# Class 8 Science Lesson 3 Force and Pressure answers

Std 8 Science Chapter 3 Force and Pressure solution

### Exercise

1. Write proper 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 )

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.

2: Make a match:

 A group B group 1. Fluid a. Higher pressure 2. Blunt knife b. Atmospheric pressure 3. Sharp needle c. Specific gravity 4. Relative density d. Lower pressure 5. Hecto Pascal e. Same pressure in all directions

 A group B group 1. Fluid a. Same pressure in all directions 2. Blunt knife b. Lower pressure 3. Sharp needle c. Higher pressure 4. Relative density d. Specific gravity 5. Hecto Pascal e. Atmospheric pressure

3. Answer the following questions in brief.

a. A plastic cube is released in water. Will it sink or come to the surface of water?

Answer: The plastic cube will come to the surface of the water. This is because the density of the plastic cube is less than that of water.

b. Why do the load carrying heavy vehicles have large number of wheels?

Answer: Pressure = Force / Area

The pressure on an object depends on the area on which the force is applied. If the area is greater the pressure is less. Therefore, the load carrying heavy vehicles have large number of wheels, so that the pressure is distributed among the tyres.

c. How much pressure do we carry on our heads? Why don’t we feel it?

Answer: At the sea level the atmospheric pressure 101 x 103 Pa. This is the air pressure we carry on our heads. However the cavities in our body are also filled with air and arteries and veins are filled with blood. Therefore we do not get crushed under water and due to atmospheric pressure, as the pressure is balanced.

4. Why does it happen?

a. A ship dips to a larger depth in fresh water as compared to marine water.

Answer: More the density of liquid, more is the force of buoyancy. The density of sea water is higher than the density of fresh water, due to salts dissolved in sea water. Hence a ship dips to a larger depth in fresh water as compared to marine water.

b. Fruits can easily be cut with a sharp knife.

Pressure = Force / Area

If area increases, pressure reduces for the same force, and if the area decreases, the pressure increases for the same force.

Since the sharp edge of the knife exerts a greater pressure at a smaller area, therefore fruits can be cut with a sharp knife.

c. The wall of a dam is broad at its base.

Answer: The pressure increases as the depth of the liquid increases.

The pressure of the water is greater at the bottom of the dam. In order to bear the heavy pressure the walls of the dam is broad at the base.

d. If a stationary bus suddenly speeds up, passengers are thrown in the backward direction.

Passengers sitting or standing in a stationary bus are in a state of rest. When the bus suddenly speeds up, the lower parts of their body in contact with the bus acquire the speed of the bus, while the upper parts of their body, continue to be in the state of rest due to inertia. Hence, if a stationary bus suddenly speeds up, passengers are thrown in the backward direction.

Question 5: Complete the following tables.

Density = Mass / Volume

 Mass (kg) Volume (m3) Density (kg/m3) 350 175 2 760 190 4

(i) Mass = 350 kg

Volume = 175 m3

Density = ?

Density = mass/volume

= 350 / 175

= 2 kg/m3

(ii) Volume = 190 m3

Density = 4 kg/ m3

Mass = ?

Density = mass/volume

Mass = Density x volume

= 4 x 190

= 760 kg

 Density of Metal (kg/m3) Density of water (kg/m3) Relative Density 5×103 103 5 8.5×103 103 8.5

(i) Density of water = 103 kg/m3

Relative Density = 5

Density of metal = ?

Relative density of substance = Density of substance / Density of water

Density of metal = Relative density of substance  x  Density of water

= 5 x 103 kg/103

(ii) Density of Metal = 8.5×103

Density of water = 103 kg/m3

Relative Density = ?

Relative density of substance = Density of substance / Density of water

Relative density of substance  = 8.5×103  /  103

= 8.5

 Weight (N) Area (m2) Pressure (Nm−2) 800 0.04 20,000 1500 500 3

(i) Area = 0.04 m2

Pressure = 20,0000 Nm-2

Weight = ?

Pressure = Force / Area

Force = Pressure x Area

= 20,000 x 0.04

= 800 N

∵ Force = Weight

∴  Weight = 800 N

(ii) Weight = 1500 N

Area = 500 m2

Pressure = ?

Pressure = Force / Area

∵ Force = Weight

∴ Pressure = Weight / Area

= 1500 / 500

3 N m2

Question 6: The density of a metal is 10.8×103 kg/m3. Find the relative density of the metal.

Given: Density of metal = 10.8×103 kg/m3

We have density of water = 1000 kg/m3

Relative density of substance =Density of substance / Density of water

Relative density of substance = 10.8×103 / 1000 = 10.8

Question 7: Volume of an object is 20 cm3 and the mass is 50 g. Density of water is 1 gcm-3. Will the object float on water or sink in water?

Given: Volume of object = 20 cm3

Mass of object = 50 g

Density of object = Mass of object / Volume of object = 50 / 20 = 2.5 g /cm3

Now, we know density of water = 1 g/cm3

Since, density of object > density of water, therefore, the object will 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?

Given: Volume of box = 350 cm3

Mass of box = 500 g

Density of object = Mass of object / Volume of object = 500 / 350 = 1.43 g/cm3

We know, density of water = 1 g/cm3

Since, density of box > density of water, therefore, the object will 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