- Fluids can be defined as any substance which is capable of flowing.
- They don’t have any shape of their own.
- For example:-water which does not have its own shape but it takes the shape of the container in which it is poured.
But when we pour water in a tumbler it takes the shape of the tumbler
- Pressure is defined as force per unit area.
- Pressure = Force/Area
- For Example:-
- Consider a very sharp needle which has a small surface area and consider a pencil whose back is very bluntand has more surface area than the needle.
- If we poke needle in our palm it will hurt as needle gets pierced inside our skin.Whereas if we poke the blunt side of the pencil into our hand it won’t pain so much.
- This is because area of contact between the palm and the needle is very small therefore the pressure is large.
- Whereas the area of contact between the pencil and the palm is more therefore the pressure is less.
- Consider a stuntman lying on the bed of nails which means there are large numbers of nails on any rectangular slab. All the nails are identical and equal in height.
- We can see that the man is not feeling any pain and he is lying comfortably on the bed. This is because there isa large number of nails and all the nails are closely spaced with each other.
- All the small pointed nails make large surface area therefore the weight of the body is compensated by the entire area of all the nails.
- The surface area increases therefore pressure is reduced.
- But even if one nail is greater than the others then it will hurt. Because then the surface area will be less as a result pressure will be more.
Problem:- A 50 kg girl wearing high heel shoes balances on a single heel. The heel is circular with a diameter 1.0 cm. What is the pressure exerted by the heel on the horizontal floor?
Mass of the girl, m = 50 kg
Diameter of the heel, d = 1 cm = 0.01 m
Radius of the heel, r = d/2 = 0.005m
Area of the heel = πr2 = π (0.005)2 = 7.85 × 10–5 m2
Force exerted by the heel on the floor:
F = mg = 50 × 9.8 = 490 N
Pressure exerted by the heel on the floor:
= 6.24 × 106 N m–2
Therefore, the pressure exerted by the heel on the horizontal floor is 6.24 × 106 Nm–2.