Solutions-Class 8-Science-Chapter-3-Force and Pressure-Maharashtra Board

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.

The plastic cube will come to the surface of water because its density is less than that of water. When it floats unbalanced force acting on it is zero.

We know, Pressure=Force / Area

Greater the area of contact between two surfaces, lesser will be the pressure.

The Load carrying heavy vehicles have large number of wheels so that the load (weight, force) is distributed over large surface area of the wheels in contact with the road and ensures that the force due to the load is shared among the tyres and no single tyre is under stress.

Hence, the pressure decreases and the tyres do not burst.

We carry atmospheric pressure of about 10⁵ Pa on our heads. We don't feel it because the cavities in our body filled with air and arteries with blood, their pressure balanced by atmospheric pressure and the pressure created by the blood inside our body. 

The density of fresh water is less than marine 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.

The effect of given force varies inversely as the area of the surface on which force is applied. 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.

We know Pressure of a point in a liquid is proportional to the height of the liquid column above it, hence pressure of liquid increases with depth and pressure is greater at the bottom than the top. 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.

 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.

Using formula - Density=Mass / Volume

Using formula - Relative density of substance=Density of substance/ Density of water

Using formula - Pressure=Force/Area

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

We know, density of water = 10³ kg/m³

Relative density of substance = \(\frac{\text{Density of substance}}{\text{Density of water}}\)

Relative density of substance = \(\frac{10.8×10^3}{10^3}\) =10.8

Relative density of Metal = 10.8

Given:
Volume of object (V) = 20 cm³  , Mass of object (m) = 50 g

Density (ρ) = \(\frac{Mass}{Volume} =\frac{50}{20}\) =2.5 g/cm³

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

Since, density of object is greater than density of water, therefore the object is going to sink in water.

Given:
Volume of box = 350 cm³  , Mass of box = 500 g

Density of object = \(\frac{\text{Mass of object}}{\text{Volume of object}}=\frac{500}{350}\) =1.43 g/cm³

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

Since, density of box is greater than density of water, therefore the object is going to sink in water.

Now, volume of water displaced by the box = Volume of the box = 350 cm³

⇒Mass of water displaced = Density of water displaced x Volume of box

∴ Mass of water (m) = 1 g.cm³ x 350 cm³ = 350g

Mass of water displaced by box is 350g