Motion and Time for class 7

Motion and Time 

SYSTEMS OF UNITS

There are seven basic units. In class 7 we will use 5 basic units but for knowledge sake we will see all the basic units.

QUANTITY	UNIT	SYMBOL
TIME	second	s
LENGTH	meter	m
MASS	kilogram	kg
TEMPERATURE	kelvin	K
ELECTRIC CURRENT	ampere	A
LUMINOUS INTENSITY	candela	cd
AMOUNT OF SUBSTANCE	mole	mol

These units are called the “SI” units. To express quantities which are way greater or smaller than the defined SI units we use prefix before the units. For example radius of earth is 6400000 m.  For convenience we use prefixes such as

Prefix	Factor
milli	1/1000
centi	1/100
deci	1/10
deca	10
hecto	100
kilo	1000

Thus if we write 1km it means 1000m.

1 km = 1000 m

Or,    1 = 1000 m/1 km

                                            

Example: The distance between Delhi and Mumbai is 1384 km. What is the distance in m?

CONVERSION OF UNITS

For converting time we use the following factors.

1 hour = 60 minutes

1 minute = 60 seconds

1 hour = 60 X 60 seconds

Conversely,

1 second =  1/60 minutes

1 minute = 1/60 hour

1 second = 1/60 x 1/60 hour = 1/3600 hour

Standard method of conversion from one unit to another unit:

First Method:

Suppose we have been give speed as 36 km/h. What will be speed in m/s?

1 km = 1000 m

So, 1 = 1000m/1km

Further 1 h = 60x60 s = 3600 s

So, 1 = 1 h/ 3600 s

Hence 36 km/h = 36 km/h x 1000m/1 km x 1h/3600 s = 10 m/s

Practice this method for other examples.

A few questions for practice:

1.   36,000 km/h = ....... km/s
2.   7.2 km/h = ............... m/s
3.   54/90 km/minutes = ................... m/s 
4.   1/10 km/minutes = ..................... km/h 
5.  0.15 km/minutes = ..................... km/h

Second Method:

Suppose we have been give speed as 36 km/h. What will be speed in m/s?

36 km/h = 36x1000 m/60x60 s =10 m/s

Practicing the first method will be helpful in more complicated problems involving different units. Anyway, practice in whatever method you are comfortable.

Types of Motion:

Motion is a change of position of an object with respect to time. Motion can be classified as per the path undertaken by the object.

1   

         Straight line motion: If motion of the object is in a straight line, it is straight line motion. Example: Motion of a train on a straight bridge; Motion of a horse pulling a cart on a straight road.

2    Circular motion: If motion of an object is in a circle, it is circular motion. Example: Motion of a child in a merry-go-round,

3    Oscillatory motion: back and forth motion is oscillatory motion. Example: Motion of a child on a see-saw; Motion of the hammer of an electric bell

Distance-Time Graph:

Distance – Time Graph says about the speed of an object. This grah will tell whether object is stationary, or moving with constant speed or speed is not constant. Our objective should be able to understand these graphs very well. We have to read the graph, and tell nature of speed instantly.

Example-1

Distance (m)	0	2	4	6	8
Time (s)	0	1	2	3	4

This is the Distance Time Graph for the tabulated data. Look at the time interval and corresponding distance. In the first second, the distance travelled is 2 m. Between 1 s and 2 s, the distance travelled is again 2 m (4 m – 2 m). Likewise for all the intervals, distance travelled is equal. This type of graph says that the speed is constant or uniform.

Example-2

Distance (m)	10	12	14	16	18
Time (s)	0	1	2	3	4

Again this is an example of constant speed. See the time intervals 0-1 s, 1-2 s, 2-3 s, 3-4 s and corresponding distance traveled in every interval. It is 2m for every interval.

Example 3:

Distance (m)	8	8	8	8	8
Time (s)	0	1	2	3	4

This graph is an example of stationary object i.e. no speed. See at any time, the distance is the same.

Example 4:

Distance (m)	0	1	4	9	16
Time (s)	0	1	2	3	4

This is an example of changing speed. During 0-1 s, distance travelled is 1m. During 1-2 s, distance traversed is 3 m; during 2-3 s, distance traversed is 5 m; and during 3-4 s distance travelled is 7 m. As the distance is changing for every interval of time, this is an example of changing speed, i.e speed is not constant.

Now let us see what is speed? Speed is distance per unit time.

speed = distance/time

In all graphs above, distance has been plotted on y-axis, whereas time has been plotted on x-axis. This convention or way of plotting is more useful as it gives us value of speed directly. Learn to read the graph.

Note: It is important and necessary to read text book and solve the problems. Only this way one can have clear understanding of the subject. Here I have tried to explain concepts that will be helpful in reading and understanding the concept. Try to solve NCERT book problems including Exemplar problems. The next lecture will deal with problems from these books.