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.
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?
