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121.  A person carrying a bucket of water on his right hand leans towards the left. Why?

As the body will be stable if its center of gravity i.e. C.G. lies within the base of the body. A person carrying a bucket of water on his right hand leans towards the left so that the vertical line passing through C.G. lies within the base and it will pass through the lower height so, that the person can enjoy stability. Hence, in order to increase the stability of the body, a person carrying a bucket of water on his hand leans towards the left.

 

122. A truck loaded with cotton or straw is likely to topple easily as compared to the one loaded with iron. Why?

As the body will be stable if its center of gravity i.e. C.G. lies within the base of the body or the base of that body is made heavy. A truck loaded with cotton or straw is fewer stables as compared to that loaded with iron, the C.G. will lie at lower height and while loaded with cotton or straw, it raised the position of C.G.. Hence, a truck loaded with cotton or straw is likely to topple easily as compared to the one loaded with iron.

 

123.  Why is it easier to hold down a 10 kg body in your hand it with your arm extended horizontally? (HSEB 1995)

When a 10 kg body is caught by hand at the side, the vertical line passing through center of gravity i.e. C.G. will lie with in the base. But when the hand is outstretched, the vertical line passing through C.G. will not lie within the base and it will lie at greater height. As the body will be stable if its center of gravity i.e. C.G. lies within the base of the body. Hence, it is easier to hold down 10 kg body than arm extended horizontally.

 

124.  Is it possible for a solid body to have no matter at its C.G.? (HSEB 1995)

Yes, it is possible for a solid body to have no matter at its C.G. For example, in a hollow sphere, the C.G. lies at the geometrical center. For the ring, the C. G. lies at geometrical center etc.

 

125. Cow (horse) is more stable than a man. Why?

The vertical lines passing through the center of gravity (C.G.) of the cow always lie within the base. As the body will be stable if its center of gravity i.e. C.G. lies within the base of the body. Further, the C.G. of cow lies at lower height than that of man. Hence, cow is more stable than man.

 

126.  Can center of mass of a body coincide with the geometrical center of the body?

Yes, center of mass i.e. C.M. of a body can coincide with the geometrical center of the body. When a body has uniform density, its center of mass shall coincide with the geometrical center.

 

127. Does the center of mass of a solid necessarily lie within the body? If not, give two examples.

The Center of mass i.e. C.M. of a solid is not necessary to lie within the body. It may also lie outside the body of the solid. For example, in a ring and hollow sphere, the center of mass (C.M.) lie at the geometrical center where there is no mass at all.

 

128. Which is more elastic, steel or rubber?

Let two rods, one of steel and the other of rubber be considered, with lengths Ls, Lr and cross sections As and Ar. If Ys and Yr are their Young Modulus. The expressions for forces on them are given by 

 

Then, and , where the extensions produced are es and er respectively.

If equal forces are applied, Fs = Fr which gives .

 

Similarly if the lengths and the cross sections are also considered equal, they cancel out and gives . The comparison is now just between extensions and Young's modulus. Since it is generally observed in daily life that  the extension in rubber is generally greater than in steel, i.e. er > es, so . This means that steel is more elastic than rubber.

 

129. Rubber pads are used as vibration absorber. Why?

 



130. Why are springs made of steel and not copper?

 



131. How does the length of a wire affect its load bearing capacity?

The load bearing capacity means the maximum load (often called the breaking load) or the force the elastic body can withhold. This is given by the relation

 

Breaking Load (Breaking Force) = Breaking Stress Cross sectional Area

 

The relation does not show any relation of the Breaking load to the length. So it does not depend on the length. However, it does depend on the Cross Section.



132. Where does the work go, which is used to stretch or compress a wire go?

 



133. What do you mean by breaking stress?

It is the amount of stress required to snap or break a long cylindrical wire of a substance by applying longitudinal force. In other words, it is the amount of stress required to bring up fracture of such bodies. It resembles the condition in which the molecules are carried so far away that there is very less, almost no more attractive forces to hold them. It is related to the breaking force or the maximum allowed force as

 

 

Or,      



134. Water is more elastic than air. Explain.

     Same as in No. 128.

 



135. What is elastic fatigue?

 



136. Why are bridges declared unsafe after a long use?

 



137. Why are steel rods embedded in concrete in housing structures?

 



138. What is elastic after effect?

Ans: When an elastic substance is deformed within elastic limit, it returns to its original dimensions. However, the return takes some time. This phenomenon is called as Elastic After Effect.



139. Why does a spring balance used for long time show wrong reading?

 



140. What happens to the (i) extension (ii) the maximum load, a wire can bear, when the wire is cut into half?

 

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