Measurement of Time

Our ancestors noticed that many events in nature repeat themselves after definite intervals of time. For example, they found that the sun rises everyday in the morning. The time between one sunrise and the next was called a day. Similarly, a month was measured from one new moon to the next. A year was fixed as the time taken by the earth to complete one revolution of the sun.

● The S.I. unit of time is second (s).

● Commonly used units/conversion factors for time

1 minute = 60 s

1 hour = 60 minutes

1 hour = 3600 s

1 decade = 10 years

1 millennium = 10 centuries

1.0Graphs in Motion

Motion graphs are an important tool used to show the relationships between position, speed, and time.

Distance-Time Graph

Imagine you are helping a friend who is training for a track meet. She wants to know if she is running at a constant speed. You mark the track every 50 meters. Then you measure her time at each mark as she runs.

Position and time data for a runner

Position (m)	0	50	100	150
Time (s)	0	10	20	30

Important point related to distance time graph

Horizontal lines mean the object is stationary (its distance from the starting point is not changing) [See figure (a)]

Straight, sloping lines mean the object is travelling at constant speed [See figure (b)]

The steeper the line, the faster the object is travelling [See figure (c)]

Distance-time graph is always positive; it is always increasing NEVER decreasing for moving body.

Distance-time graph is a curved line for a body having non-uniform motion.

2.0How to Graph the Data

To graph the data, put position on the vertical (y) axis and time on the horizontal (x) axis. Each row of the data table makes one point on the graph. Notice the graph goes over 10 seconds and up 50 meters between each point. This makes the points fall exactly in a straight line. The straight line tells you the runner moves the same distance during each equal time period. An object moving at a constant speed always creates a position vs. time graph that is a straight line.

This is a position vs. time graph because it tells you her position at different points in time. She is at 50 meters after 10 seconds, 100 meters after 20 seconds, and so on.

3.0Speed-Time Graph

Here, speed is taken on y-axis and time is taken on x-axis.

Speed-time graph is always positive, it can be increasing or decreasing.

Speed-time graph is always positive, it can be increasing or decreasing.

4.0Also Read

Scattering of Light	Relative Density	Power of Accommodation
Pressure	Buoyancy Force	Defects of Vision and Their Correction
Pascal’s law	Human Eye	Refraction of Light Through A Prism