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Physics - College, section 1, Fall 2019
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Salu - Physics for Architects, 2/e (Homework)

James Finch

Physics - College, section 1, Fall 2019

Instructor: Dr. Friendly

Current Score : 0 / 24

Due : Monday, January 28, 2030 00:00 EST

Last Saved : n/a Saving... ()

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Instructions

Physics for Architects, 2nd edition by Yehuda Salu focuses on architecture-related applications of physics concepts and principles. The WebAssign component for this title provides students with immediate feedback and includes question links to the eBook. Also included is a selection of questions from the exclusive WebAssign College Physics collection, which are organized to match the book’s table of contents. These questions feature integrated feedback for every question part and detailed solutions for every question. Additionally, a series of multi-step tutorials are available to help strengthen problem-solving skills and conceptual understanding. The assignment below presents a sample of questions from chapter 4.

Question 1 demonstrates numeric randomization (in red) and numeric answer entries with units.

Questions 2, 6, and 7 shows numeric randomization and numeric answer entries.

Question 3 features our physPad for entering symbolic answers, provides helpful feedback, and includes a detailed solution.

Question 4 demonstrates substance and numeric randomization, provides helpful feedback, and includes both a tutorial and a detailed solution.

Question 5 includes helpful feedback and a detailed solution.

Question 8 features terminology randomization and includes a detailed solution.

Question 9 includes a detailed image and a detailed solution.

Question 10 provides both a tutorial and a detailed solution. This demo assignment allows many submissions and allows you to try another version of the same question for practice wherever the problem has randomized values.

The answer key and solutions will display after the first submission for demonstration purposes. Instructors can configure these to display after the due date or after a specified number of submissions.

Assignment Submission

For this assignment, you submit answers by question parts. The number of submissions remaining for each question part only changes if you submit or change the answer.

Assignment Scoring

Your last submission is used for your score.

1. –/6 points SaluPhysArch2 4.3.P.011. My Notes

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A 75 lb ladder of length 14 ft leans against a frictionless wall. Find what horizontal force is needed to hold the ladder in place when the angle that it makes with the floor is 70°, 45°, and 38°. (Include units in your answers. More information. Enter the magnitude.)

F_70° = F_45° = F_38° =

If the horizontal force between the ladder and the floor is due only to friction, what should be the smallest friction coefficient needed to hold the ladder in place in each of those three angles?

μ_70° = μ_45° = μ_38° =

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2. 0/4 points | Previous Answers SaluPhysArch2 4.3.P.008. My Notes

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A horizontal truss, symmetric around its center, of mass 17,000 kg and total length 22 m, is placed on two posts as shown in the figure. (Enter the magnitude.)

(a)

Find the downward force (in N) that it exerts on each post.

F_left=
72900
N F_right=
93700
N

(b)

Find the additional downward force (in N) on each post when a load of 2,700 kg is placed at the center of the truss.

F_{additional, left}=
11600
N F_{additional, right}=
14900
N

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3. –/1 points SaluPhysArch2 4.1.WA.012. My Notes

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As shown in the figure below, a bicycle wheel is at rest against a curb. If the wheel has a radius R, mass M and is at rest against a curb of height
h = 0.18R,
determine the minimum horizontal force
F
(in terms of M and g) that must be applied to the axle to make the wheel start to rise up over the step.
F =

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4. –/1 points SaluPhysArch2 4.4.WA.004.Tutorial. My Notes

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A rod has a 1.3-m-long section that is cast iron and a 2.6-m-long section that is aluminum as shown in the diagram below. The diameter of the cylindrical rod is 0.40 cm.
Y_{cast iron} = 100 ⨯ 10¹⁰ N/m²
and
Y_aluminum = 7 ⨯ 10¹⁰ N/m².
How much elongation is produced in the rod when it is subjected to a force of 6.1 10³ N?
cm

Tutorial

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5. –/2 points SaluPhysArch2 4.4.WA.003. My Notes

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Steel beams are used for load bearing supports in a building. Each beam is 4.0 m long with a cross-sectional area of 7.7 10^-3 m² and supports a load of 4.7 10⁴ N. Young's modulus for steel is 210 ⨯ 10⁹ N/m².

(a) How much compression does each beam undergo along its length?
mm

(b) Determine the maximum load one of these beams can support without any structural failure if the compressive strength of steel is 1.50 ⨯ 10⁸ N/m².
N

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6. 0/1 points | Previous Answers SaluPhysArch2 4.4.P.023. My Notes

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A brick of base area
3.2" ✕ 6.0"
is cemented by concrete to the floor. What horizontal force F (in N) is needed in order to break the concrete? Assume that both the brick and the bonding between the brick and the concrete are stronger than the concrete. (Enter the magnitude.)

24800
N

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7. –/2 points SaluPhysArch2 4.5.P.028. My Notes

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A reservoir at the top of a high-rise building is directly connected to faucets in floors below. The water level in the reservoir is 86 m above street level. What would be the pressure (in Pa) at a closed faucet in the following locations?

(a)

65 m above street level

Pa

(b)

4 m above street level

Pa

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8. 0/3 points | Previous Answers SaluPhysArch2 4.5.WA.004. My Notes

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You inflate the tires of your car to a gauge pressure of 36.0 lb/in². If your car has a mass of 1550 kg and is supported equally by its four tires, determine the following.
(a) Contact area between each tire and the road

0.0153

Can you write an expression for the pressure in terms of the force and area? Which pressure are we interested in, gauge, atmospheric or absolute? Notice that the mass rather than weight is given for the car. m²

(b) Will the contact area increase, decrease, or stay the same when the gauge pressure is increased?

increase decrease stay the same

If the tire is not perfectly round but rather slightly flat on the bottom and you pump air into it, does it get flatter or more round? If it gets more round, what happens to the flat region (the contact area) of the tire?
(c) Gauge pressure required to give each tire a contact area of 108 cm²

51

See if you can develop an expression for the gauge pressure of a tire in terms of the mass of the car and the contact area of each tire. lb/in²

Solution or Explanation
Note: We are displaying rounded intermediate values for practical purposes. However, the calculations are made using the unrounded values.

(a) The relationship between pressure P, area A, and force F is
P = F/A.
In this case the force supported by each tire is one fourth the weight of the car or
F = Mg/4,
where M is the mass of the car and g is the magnitude of the acceleration due to gravity. Combining the above, an expression for the contact area of the tire may be written as
A = Mg/(4P).
Inserting values we have

A =
Mg
4P_g
=
(1.55 10³ kg)(9.80 m/s²)
4(36.0 lb/in²)
= 105
N
lb/in²
.

Converting lb/in² into Pa and then N/m², we have

A =
105
N
lb/in²
1 lb/in²
6.895 ⨯ 10³ Pa
1 Pa
1 N/m²
= 0.0153 m².

(b) Due to the weight of the car, the tire is not perfectly round and the part of the tire in contact with the road is slightly flattened. If air is pumped into the tire, the gauge pressure increases, the tire becomes slightly more round, and the area in contact with the road decreases. Looking at this in a more quantitative manner, knowing that the relation between pressure, area, and force is
P = F/A,
we may solve for the area to obtain
A = F/P.
Since the mass of the car and hence the force is constant, the area is inversely proportional to the pressure. If the pressure increases, the contact area decreases.

(c) In part (a) of the problem, we developed an expression which relates the contact area and the gauge pressure. Using this expression we may write the gauge pressure as
P_g = Mg/(4A).
Inserting values, we obtain

P_g =
Mg
4A
=
(1.55 10³ kg)(9.80 m/s²)
(4)(108 cm² )(10⁻² m/cm)²
= 3.52 10⁵ N/m².

Converting from N/m² to Pa and then lb/in² we have

P_g = 3.52 10⁵ N/m²
1 Pa
1 N/m²
1 lb/in²
6.895 ⨯ 10³ Pa
= 51.0 lb/in².

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9. 0/3 points | Previous Answers SaluPhysArch2 4.5.WA.008. My Notes

Question Part

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0/3

You inflate the tires of your car to a gauge pressure of 35.5 lb/in². If your car has a mass of 1525 kg and is supported equally by its four tires, determine the following.
(a) Contact area between each tire and the road

0.0153

Can you write an expression for the pressure in terms of the force and area? Which pressure are we interested in, gauge, atmospheric or absolute? Notice that the mass rather than weight is given for the car. m²

(b) Will the contact area increase, decrease, or stay the same when the gauge pressure is decreased?

increase decrease stay the same

(c) Gauge pressure required to give each tire a contact area of 114 cm²

47.5

See if you can develop an expression for the gauge pressure of a tire in terms of the mass of the car and the contact area of each tire. lb/in²

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10. –/1 points SaluPhysArch2 4.3.WA.016.Tutorial. My Notes

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1

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–/1

A deli owner creates a lunch special display board by taking a uniform board which has a weight of 207 N, cutting it in half, hinging the halves together with a frictionless hinge, and setting it up as an inverted "V". Determine the minimum coefficient of static friction needed between the board and the ground in order for her to set the display board up with an angle of 30° between the two sides.

Tutorial

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