Page 73 - Science Class 06

than one piece, the knife lose it sharpness. Involvement of Energy
But remember that for the apple, the When we cut the apple using a knife, we

change is very fast while for the knife, it is use force. This force comes from the energy
very slow. of the body through the hand holding the
knife. Evidently, muscular energy is involved

in the change that occurred in the apple
and the knife.
Similarly, when we boil water, heat energy is

used up. As a result, water changes into
steam (vapour). Inversely, when steam
When you cut an apple its inner surface turns touches a cold surface it changes back to
into red as it comes in contact with air.
water-droplets a er losing its heat energy.
In other words, heat energy is given out.

1. Why is a wet cloth placed on the forehead of a person in running high fever?
2. Why does water put in an earthen pot gets cooler during the high summer season?

CONTROLLING THE CHANGES of seasons, etc. going on in nature are not in

We have studied the various types of our control. But scien sts are not ready to
changes going on around us almost every give up. They are trying to know more and
moment. We have also followed the causes more about the secrets behind these
that bring about these changes in their changes.
influences. We have been trying our best to

control more and more changes and use the
desirable ones for our good and also to do
away with or slow down those changes that

are undesirable.
But it is a harsh truth that some changes take
place beyond our control. Random changes

like natural disasters—earthquakes, floods
and landslides, etc. and also regular changes A storm has a great amount of energy
like the various cycles—day-night cycle, cycle which brings devasta ng changes.

Periodic : Something that happens after regular periods
Reversible : Any change that can be reversed to obtain the original position

Desirable : Anything that is worth desiring because it is useful to one and all

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