Engineering Fundamentals: An Introduction to Engineering, 4th ed.c

296 Chapter 10 Force and Force-Related Parameters

10.35.The coefficient of static friction between a concrete

block and a surface is 0.8. The block weighs 20 lbf. If

a horizontal force of 15 lbfis applied to the block,

would the block move? And if not, what is the magni-

tude of friction force? What should be the magnitude

of the horizontal force to set the block in motion?

10.36.Calculate the shear modulus for a given cylindrical

metal speciman and test results of T1700 Nm,

L20 cm, D5cm, 0.02 rad. Can you tell

what the material is?

10.37.Determine the amount of work done by you and a

friend when each of you push on a car that has run out

of gas. Assume you and your friend each push with a

horizontal force with a magnitude of 200 N, a distance

of 100 m. Also, suggest ways of actually measuring the

magnitude and the direction of the pushing force.

10.38.Determine the work done by an electric motor lifting

an elevator weighing 1000 lb through five floors;

assume a distance of 15 ft between each floor. Com-

pute the power requirements for the motor to go from

the first to the fifth floor in

a. 5 s

b. 8

Express your results in horsepower (1 hp  550

ftlb/s).

10.39.If a laptop computer weighing 22 N is dropped from

a distance of 1 m onto a floor, determine the average

reaction force from the floor if the time of contact is

changed by using different cushion materials, as shown

in the accompanying table.

Average

Time of Reaction

Contact (s) Force (N)

0.01

0.05

0.1

1.0

2.0

5 cm

A

B

C

D

3 cm

2 cm

35

50 N

50 N

100 N

Problem 10.34

10.34.Determine the sum of the moments created by the

forces shown in the accompanying figure about points

A,B,C, andD.

10.33.Determine the moment created by the weight of the

traffic light about pointO. The dimensions of the

traffic light post and arm are shown in the accompa-

nying figure. The traffic light weighs 40 lb.

O

18 ft

24 ft

Problem 10.33

10.40.Obtain the values of vapor pressures of alcohol, water,

and glycerin at a room temperature of 20C.

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