Page 167 - Science Class 07

Digital Clock
v A stopwatch is used to know the me taken
Pendulum
v The quartz clock was invented in 1926 to
know me more accurately. These clocks
work on the basis of the vibra on of
certain crystals of a substance. These
crystals are also called quartz and the
watch in which these quartz are used are
called quartz watches. These watches and
clocks are widely in use. They can give me
in frac on of second because the quartz
used in such watches vibrate very fast and
with a very precise rate.
Quartz Clock
v Digital clocks or watches are the most
advanced. They display me in digits. They
do not have hour, minute or second hands.
Now a days, most of the scien fic equip-
ment consists of digital watches. You can
see digital watches a ached with
computer, VCD player, digital cameras.
These watches are also used in air cra s.
Science-7
in comple ng a certain piece of work. It
also displayed me in digits. It has a start
and a stop bu on. Suppose, you want to
know how much me you take to complete
100 meters race. For this, you have to push
start bu on when you start running and
you have to push stop bu on just when you
touch the 100 metres marked point. It will
display the me taken by you in comple ng
the 100 metre race. Stopwatches usually
read me up to one-tenth of a second.
However, scien sts have developed some
stop watches which can measure me up to
one-hundredth of a second.
Stopwatch
1. How did the ancient people know about
me?
2. On what principle does the sundial work?
3. Name any three modern me
measurement devices.
4. How many seconds are there in one day?
5. What is the use of stopwatch?
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