Question 1

Which of the following is a necessary condition for an object to be in static equilibrium?

Accepted Answer

Both the net force and the net torque on the object must be zero.

Question 2

A 910 kg bucket is suspended 0.85 m from a pivot on a beam. A person lifts the beam at its end, 3.6 m from the pivot. What force must the person apply to keep the beam in equilibrium? (Assume $$g = 9.8\ \mathrm{m/s^2$})$$

Accepted Answer

$$F = 2100\ \mathrm{N}$$

Question 3

A ladder leans against a wall at a $$60^\circ$$ angle with the floor. If the coefficient of static friction between the ladder and the ground is just enough to prevent slipping, what is the minimum value of $$\mu_s$$?

Accepted Answer

$$\mu_s = 0.29$$

Question 4

A car has a track width $$t$$ and a center-of-gravity height $$h. $$The critical angle $$	heta_c at $$which the car will tip is given by $$	an 	heta_c = t/(2h). If$$ $$h$$ increases while $$t$$ stays the same, what happens to $$	heta_c$$?

Accepted Answer

$$	heta_c$$ decreases

Question 5

A 2-m-long board weighing 50 N extends over the edge of a table, with 40% of its length off the table. How far beyond the table edge can a 25-N cat walk before the board begins to tilt?

Accepted Answer

0.8 m

Question 6

Which of the following best describes the relationship between the restoring force of a spring and its displacement according to Hooke’s Law?

Accepted Answer

$$F_{sp} = -kx$$

Question 7

A spring with a spring constant $$k = 125\ \mathrm{N/m} is $$used to pull a 25 N block across a table with a coefficient of friction $$\mu_k = 0.20. By $$how much does the spring stretch from its equilibrium length?

Accepted Answer

$$0.04\ \mathrm{m}$$

Question 8

Which of the following materials has the highest Young’s modulus?

Accepted Answer

Steel

Question 9

A steel cable 6.0 m long and 2.5 mm in diameter supports a 120 kg mass. Given $$Y = 2.0 	imes 10$$^{11}$$\ \mathrm{N/m^2$}$$, by how much does the cable stretch? ($$g = 9.8\ \mathrm{m/s^2$})$$

Accepted Answer

7.2 mm

Question 10

Which of the following statements about the stability of an object is correct?

Accepted Answer

A wider base of support and a lower center of gravity both increase stability.

Question 11

A 20-cm-long spring is stretched to 22 cm by a 100 N force. What is the spring constant $$k$$?

Accepted Answer

$$k = 500\ \mathrm{N/m}$$

Question 12

A rod of length $$L$$ and cross-sectional area $$A is $$stretched by a force $$F. $$The ratio $$F/A is $$called:

Accepted Answer

Stress

Question 13

A 61 kg woman stands on tiptoe, and the tension in her Achilles tendon is found to be 1800 N. By what factor does this force exceed her weight? ($$g = 9.8\ \mathrm{m/s^2$})$$

Accepted Answer

3 times

Question 14

Which of the following best explains why a soda can with a small amount of water can balance on its bevel, but a full or empty can cannot?

Accepted Answer

With a small amount of water, the center of gravity shifts directly above the bevel, increasing stability.

Question 15

A nutcracker has a mechanical advantage of 3. If you apply a force of 50 N, what force is applied to the nut?

Accepted Answer

150 N

Question 16

If a rod is stretched beyond its elastic limit, what happens?

Accepted Answer

It undergoes permanent deformation and does not return to its original length.

Question 17

Which of the following biological materials has the lowest Young’s modulus?

Accepted Answer

Cartilage

Question 18

A seesaw is balanced with a 26 kg child sitting 1.0 m from the pivot. Where should a 32 kg child sit to balance the seesaw?

Accepted Answer

0.81 m from the pivot

Chapter 8: Equilibrium and Elasticity – Physics with Algebra Study Notes

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