Page 168 - Science Class 07

REST AND MOTION Example : A train cover 560 km in 7 hours.

Rest and mo on are defined on the basis of the What will be its speed?
change of the posi on of an object with me. Distance covered = 560 km
Rest Time taken = 7 hours

An object is said to be at rest if its posi on Speed = Distance = 560 = 80 km/hour

does not change with me and with respect to Time 7
the observer. How to convert the unit of speed?
Mo on Example : A car is moving with a speed of 54
km/hour. What will be its speed in
Mo on can be defined as the con nuous
change of posi on of a body with respect to metre/second or m/s?
the observer. Convert 54 km in metre and 1 hour into
second.
Let’s understand it with an example:
A person is standing on a pla orm and a train 54 km = 54 × 1000 m
moves on the track. Then, the people si ng in = 54000 m
the train are said to be in mo on. But, if the 1 hours = 60 minutes
person is also si ng in the same train the 60 × 60 second = 3600 second
people are at rest with respect to that person. Distance 54000
Speed = = = 15 m/s
SLOW AND FAST MOTION Time 3600
You might have seen various vehicles running Example : A bus is moving with a speed to
on the road. Some move very fast while some 20 m/s. What will be its speed in km per hour?
move slowly. A car moves faster than an Convert 20 m into kilometer and 1 second into
autorikshaw. It is our observa on. But, to hour.
represent our observa on on paper or to 20
compare which one is faster and by how much, 20 m = km
1000
we have a method. According to this method, 1
we find the distance covered by the two bodies 1 second = hour
3600
and the me taken. The distance covered by a 20 3600
body in a unit me is called its speed. Speed = × = 72 km/hour
1
1000
Let’s understand speed: Finding Distance
Speed Example : If a cyclist is moving with a speed of

Different moving objects cover different 18 km/ hour. How much distance he will cover
distances and in different mes. But, if we find in 20 minutes?
out the distance covered by them in a unit Here, me is given in minutes, so we have to
me, we will be able to know which one is fast convert 18 km /h into metre/minute.
and by how much. The distance covered by an Distance = 18 km
object in a unit me is called its speed. So, to
compare two moving objects we should first = 18000 m
know their speed. Time = 1 hour

–1
The unit of speed is metre/second or ms . = 60 minutes
168 Science-7