# Revision Notes | MOTION IN A STRAIGHT LINE | part 1

XI PHYSICS
Revision Notes
CHAPTER 3
MOTION IN A STRAIGHT LINE

1. An object is said to be in motion if its position changes with time. The position of the object
can be specified with reference to a conveniently chosen origin. For motion in a straight line,
position to the right of the origin is taken as positive and to the left as negative.

2. Path length is defined as the total length of the path traversed by an object.

3. Displacement is the change in position: . Path length is greater or equal to
the magnitude of the displacement between the two points.

4. An object is said to be in uniform motion in a straight line if its displacement is equal
in equal intervals of time. Otherwise, the motion is said to be non-uniform.

5. Average velocity is the net displacement divided by the time interval in which the
displacement occurs: . It is a vector quantity and its SI unit is m/s.
On an x-t graph, the average velocity over a time interval is the slope of the line connecting
the initial and final positions corresponding to that interval.

6. Average Speed is the ratio of total path length traversed and the corresponding time
interval. It is a scalar quantity and its SI unit is same (m/s) as that of velocity.
The average speed of an object may be greater or equal to the magnitude of the average
velocity over a given time interval.