Download 1 - Andes Physics Tutor

Name:_______________________________
Instructor:____________________________
Section:______________________________
UNITED STATES AIR FORCE ACADEMY
DEPARTMENT OF PHYSICS
Physics 110
Homework Journal #6
Fall 2006
Date:
21-22 Sep 2006
Points:
28
AUTHORIZED RESOURCES: Full collaboration is authorized on this assignment. You MUST document any assistance you
receive from another individual in completing this assignment. (You DO NOT need to document use of the text, class handouts, or your
own classroom instructor. Any published or unpublished material, or internet source, does not need to be documented unless the
material specifically aims at answering problems in Mastering Physics.)
DOCUMENTATION: Please document any instructor(s) and/or student(s) you consulted and the precise nature of the assistance
you received in completing this assignment (continue on the opposite side of this sheet if necessary):
____________________________________________________________________________________________
__________________________________________________________________________________________________
__________________________________________________________________________________________________
__________________________________________________________________________________________________
__________________________________________________________________________________________________
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INSTRUCTIONS:
1.
Put your full name, your instructor’s name, and section in the upper right hand corner of this page.
2.
Show enough work to justify your answers. Solutions to all problems must start from first principles, including assumptions,
diagrams, known/unknowns, and equations from the constants and equations sheet. Answers to problems with no justification
receive zero credit. Please box your answers to receive full credit. NOTE: The homework journal frequently has added parts to
the problem than may be found in Mastering Physics.
3.
GIVE ALL ANSWERS ON THE WORKOUT PROBLEMS TO AT LEAST 3 SIGNIFICANT FIGURES
4.
Workout the following problems completely & carefully—your goal is to effectively communicate your understanding of
the physics. One of the following problems will be fully graded by your instructor and will be worth 20 points. Other problems
will be “spot-checked” for effort and will be worth 2 points each.
Problem Name
2.68
Points Possible
PSS 5.1
Hanging Chandelier
5.39
5.41
TOTAL
28
Points Earned
LESSON 12: Problem 2.68 (Just in time at the Stop Light)
A motorist is driving at 20 m/s when she sees that a traffic light 200 m ahead has just turned red. She knows that this
light stays red for 15 s, and she wants to reach the light just as it turns green again. It takes her 1.0 s to step on the
brakes and begin slowing.
What is her speed as she reaches the light at the instant it turns green?
Sketch a picture of the problem, include a coordinate system, and label
important points in the motion:
Known:
Find:
Solve using the kinematic equations:
Assess if you answer is reasonable
 and has the correct units 
B. (Not in Mastering Physics) Consider the following statement: “A numerical value of acceleration, given alone, tells us
nothing about how fast the object in question is moving.” Is this statement correct, or incorrect? Explain your answer
carefully by referring to the definitions of both velocity and acceleration.
This problem targets Core Program Outcomes 2.2, 2.3, 2.4, 2.5, 3.4, 4.1, 4.2, 4.3, 5.1, & 5.2
LESSON 13: PSS 5.1: Don't Rock the Block
A pair of students is lifting a heavy trunk on move-in day. Using two ropes tied to a small ring at the center of the top of
the trunk, they pull the trunk straight up at a constant velocity v. Each rope makes an angle θ with respect to the
vertical. The magnitude of the weight of the trunk is w.
Find the tension in each rope.
MODEL: Make simplifying assumptions. The trunk should be treated as _____________________________________.
VISUALIZE: The free-body diagram is usually sufficient as a picture for ___________________________ problems, but
you still must translate words into symbols and identify what the problem is trying to find. Draw a free-body diagram
(see Tactics Box 4.3, p. 114) that includes all the forces acting on the trunk. Use a coordinate system with the y axis
vertical and the ropes both in the xy plane.
Known:
Find:
SOLVE: Starting with Newton's 1st law, construct the necessary mathematical expressions and derive the solution. Find
the force of tension in each rope.
ASSESS: Check if your result has the correct units, is reasonable, and answers the question. Intuitively, what will
happen to the value of tension if the students try to stand farther apart so that the ropes make a greater angle with the
vertical? Explain your answer. Assume that all other conditions remain the same. Does your intuitive understanding
make sense with your mathematical result? Explain.
This problem targets Physics Core Program Outcomes 2.3, 2.4, 2.5, 3.1, 3.3, 3.4, 4.1, 4.2, 4.3, 5.1, & 5.2
LESSON 13: Hanging Chandelier
A chandelier with mass m is attached to the ceiling of a large concert hall by two cables. Because the ceiling is covered
with intricate architectural decorations (not indicated in the figure, which uses a humbler depiction), the workers who
hung the chandelier couldn't attach the cables to the ceiling directly above the chandelier. Instead, they attached the
cables to the ceiling near the walls. Cable 1 has tension T1 and makes an angle of θ1 with the ceiling. Cable 2 has tension
T2 and makes an angle θ2 of with the ceiling. Find an expression for T1, the tension in cable 1; this expression shoud not
depend on T2. Express your answer in terms of some or all of the variables m, θ1, and θ2, as well as the magnitude of
the acceleration due to gravity g.
Draw a free body diagram (see Tactics Box 4.3, p. 114) & include a coordinate system:
Known:
Find:
Solve (start with Newton’s 1st and 2nd laws; sum in both the x and y directions. You will get 2 equations and 2
unknowns: T1 & T2. Then solve the simultaneous equations):
Assess if you answer is reasonable
 and show in the space below that it has the correct units:
This problem targets Physics Core Program Outcomes: 2.3, 2.4, 2.5, 3.4, 4.1, 4.2, 4.3, 4.4, 5.1, & 5.2
LESSON 14: Problem 5.39 (Block on Table)
A ___________________ kg steel block is at rest on a steel table. A horizontal string pulls on the block. The coefficient of
static friction of dry steel on steel is μs = .800, the coefficient of kinetic friction of dry steel on steel is μk,dry = .600, and
the coefficient of kinetic friction of lubricated steel on steel is μk,lube = .050.
A. What is the minimum string tension needed to move the block?
Draw a free body diagram (see Tactics Box 4.3, p. 114) and include a coordinate system:
Known:
Find:
Solve (start with Newton’s 2nd law):
Assess if you answer is reasonable

and has the correct units

B. If the string tension is _______________ N, what is the block's speed after moving _________________ m.
Draw a free body diagram (see Tactics Box 4.3, p. 114) and include a coordinate system:
Known:
Find:
Solve (start with Newton’s 1st law or 2nd law and determine the ________________. Then use kinematics to find
the final answer):
Assess if you answer is reasonable

and has the correct units

C. If the string tension is ____________N and the table is coated with oil, what is the block's speed after moving
_____________?
Use the same FBD as Part B. Solve (start with Newton’s 2nd law to determine the value of the
________________. Then use ____________________ to find the final answer):
Assess if you answer is reasonable

and has the correct units

This problem targets Physics Core Program Outcomes: 2.2, 2.3, 2.4, 2.5, 4.1, 4.2, 4.3, 4.4, 5.1, & 5.2
LESSON 14: Problem 5.41 (Downhill Jet Skiing)
Sam, who’s mass is ______________ kg, takes off down a _________________ m high, ________________ slope on
his jet-powered skis. The skis have a thrust of ________________ N. Sam's speed at the bottom is ______________m/s.
What is the coefficient of kinetic friction of his skis on snow?
Draw a free body diagram (see Tactics Box 4.3, p. 114) and include a coordinate system:
Known:
Find:
Solve (for this problem, we need to start with kinematics to determine the ______________________. Then we
can use Newton’s ________ law to determine the friction force and its coefficient):


Assess if you answer is reasonable
and has the correct units
This problem targets Physics Core Program Outcomes: 2.2, 2.3, 2.4, 2.5, 4.1, 4.2, 4.3, 4.4, 5.1, & 5.2