Page 131 - Physics

Mechanical properties of matter
(d) rubber bands that hold things together;
and
(e) toys like balloons and balls.
In transport, we fi nd application of
elasticity in:
(a) hoses, belts and shock-absorbing
springs for cars and trucks;
(b) rubber tyres; and
(c) support cables for bridges.
In industry, the application of elasticity is
found in:
(a) steel beams used in construction;
(b) conveyor belts;
(c) measuring weight;
(d) insulation against vibration and sound;
and
(e) mechanical control devices.
Task 6.1
Identify some elastic objects in your
environment and explain how the
concept of elasticity has made them
useful in your daily life.
Adhesion and cohesion
If you pass through a grass fi eld in the
morning you will notice some drops of
water stuck on grass leaves. Water drops
are formed by the molecules that stick
together because of cohesive forces. On
the other hand, drops of water stick to
the grass leaves because of the adhesive
forces between water molecules and grass
molecules. Thus, cohesion and adhesion are
the two important concepts in explaining
molecular interactions.
Physics Form 1 Final.indd 125
Cohesion refers to force of attraction
between molecules of the same type
whereas adhesion refers to the force
of attraction between molecules of
different kinds.
Both cohesion and adhesion play important
roles in explaining various physical
phenomena. For example, solids are known
to have defi nite shapes because of the strong
cohesive forces among their molecules
which are in fi xed positions and close
together. Water wets glass surfaces since
adhesive forces between water and glass
molecules are stronger than cohesive forces
between water molecules. Conversely,
mercury does not wet glass surfaces
since cohesion between its molecules is
stronger than adhesion between mercury
and glass molecules. Furthermore, cohesion
and adhesion explain why liquids form
meniscus when poured into glass tubes as
shown in Figure 6.5.
Concave meniscus
Convex
meniscus
Water
Mercury
Figure 6.5: Shapes of menisci of mercury
and water
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