Pressure
# Pressure: The force acting per unit area is called pressure. Its SI unit is Pascal (pa) N/m2.
p = F/A
# Liquid Pressure: Force exerted per unit area of a container due to the weight of the liquid in it is called liquid pressure.
1) What is Density? What will happen when an un-boiled egg is placed in the beaker of fresh water and salt water? Explain it.
Ans: The mass per unit volume is called density. Its SI unit is kg/m3 or g/cm3.
An egg sinks in the beaker of fresh water because egg’s density is more than that of fresh water. Whereas egg floats in salt water because salt water has more density than that of egg.
2) Prove that liquid pressure p = dgh
Soln:
Let us suppose a cylindrical vessel of base area (A) filled with a liquid of density (d) to a height (h).
Then,
Pressure (p) = Force (F) / Area (A) = weight of liquid / base area
p = F / A
p = m x g / A ............................. [Since f = m x g]
p = (v x d x g)/A .......................[Since m = v x d]
p = (A x h x d x g) / A ..............[Since v = A x h]
p = dgh
Hence proved.
Ans: Egg floats on the solution of salt and water because the density of egg is less than the density of solution of salt and water.
Use of Atmospheric Pressure:
a) To balance blood pressure by living things.
b) We can measure altitude with the help of atmospheric pressure.
c) We can suck water or cold drinks with the help of pipe due to atmospheric pressure.
4) What is one Pascal? An egg sinks in pure water but floats in salt solution. Why?
Ans: If 1 N force is acted on 1 square meter area then the pressure is said to be 1 Pascal pressure.
Use of Atmospheric Pressure:
a) To balance blood pressure by living things.
b) We can measure altitude with the help of atmospheric pressure.
c) We can suck water or cold drinks with the help of pipe due to atmospheric pressure.
4) What is one Pascal? An egg sinks in pure water but floats in salt solution. Why?
Ans: If 1 N force is acted on 1 square meter area then the pressure is said to be 1 Pascal pressure.
An egg sinks in pure water because egg has more density then that of pure water where as floats in salt solution because salt solution has more density that egg.
5) Define relative density. Write its formula.
Ans: The ratio of density of substance to the density of water at 4oc is called relative density.
Relative Density = Density of substance / density of water at 4oc.
6) Write the difference between force and pressure.
Ans:
|
S.N |
Foce |
Pressure |
|
1 |
Pull or push is called force |
The force acting per unit area is called pressure. |
|
2 |
Its SI unit is Newton. |
Its SI unit is Pascal. |
7) What are the factors affecting liquid pressure?
Ans: The following are the factors affecting liquid pressure;
a) Density of liquid (d)
b) Acceleration due to gravity (g)
c) Height or depth of liquid (h)
8) A piece of wood floats on water but a piece of iron sinks. Give reason.
Ans: Wood has less density than water, so it floats on water. Whereas, iron has more density than water, so it sinks.
9) When does the body floats on water?
Ans: The body floats on water when it has less density than that of water.
10) Write the formula to calculate the pressure of liquid?
Ans: Liquid of Pressure (p) = dgh
11) Give an example of application of pressure in our daily life.
Ans: the following are the application of pressure in our daily life;
a) Cutting appliances are made sharper.
b) The base of building is made wider.
12) It is easier to cut with a sharp knife than a blunt one, why?
Ans: It is easier to cut with a sharp knife than a blunt one because effect of force will be more on small area and then large area. So, pressure due to sharp knife is more and blunt knife is less.
13) A pin or needle is provided with sharp tip, why?
Ans: A pin or needle is provided with sharp tip because sharp tip covers less area so that pressure exerted will be more and easy to insert into an object.
14) The bases of buildings are made broader or wider, why?
Ans: The base of buildings are made boarder because if it is made boarder area increases which leads to decrease the pressure on the ground due to weight of building so that buildings can withstand for long time.
15) As we go up from the earth surface, atmospheric pressure decreases, why?
Ans: As we go up from the earth surface gravity of the earth decreases and air molecules are less in number and molecules are far apart. So, atmospheric pressure decreases.
16) Nose bleeding occurs at higher altitude, why?
Ans: As we move up atmospheric pressure decreases so that our internal pressure becomes more than external pressure and blood starts to come out through nose. Hence, nose bleeding occurs at higher altitude.
17) A doctor pushes the plunger of a syringe down and then pulls up to draw liquid into a syringe, why?
Ans: When a doctor pushes the plunger of a syringe down all the air inside syringe goes out and creates partial vacuum. After that he dipped into a liquid and then pull outwards such that liquid pressure is more than the pressure inside syringe and easily liquid draws into it.
18) Bucket of water fills faster in downstairs tap than in top stairs tap. Why?
Ans: As we know that pressure of water (liquid) increases with depth. Downstairs tap has greater depth of water than topstairs. So that pressure on downstairs has more than topstairs and with more force water comes out in downstairs. Hence, bucket of water fills faster in downstairs than in top stairs tap.
19) Even the atmospheric pressure is large though we do not feel it, why?
Ans: We do not feel the atmospheric pressure because the pressure inside our body is equal to the pressure outside the body (atmospheric pressure).