Conceptual Physics

wheels is 0.85 m, how far is her center of gravity from the front wheels? Ignore the weight of this rather long skateboard.

m

3.8 The figure shows a uniform disk with a circular

hole cut out of it. The disk has a radius of

4.00 m. The hole's center is at 2.00 m, and its

radius is 1.00 m. What is the x-coordinate of

the center of gravity of the system?

m

Section 4 - Interactive problem: achieve equilibrium

4.1 Use the data and the diagram in the interactive problem in this section to answer the following question. If the pivot is placed

at the left end of the rod, what is (a) the magnitude and (b) direction of the unknown force? (c) How far should it be placed

from the pivot? Use the simulation to test your answer.

(a) N

(b) i. Positive x

ii. Negative x

iii. Positive y

iv. Negative y

(c) m

Section 6 - Stress and strain

6.1 (a) Suzy the elephant weighs 65,000 N. The cross-sectional area of each foot is 0.10 m^2. When she stands on all fours, what

is the average stress on her feet? (b) Suzanne the stockbroker weighs 610 N. She wears a pair of high-heeled shoes whose

heels each have a cross-sectional area of 1.5×10í^4 m^2 in contact with the floor. If she stands with all her weight on her heels,

what is the stress on the heels of the shoes?

(a) N/m^2

(b) N/m^2

Section 7 - Tensile stress

7.1 A 85.0 kg window washer hangs down the side of a building from a rope with a cross-sectional area of 4.00×10í^4 m^2. If the

rope stretches 0.740 cm when it is let out 7.50 m, how much will it stretch when it is let out 26.0 m?

m

7.2 A building with a weight of 4.10e+7 N is built on a concrete foundation with an area of 1300 m^2 and an (uncompressed)

height of 2.82 m. By what vertical distance does the foundation compress?

m

7.3 Many gyms have an exercise machine called a vertical leg press. To use this machine, you lie on your back and press a

weight upward with your feet until your legs are perpendicular to the ground. Find the amount the femur bone compresses

when a person with a 0.385 m femur lifts 525 N with a vertical leg press, using one leg, until the leg is fully extended straight

up. Assume that the cross sectional area of the femur is 5.30×10í^4 m^2.

m

7.4 (a) An engineer is designing a bridge with four supports made of cylinders of concrete. The cylinders are 3.5 m tall, and they

cannot compress by more than 1.6×10–4 m. If the bridge must be able to hold 7.5×10^6 N, what should be the radius of the

cylinders? (b) Suppose the restriction on the amount of compression is removed. If the yield strength of the concrete is

5.7×10^6 N/m^2 , what is the smallest radius that will prevent the concrete from deforming under the weight?

(a) m

(b) m

7.5 A climber with a mass of 75 kg is attached to a 6.5 m rope with a cross sectional area of 3.5×10í^4 m^2. When the climber

hangs from the rope, it stretches 3.4×10í^3 m. What is the Young's modulus of the rope?

N/m^2

7.6 A helicopter is working to bring water to a forest fire. A steel cable hangs from the helicopter with a large bucket on the end.

The cable is 8.5 m long and has a cross-sectional area of 4.0×10í^4 m^2. Together, the bucket and the water inside it weigh

(^218) Copyright 2007 Kinetic Books Co. Chapter 11 Problems