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Pressure is force acting per unit of area
Pressure = Force (Thrust) / Area
Note
Lesser the area,more the pressure
More the area,less the pressure
Question 1
Suppose a square box of mass 100 kg is placed on ground
It has a width of 5 meters
What is Thrust(Force) and Pressure
Take value of g to be 10 m/s 2
Ans
Mass = m = 100 kg
g = 10 m/s 2
Thrust = m × g
= 100 × 10
= 1000 Newtons
Side of square box = 5 meters
Area of square box = 5 × 5 = 25 m 2
Pressure = Force/Area
= 1000/25
= 40 N/M 2
= 40 Pa
Question 2
What happens if we reduce size of box to 2 meters
Suppose a square box of mass 100 kg is placed on ground
It has a width of 2 meters
What is Thrust(Force) and Pressure
Take value of g to be 10 m/s 2
Mass = m = 100 kg
g = 10 m/s 2
Thrust = m × g
=100 × 10
= 1000 Newtons
Side of square box = 2 meters
Area of square box = 2 × 2 = 4 m 2
Pressure = Force/Area
= 1000/4
= 250 N/M 2
= 250 Pa
(Note SI unit of Pressure is Pascal ,denoted by Pa)
Summary
Lesser the area,more the pressure
More the area, less the pressure
Examples
When we stand on loose sand,our feet sink into it
But when we lie down, we do not sink
This is because when we are standing,
Force of our body is exerted through our feet
which have smaller area
and hence more pressure is exerted on sand
But when we are lying
More area of our body is in contact with stand
Since Area increases, Pressure is reduced
So our body does not sink
Pressure in Fluids
What are 3 forms of Matter
Three forms of Matter are
- Solid (Example - Ice)
- Liquid (Example - Water)
- Gas (Example - Water Vapor)
What are fluids ?
it is a common name given to liquids and gases.
So liquid and gases are both called fluids
Is pressure exerted by fluids
yes
All forms of matter exert pressure
Just like solids have weight and exert pressure
Similarly fluids also have weight and exert pressure
Examples of Fluid Exerting Pressure
Suppose we fill a balloon with water, it bulges. This is because of pressure of water on surface of balloon
Similarly if we fill a balloon with air, it bulges. This is because of pressure of air on surface of balloon
Pressure = Force/ Area
Hence pressure is inversely proportional to the area in contact.
Let's look at some example of pressure
School bags have wide straps
Pressure = Force/ Area
Hence pressure is inversely proportional to the area in contact.
If the straps of the bag would not be wide, the pressure exerted on the shoulders would be large.
Therefore, straps of school bags are made wide in order to reduce the pressure exerted on the shoulders.
Sharp knife cuts better than a blunt knife.
A sharp knife has a very thin edge as compared to a blunt knife.
When we cut an object using a sharp knife, the force is exerted on a very less area.
Since the area of contact is less, the pressure exerted is very large.
While, in case of a blunt knife the area in contact is relatively more and hence the pressure exerted is less.
Hence, a sharp knife cuts better than a blunt knife.
Tip of a needle is sharp
The tip of a nail is sharp because the sharper the tip, the lesser will be the area of the tip.
When the needle would be hammered, the large force exerted by the hammer would fall on the very small area of the tip.
Due to small area of the needle in contact with the object, the pressure exerted on the object would be very large.
Hence, the needle would pierce in to the object easily.
Pressure on ground is more when we are walking than when we are standing
When we are standing, both are feet are in contact with the ground while when we are walking, one of our feet is always in air.
Hence, the area in contact with the ground is more when we are standing than when we are walking.
Greater the area in contact, lesser would be the pressure.
Hence, pressure on ground is more when we are walking than when we are standing.
Depression is much more when a man stands on a cushion than when he lies down.
When a man is standing on a cushion the area in contact is much less than when he lies down.
Lesser the area in contact, more would be the pressure.
Hence, the pressure would be more when the man is standing on the cushion than when he is lying on it.
Tractors have broad tyres
Tractors have broad tyres, hence the area in contact with the ground is large.
Due to this large area in contact, the pressure exerted would be less.
Hence, the tyres of the tractors don not sink into comparatively soft ground in fields.
Questions
Q 1 Page 141 - Why is it difficult to hold a school bag having a strap made of a thin and strong string?
Example 10.6 - A block of wood is kept on a tabletop. The mass of wooden block is 5 kg and its dimensions are 40 cm × 20 cm × 10 cm. Find the pressure exerted by the wooden block on the table top if it is made to lie on the table top with its sides of dimensions (a) 20 cm × 10 cm and (b) 40 cm × 20 cm.
