gravity is 9
.
8 m
/
s
2
.
What is the normal force exerted on the
cart by the floor.
Correct answer: 75
.
4493 N.
Explanation:
F
20
◦
18
.
2 kg
μ
= 0
.
5
Summing the components of the forces
along the vertical direction provides
X
F
y
=
F
sin
θ
+
N 
m g
= 0
.
For the horizontal direction we have
X
F
x
=
F
cos
θ

μ
k
N
= 0
N
=
F
cos
θ
μ
k
,
so that
F
sin
θ
+
F
cos
θ
μ
k

m g
= 0
F μ
k
sin
θ
+
F
cos
θ
=
μ
k
m g
F
=
μ
k
m g
μ
k
sin
θ
+ cos
θ
.
Since
μ
k
sin
θ
+ cos
θ
= (0
.
5) sin 20
◦
+ cos 20
◦
= 1
.
1107
,
the force on the cart by the floor is
N
=
F
cos
θ
=
μ
k
m g
cos
θ
μ
k
sin
θ
+ cos
θ
=
(0
.
5) (18
.
2 kg) (9
.
8 m
/
s
2
) cos 20
◦
(1
.
1107)
=
75
.
4493 N
.
003
How much work is done on the cart by the
rope?
Correct answer: 1
.
41798 kJ.
Explanation:
This preview has intentionally blurred sections. Sign up to view the full version.
View Full Document
Create assignment, 00223, Homework 5, Oct 02 at 10:06 am
2
The work done on the cart is given by
W
F
=
F d
cos
θ
=
μ
k
m g
μ
k
sin
θ
+ cos
θ
d
cos
θ
=
(0
.
5) (18
.
2 kg) (9
.
8 m
/
s
2
)
(1
.
1107)
×
(20 m) cos 20
◦
=
1
.
41798 kJ
.
004
Note:
The energy change due to friction is a
loss of energy.
What is the energy change
W
f
due to fric
tion?
Correct answer:

1
.
41798 kJ.
Explanation:
Since the cart does not accelerate, the force
due to friction is found from our horizontal
equilibrium.
f
k
=
F
cos
θ
The energy lost due to friction is just the work
done by friction on the cart:
W
f
=
f
k
d
cos 180
◦
Substituting the frictional force and simplify
ing, cos (180
◦
) =

1, we obtain the result,
W
f
=

F d
cos
θ
=

(75
.
4493 N)(20 m) cos 20
◦
=

1
.
41798 kJ
.
This is the end of the preview.
Sign up
to
access the rest of the document.
 Spring '11
 Chiu
 Physics, Energy, Force, Friction, Potential Energy, Work, Correct Answer, Uadd

Click to edit the document details