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Chapter 7 Homework
Due: 9:00am on Wednesday, September 30, 2009
Note: To understand how points are awarded, read your instructor's Grading Policy.
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A Space Walk
Part A
An astronaut is taking a space walk near the shuttle when her safety tether breaks. What should the astronaut do to get back to the shuttle?
Hint A.1
How to approach the problem
Hint not displayed
ANSWER:
Attempt to "swim" toward the shuttle.
Take slow steps toward the shuttle.
Take a tool from her tool belt and throw it away from the shuttle.
Take the portion of the safety tether still attached to her belt and throw it toward the shuttle.
Correct
As the astronaut throws the tool away from the shuttle, she exerts a force in the direction away from the shuttle. Then, by Newton's 3rd law, the tool will exert an opposite force on her. Thus, as she throws the tool,
a force directed toward the shuttle will act on the astronaut. Newton's 2nd law stipulates that the astronaut would acquire an acceleration toward the shuttle.
Part B
Assuming that the astronaut can throw any tool with the same force, what tool should be thrown to get back to the shuttle as soon as possible?
Hint B.1
How to approach the problem
Hint not displayed
Hint B.2
Newton's 2nd law
Hint not displayed
ANSWER:
The tool with the smallest mass.
The tool with the largest mass.
Any tool, since the mass of the tool would make no difference.
Correct
The force that acts on the astronaut must equal in magnitude the force that she exerts on the tool. Therefore, if she exerts the same force on any tool, the force acting on her will be independent of the mass of the
tool. However, the acceleration that the astronaut would acquire is inversely proportional to her mass since she is acted upon by a constant force. If she throws the tool with the largest mass, the remaining mass
(the astronaut plus her remaining tools) would be smallest—and the acceleration the greatest!
Part C
If the astronaut throws the tool with a force of 16.0
Hint C.1
, what is the magnitude of the acceleration
of the astronaut during the throw? Assume that the total mass of the astronaut after she throws the tool is 80.0
.
Find the force acting on the astronaut
Hint not displayed
Hint C.2
Newton's 2nd law
Hint not displayed
Express your answer in meters per second squared.
ANSWER:
= 0.200
Correct
Conceptual Question 7.7
A small car is pushing a large truck. They are speeding up.
Part A
Is the force of the truck on the car larger than, smaller than, or equal to the force of the car on the truck?
ANSWER:
The force of the truck on the car is smaller than the force of the car on the truck.
The force of the truck on the car is larger than the force of the car on the truck.
The force of the truck on the car is equal to the force of the car on the truck.
Correct
Pulling Three Blocks
Three identical blocks connected by ideal strings are being pulled along a horizontal frictionless surface by a horizontal force
tension in the string between blocks B and C is
= 3.00
. Assume that each block has mass
= 0.400
. The magnitude of the
.
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Part A
What is the magnitude
Hint A.1
of the force?
Find the acceleration of block B
What is the magnitude
Hint A.1.1
of the acceleration of block B?
Consider blocks A and B as a unit
Hint not displayed
Express your answer numerically in meter per second squared.
ANSWER:
Hint A.2
= 3.75
Correct
Find the acceleration of all three blocks
Which of the following expressions gives the magnitude
Hint A.2.1
of the acceleration of the three blocks?
Consider all three blocks as a unit
Hint not displayed
ANSWER:
Correct
Since all three blocks have the same acceleration, set , which you just found, equal to the acceleration of block B, found in the previous hint. Then solve the equation obtained for
in terms of 3.00
.
Express your answer numerically in newtons.
ANSWER:
= 4.50
Correct
Part B
What is the tension
Hint B.1
in the string between block A and block B?
How to approach the question
Hint not displayed
Express your answer numerically in newtons
ANSWER:
= 1.50
Correct
Problem 7.42
The 2090
cable car shown in the figure descends a 200-m-high hill. In addition to its brakes, the cable car controls its speed by pulling an 1860
counterweight up the other side of the hill. The rolling friction of
both the cable car and the counterweight are negligible.
Part A
How much braking force does the cable car need to descend at constant speed?
ANSWER:
4010
N
Correct
Part B
One day the brakes fail just as the cable car leaves the top on its downward journey. What is the runaway car's speed at the bottom of the hill?
ANSWER:
28.5
Correct
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Score Summary:
Your score on this assignment is 99.8%.
You received 29.93 out of a possible total of 30 points.
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