Force And Pressure - Chapter Wise CBSE Solved Question and Answer Based On NCERT
Long Q&A:
Q1: Explain contact and non-contact forces. Give two examples for each.
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Ans:
Contact forces:
Forces which act only when there is physical contact between two interacting objects are known as Contact forces.
Example-Muscular force: This is the force we can exert with our bodies by using our muscles, e.g.push, pull etc.
Frictional force : The force acting against the relative motion of surfaces in contact is called frictional force or friction.
Non-contact forces:
Forces which can act without physical contact between objects, i.e. those that can act from a distance, are called non-contact forces or field forces.
Example-magnetic force: Magnets exert forces of attraction or repulsion on other magnets
Electrostatic force: The force exerted by a charged body on another charged or uncharged body is known as electrostatic force.
Q2:
How can friction be reduced
How can it be increased? Give examples.
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Ans:
Reducing Friction
By using wheels and ball bearings.Use of wheels between surface moving over each other reduces friction. Ball bearingshave small balls of steel between steel surfaces. Because of the balls the steel surfaces can easily moves over each other.
By making the rubbing surfaces smooth by polishing them.
By using a suitable lubricant, like oil (for light machinery) or grease (for heavy machinery). This helps because fluid friction is less than solid friction.
Friction due to air (air resistance) or water is reduced by using streamlined shapes inaeroplanes or ships. A streamlined shape is narrow in front and broader at the back. Birds and aquatic animals have streamlined shapes which held them in flying or swimming.
Increasing Friction
Sand and gravel is strewn on slippery ground during the rainy season to increase friction. It is them easier to walk on the ground.
By making the moving surfaces rough, e.g. tyres have designs and patterns with grooves on the surface to increase resistance with the road. This prevents slipping of the tyres on a wet road.
To increase friction, spikes are provided in the soles of shoes used by players and athletes.
Q3: Do liquid and gases exert pressure on the walls of container in all direction? Give example to justify your statement.
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Ans:
Liquid and gases exerts pressure on the walls of container. For example-If we take a plastic bottle and drill four holes around near the bottom of bottle at the same height. After filling the water in that bottle, we observe that water comes out of the holes and falls at the same distance. This shows that liquid exerts pressure on the walls of container in all directions.
Similarly, we are not able to inflate a balloon which has holes because air inside balloon exerts pressure in all direction.Hence we can say that gases exert pressure on the walls of container in all directions.
Q4: Why is it easy to push a nail into a wooden plank by the pointed end?
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Ans:
It is easy to push a nail into a wooden plank by pointed end because the smaller the area, larger the pressure on a surface for the same force. The area of the pointed end of the nail is much smaller than that of its head. The same force, therefore, produces a pressure sufficient to push the pointed end of the nail into the wooden plank.
