General Science Solutions Solutions for Class 8 Science Chapter 3 - Force And Pressure

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.

Answer:

Answer:

(a) The plastic cube is going to float on the surface of water as its density is less than that of water.

(b) We know, $\mathrm{Pressure}=\frac{\mathrm{Force}}{\mathrm{Area}}$
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) 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. 

Answer:

A. 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. 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. 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. 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.

Answer:

We know, $\mathrm{Density}=\frac{\mathrm{Mass}}{\mathrm{Volume}}$
So, using above formula, we can find one quantity if other two quantities are given.

Answer:

Given:
Density of metal = $10.8\times {10}^{3 }\mathrm{kg}/{\mathrm{m}}^3$
We know, density of water = 1000 kg/m³
$\mathrm{Relative} \mathrm{density} \mathrm{of} \mathrm{substance} =\frac{\mathrm{Density} \mathrm{of} \mathrm{substance}}{\mathrm{Density} \mathrm{of} \mathrm{water}}$
$\mathrm{Relative} \mathrm{density} \mathrm{of} \mathrm{substance} =\frac{10.8\times {10}^3}{1000}=10.8$

Answer:

Given:
Volume of object = 20 cm³
Mass of object = 50 g
Density of object = $\frac{\mathrm{Mass} \mathrm{of} \mathrm{object}}{\mathrm{Volume} \mathrm{of} \mathrm{object}}=\frac{50}{20}=2.5 \mathrm{g}/{\mathrm{cm}}^3$
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.

Answer:

Given:
Volume of box = 350 cm³
Mass of box = 500 g
Density of object = $\frac{\mathrm{Mass} \mathrm{of} \mathrm{object}}{\mathrm{Volume} \mathrm{of} \mathrm{object}}=\frac{500}{350}=1.43 \mathrm{g}/{\mathrm{cm}}^3$
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 cm^{3
$$\begin{gathered} \Rightarrow \frac{\mathrm{Mass} \mathrm{of} \mathrm{liquid} \mathrm{displaced}}{\mathrm{Density} \mathrm{of} \mathrm{liquid} \mathrm{displaced}}= 350 {\mathrm{cm}}^3 \\ \Rightarrow \mathrm{Mass} \mathrm{of} \mathrm{liquid}=350\times 1=350 \mathrm{g} \end{gathered}$$}