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Class 8 MAHARASHTRA STATE TEXTBOOK BUREAU Solutions Science Chapter 3: Force and Pressure

Force and Pressure Exercise Exercise

Solution 1

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.

Solution 2

Solution 3

a. The plastic cube will float on the surface of the water as its density is less than that of water.

Pressure = Force/Area

b. So, greater the area of contact between two surfaces, lesser will be the pressure. So, load - carrying heavy vehicles have large number of wheels so that pressure on the road is reduced due to larger contact area. Also, larger number of wheels ensure that 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 air pressure gets balanced by the pressure which is created by air and blood inside our body.

Solution 4

a. The density of marine water is more than the freshwater. Thus, buoyant force on the ship in marine water is more than the freshwater.

b. Sharp edge knife makes less contact with fruits to be cut, thus, the pressure exerted by it on fruits is greater. Due to this, fruits get easily cut by sharp knife.

c. The pressure of liquid increases with depth. Thus, wall of dam is broader at its base so as to withstand the heavy pressure of water.

d. When bus suddenly speeds up, passengers get backward jerk. This happens because the whole body of passenger inside bus was in state of rest. When bus suddenly starts or speeds up, the lower half of passengers' body comes in motion in forward direction while the upper body is still in state of rest due to inertia of rest. Thus, if a stationary bus suddenly speeds up, passengers are thrown in the backward direction.

Solution 5

Density = Mass/Volume

 

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

 

Pressure = Force/Area

Solution 6

Density of metal = 10.8 x 103 kg/m3

Density of water = 103 kg/m3

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

= 10.8 x 103/103 = 10.8

Solution 7

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

Density of water = 1 g/cm3

Density of object > density of water

Thus, the object is going to sink in water.

Solution 8

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

Density of water = 1 g/cm3

Density of box> density of water

Thus, object is going to sink in water

Now, volume of liquid displaced = volume of object = 350 cm3

Mass of liquid displaced/ density of liquid displaced = 350 cm3

Mass of liquid = 350 x 1 = 350 g