# Science Textbook Solutions for Class 8 Science Chapter 3 - Force and Pressure

Important points to remember :
1. a force is applied between two objects even if the two objects are not in contact,  such a force  is called  a ‘Non contact force’.
2. Muscular  force  is  an  example  of  contact force and is applied to objects with the help of our muscles. It is applied  in several cases such as lifting,  pushing, pulling.
3. The tendency of an object to remain in its existing state is called its inertia. This is why an object  in stationary state remains  in  the  same  state  and  an  obect  in motion  remain in the  state of motion  in the absence of an external force.
4. Air surrounds the earth from all sides. This layer of air is called atmosphere. Archemedes  was a Greek Scientist  and a mathematician with sharp intelligence.  He found out the value of  p  by numerical calculations. When an object  is partially  or  fully immersed  in a  fluid, a  force  of buoyancy acts on it  in  the  upward direction.  This force  is equal to the weight of the fluid displaced by the object. density  of a substance is called its ‘specific gravity.

Question 1: Write the word in the blank space.
A. The SI unit of force is_________ (Dyne, Newton, Joule)

B. The air pressure on our body is equal to __________ pressure (Atmospheric, sea bottom, space)

C. For a given object, the buoyant force in liquids of different ________ is__________ (the same, density, different, area)

D. The SI unit of pressure is_______ (N/m3 , N/m2, kg/m2, pa/m2)

Answer :
A. The SI unit of force is Newton.

B. The air pressure on our body is equal to atmospheric pressure.

C. For a given object, the buoyant force in liquids of different density is different.

D. The SI unit of pressure is N/m2.

Question 2: Make a match
A group                       B group
1. Fluida.                     Higher pressure

2. Blunt knifeb.          Atmospheric pressure

3. sharp needlec.        Specific gravity

4.Relative densityd.   Lower pressure

5. Hecto pascale. Same pressure in all directions

Answer :
1. Fluide. - Same pressure in all directions
2. Blunt knifed. - Lower pressure
3. sharp needlea. - Higher pressure
4. Relative densityc. - Specific gravity
5. Hecto pascalb. - Atmospheric pressure

Question 3: Answer the following questions in brief.
A. A plastic cube is released in water.  Will it sink of come to the surface of watet?
Answer : The plastic cube is going to float on the surface of water as its density is less than that of water.

B. Why do the load carrying heavy vehicles have large number of wheels?
Answer : As the load carrying large and heavy vehicle more and wide area. Hence, it will apply more pressure in ratio to its area and weight. To distribute this pressure, there will be obvious need of force. This force is given by the tires. Hence, more the pressure, it will need more force. And thus, more tires (large number of tires) would be used.

C. How much pressure do we carry on our heads? why don't we feel it?
Answer : The pressure that gets exerted on our heads is at 14 pounds which is per square inch at the sea level. It acts not only on our heads, but on our whole body right from the moment that we are born. The reason why we don't feel it is because we get accustomed to the pressure.

Question 4: Why does it happen?
A. A ship dips to a larger depth in fresh water as compared to marine water.
Answer : The higher water density means lower sinking level of the ship. The marine water is more dense than the fresh water because of the presence of salt substances in the marine water. That's why ships sink deeper in the freshwater than the marine water.

B. Fruits can easily be cut with a sharp knife.
Answer : A sharp knife can effectively cut the fruit because the area under contact vis quite small when compared to the blunt knife. Since the force applied is the same, the pressure on the fruit increases to a great extent. It leads to more thrusts and the effective cutting of the fruit within a very short time span.

C. The wall of a dam is broad at its base.
Answer : The wall of a dam is broad at its base. The reason behind this is, pressure exerted by the liquid increases with increase in depth. The dam contains huge amount of water and as we go deep the pressure exerted by such a huge amount of water increases to very high.

D. If a stationary bus suddenly speeds up, passengers are thrown in the backward direction.
Answer : if a stationary bus suddenly speeds up passengers are thrown backward direction and it happens due to the Newton's Motion Law. When a stationary bus moves suddenly the inertia acts upon a person and he will be thrown out of the bus directly.

Question 6: The density of a metal is 10.8×103 kg/m310.8×103 kg/m3. Find the relative density of the metal.
Answer :
Given:
Density of metal = 10.8×103 kg/m310.8×103 kg/m3

We know, density of water = 1000 kg/m3
Relative density of substance =Density of substance / Density of water / Relative density of substance =Density of substance/Density of water
Relative density of substance =10.8×1031000=10.8

Question 7: Volume of an object is 20 cm3 and the mass in 50 g. Density of water is 1 g cm−-3. Will the object float on water or sink in water?
Answer :
Given:
Volume of object = 20 cm3
Mass of object = 50 g
Density of object = Mass of object / Volume of object=5020=2.5 g/cm3 Mass of object Volume of object=5020=2.5 g/cm3

Now, we know density of water = 1 g/cm3
Since, density of object > density of water, therefore the object is going to sink in water.

Question 8: The volume of a plastic covered sealed box is 350 cm3 and the box has a mass 500 g. Will the box float on water or sink in water? what will be the mass of water displaced by the box?
Answer :
Given:
Volume of box = 350 cm3
Mass of box = 500 g
Density of object = Mass of object / Volume of object=500350=1.43 g/cm3
Mass of object/Volume of object=500350=1.43 g/cm3

Now, we know density of water = 1 g/cm3
Since, density of box > density of water, therefore the object is going to sink in water.
Now, volume of liquid displaced = Volume of the object = 350 cm3
⇒Mass of liquid displaced / Density of liquid displaced= 350 cm3
⇒Mass of liquid=350×1=350 g
⇒Mass of liquid displaced / Density of liquid displaced= 350 cm3
⇒Mass of liquid=350×1=350 g

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