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Chapter 9: Q9-11Q (page 230)

Place yourself facing the edge of an open door. Position your feet astride the door with your nose and abdomen touching the door鈥檚 edge. Try to rise on your tiptoes. Why can鈥檛 this be done?

Short Answer

Expert verified

Rising on your tiptoes cannot be done because gravity will exert torque on the body.

Step by step solution

01

Understanding the center of mass

Whenever a person is standing on their tiptoes, the center of mass of their body changes in the forward direction and the chances of falling are more in this case.

02

Explanation for the position of the person if they try to rise on their tiptoes

When a person is in a position where their feet astride the door with their nose and abdomen touching the edge of the door, the center of mass cannot shift toward the forward direction.

So, gravity will apply some torque on the person, and they will not be able to stand on their tiptoes and return to the floor flat-footed.

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Most popular questions from this chapter

A 10.0 N weight is suspended by two cords, as shown in Fig. 9鈥�44. What can you say about the tension in the two cords?

(a) The tension in both cords is 5.0 N.

(b) The tension in both cords is equal but not 5.0 N.

(c) The tension in cord A is greater than that in cord B.

(d) The tension in cord B is greater than that in cord A.

A heavy ball suspended by a cable is pulled to the side by a horizontal force \(\vec F\), as shown in Fig. 9鈥�43. If angle \(\theta \) is small, the magnitude of the force F can be less than the weight of the ball because

(a) the force holds up only part of the ball鈥檚 weight.

(b) even though the ball is stationary, it is not really in equilibrium.

(c) \(\vec F\) is equal to only the x component of the tension in the cable.

(d) the original statement is not true. To move the ball, \(\vec F\) must be at least equal to the ball鈥檚 weight.


In a mountain-climbing technique called the 鈥淭yrolean traverse,鈥� a rope is anchored on both ends (to rocks or strong trees) across a deep chasm, and then a climber traverses the rope while attached by a sling as in Fig. 9鈥�91. This technique generates tremendous forces in the rope and anchors, so a basic understanding of physics is crucial for safety. A typical climbing rope can undergo a tension force of perhaps 29 kN before breaking, and a 鈥渟afety factor鈥� of 10 is usually recommended. The length of rope used in the Tyrolean traverse must allow for some 鈥渟ag鈥� to remain in the recommended safety range. Consider a 75-kg climber at the center of a Tyrolean traverse, spanning a 25-m chasm. (a) To be within its recommended safety range, what minimum distance x must the rope sag? (b) If the Tyrolean traverse is set up incorrectly so that the rope sags by only one-fourth the distance found in (a), determine the tension in the rope. Ignore stretching of the rope. Will the rope break?

A parking garage is designed for two levels of cars. To make more money, the owner decides to double the size of the garage in each dimension (length, width, and the number of levels). For the support columns to hold up four floors instead of two, how should he change the columns' diameter?

(a) Double the area of the columns by increasing their diameters by a factor of 2

(b) Double the area of the columns by increasing their diameters by a factor of \(\sqrt 2 \)

(c) Quadruple the area of the columns by increasing their diameters by a factor of 2

(d) Increase the area of the columns by a factor of 8 by increasing their diameters by a factor of \(2\sqrt 2 \)

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(II) A steel wire 2.3 mm in diameter stretches by 0.030% when a mass is suspended from it. How large is the mass?
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