class 9 physics note: motion part 1

An object changes its position with respect to a reference point with elapse of time, the object is said to be in motion.

Rest:-

When an object does not change its position with respect to a reference point with elapse of time, the object is said to be in rest.

Example – When a vehicle changes its position with respect to an electric pole (a reference point) with elapse of time, then vehicle is called in motion. And if the same vehicle does not change its position with respect to that electric pole, the vehicle is called in the state of rest.

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Thus to observe the motion of any object; two things are necessary – a reference point and time.

In the above example; vehicle is the object and electric pole is the reference point. A building, tree, or any other static thing can be taken as reference point to observe the motion of an object.

Motion along a straight line:

When an object moves along a straight line, the motion of the object is called rectilinear motion. For example; motion of a vehicle along a straight road.

Distance and Displacement:

Distance is the length of path covered by a moving object in the given time irrespective of direction. Distance has only magnitude and no direction.

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SI unit of distance is meter (m).

Kilometer is used to measure bigger distance and it is abbreviated as ‘km’.

Displacement is the shortest possible distance covered by a moving object from initial point in a particular direction. In other words, shortest distance between initial point and final point is called the displacement.

Displacement has both magnitude and direction while distance has only magnitude.

Suppose, a ball is rolling along a straight line.

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Case : 1

Suppose, the ball starts moving from point A and reaches at point B.

Thus, distance covered by ball = 10 m

Displacement of ball = 10 m towards west.

Case : 2

Suppose, ball starts moving from point A and reaches to B. Again it returns on the same path from point B and reaches at A.

Thus, distance covered by the ball = distance from A to B + Distance from B to A

= 10 m + 10 m = 20 m

In this condition, distance covered by ball = 20 m.

Since, ball returns at point A, thus displacement of the ball = 0

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Case : 3

Suppose, the ball starts moving from point A, reaches point B and returns back to point C.

Then, the distance covered by ball = distance from A to B + Distance from B to C

= 10 m + 7 m = 17 m

Displacement of ball = Distance of point C from A = 3 m towards west.