Scientific Reasons From Past Papers 18

➽ It is easier to swim in sea water than in river water. Explain.
Answer: The density of sea water is more than that of the river water. Therefore, the weight of sea water displaced by the swimmer is more. Thus, buoyant force of upthrust on the swimmer increases, making it easier to swim in sea water.

➽ A cold compress is applied on the forehead of a person suffering from high fever. Why?
Answer: Evaporation causes cooling. As the water evaporates, it absorbs heat from the forehead and helps in reducing the temperature.

➽ A man with a load his head jumps from a high building. What will be the load experienced by him.
Answer: Zero, because the acceleration of his fall is equal to the acceleration due to gravity of the earth.

➽ Why is spring made of steel and not copper?
Answer: The elasticity of steel is greater than that of copper.

➽ Why is it easier to spray water to which soap is added?
Answer: Addition of soap decreases the surface tension of water. The energy for spray is directly proportional to surface tension.

➽ A piece of chalk when immersed in water, emits bubbles. Why?
Answer: Chalk consists of pores forming capillaries. When it is immersed in water, the water begins to rise in the capillaries and air present there is expelled in the form of bubbles.
Why does a liquid remain hot or cold for a long time inside a thermo flask?
Answer: Because of the presence of air, which is poor conductor of heat, in between the double glass walls of a thermos flask.

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