YADNYA e-Learning
General Science - 8th Standard
3. Force and Pressure
Question & Answer
1. Write proper word in the blank space.
a. The SI unit of force is Newton
(Dyne, Newton, Joule)
b. The air pressure on our body is equal to atmospheric pressure.
(Atmospheric, Sea bottom, Space)
c. For a given object, the buoyant force in liquids of different density is different.
(the same, density, different, area)
d. The SI unit of pressure is N/m2
(N/m3 , N/m2 , kg/m2 , Pa/m2)
2. Make a match.
A group | B group (Ans) |
Fluid | Same pressure in all directions |
Blunt knife | Lower pressure |
Sharp needle | Higher pressure |
Relative density | Specific gravity |
Hecto Pascal | 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?
Ans: 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?
Ans: We know,
Pressure= | Force |
Area |
So, greater the area of contact between two surfaces, lesser will be the pressure. So, the load carrying heavy vehicles have large number of wheels so that the pressure on the road is reduced due to larger contact area. Also, using large number of wheels ensures that the force due to the load is shared among the tyres and no single tyre is under stress.
c. How much pressure do we carry on our heads? Why don’t we feel it?
Ans: We carry atmospheric pressure of about 105 Pa on our heads. We don't feel it because this atmospheric pressure is balanced by the pressure created by the air and blood inside our body.
4. Why does it happen?
a. A ship dips to a larger depth in fresh water as compared to marine water.
Ans: The density of marine water is more than the fresh water due to which the buoyant force on the ship in marine water is more than the fresh water. Hence, the ship dips to a larger depth in fresh water as compared to marine water.
b. Fruits can easily be cut with a sharp knife.
Ans: Since the sharp edged knife makes lesser contact with the fruits to be cut, thus the pressure exerted by it on the fruit is very large. Because of this large pressure, fruits are easily cut with a sharp knife.
c. The wall of a dam is broad at its base.
Ans: We know pressure of liquid increases with depth. So, the wall of a dam is made broader at its base so that it is able to withstand the heavy pressure exerted by the river water.
d. If a stationary bus suddenly speeds up, passengers are thrown in the backward direction.
Ans: If a stationary bus suddenly speeds up, passengers are thrown in the backward direction. This is because initially the whole body of a passenger inside the bus was in the state of rest. But, when the bus suddenly starts or speeds up, the lower half of the passenger's body comes in motion in the forward direction but the upper half still remains at rest due to inertia of rest. Hence, the passengers are thrown backward when a stationary bus suddenly speeds up.
4. Complete the following tables.
Ans:
We know,
Density= | Mass |
Volume |
So, using above formula, we can find one quantity if other two quantities are given.
We know,
Relative Density of substance = | Density of substance |
Density of Water |
So, using above formula, we can find one quantity if other two quantities are given.
We know,
Pressure= | Force |
Area |
So, using above formula, we can find one quantity if other two quantities are given.
6. The density of a metal is 10.8 x 103 kg/ m3 . Find the relative density of the metal.
Ans:
Given:
Density of metal = 10.8×103 kg/m3
We know, density of water = 1000 kg/m3
7. Volume of an object is 20 cm3 and the mass in 50 g. Density of water is 1 gcm-3. Will the object float on water or sink in water?
Ans:
Given:
Volume of object = 20 cm3
Mass of object = 50 g
Since, density of object > density of water, therefore the object is going to sink in water.
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?
Ans: Given:
Volume of box = 350 cm3
Mass of box = 500 g
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 | = 350 cm3 |
Density of liquid displaced |
Mass of liquid= | 350 x 1 = 350 g |
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