Block 1
Concepts in MechanicsThe DISTANCE travelled by an object in a given time interval is THE TOTAL LENGTH OF THE ACTUAL PATH it covers during that time.
DISPLACEMENT of an object in a given time interval from ti to tf refers to its CHANGE IN THE POSITION during that time. It includes the direction of motion.
Displacement = final position of the object (at time tf ) − initial position of the object (at time ti).
You have learnt that the position of a moving object is a function of time. Let us now explain how to describe how its position changes with time.
1.3.2 Average Speed and Average Velocity
Study the position-time graph of a bus moving on a straight road (Fig. 1 .5).
t=0s2s4s6s8s
x = 0 m 20 m 40 m 60 m 80 m
Fig. 1.5: The graph showing the positions of a moving bus at different instants of time with respect to the point x = 0, which is the origin.
Suppose you want to know: How fast is the bus moving? You can find the answer to this question by calculating the distance travelled by the bus in 1 second or in 1 hour. Try doing so!
You know the word “speed” very well as you use it in your daily life. You know that the speed of an aeroplane is much more than the speed of a train. The speed of a bus is much more than the speed of a cyclist. A fast moving object has high speed; a slow moving object has low speed. In Fig. 1.5, the bus moves 80 m in 8 s. So it moves 10 m in 1 second or 36 km in 1 hour. This brings us to the concept of average speed. You may ask: How do we define AVERAGE SPEED in physics?
There is another term you need to know and that is AVERAGE VELOCITY. Let us define these terms.