Concept of Rest and Motion Physics Notes
Concept of Rest and Motion:
Rest: An object is said to be at rest if it does not change its position with respect to time, with respect to its surrounding. For example, mountain, house, tree etc. are in rest position with respect to the person standing on earth. A book lying on a table or a person sitting on a chair are also examples of rest.
*Motion: An obect is said to be in motion if it changes its position w.r.t its surroundings with the passage of time.
For example: motion of planets around the sun, motion of a train, motion of the gas particle etc.
Rest and Motion:
- Motion is a combined property of an object under study and the observer.
- Everything in the world is at rest or in motion.
- There is no meaning of rest and motion without a viewer.
Consider a scenario, you are traveling in a car, seated with your friend in the back seat. Your friend is quietly minding his own business, and according to you, your friend is at rest since his position does not change with time. But for a pedestrian standing beside the road and seeing the cargo by, your friend (as well as you) are in motion since both of you are changing position as far as the pedestrian is concerned.
So, at any point of time, an object can be at rest with respect to a second object, while being in motion with respect to a third. In other words, motion is always relative, never absolute.
It can also be said that motion and rest are relative terms.
Types of Motion:
There are two types of motion:
- On the basis of coordinate system
- On the basis of the nature path of moving particle.
1. On the basis of the co-ordinate system: On the basis of a co-ordinate system, motion can be divided in the following way:
(a) One-dimensional motion or 1-D motion:
The motion of an object is called one dimensional if only one of the three-co-ordinates are required to specify the position of the object in space changes w.r.t. time.
In 1-D motion, the object moves along a straight line.In 1-D motion there are two directions (backward and forward, upward and downward. In these directions the object moves and these directions are specified by +ve and -ve signs. For example, A body running on a straight road, a body thrown upwards, motion of train along a straight railway track etc.
(b) Two-dimensional motion or 2-D motion:
The motion of an object is called two-dimensional if two of the three co-ordinates required to specify the position of the object in space change w.r.t. time or if two co-ordinates are required to specify the position of the object. In 2-D motion the object moves in a plane.
For example: A billiard ball moving over the billiard table, a satellite revolving around the earth, projectile motion, an insect crawling over the floor are two-dimensional motion.
Suppose an object is moving in X-Y plane with origin ‘O’ (figure). At time t, let the object be at P, it’s coordinates are (x, y). It means to know the position of the object, we are required to know two coordinates i.e., distance along Y-axis and Y-axis.
(c) Three-dimensional motion or 3-D motion: The motion of an object is called 3-D. If all the three co-ordinates are required to specify the position Of the object in space changes w.r.t. time. Such a motion is not restricted to a straight line or a plane but takes place in space. It is the most general form of the motion. For example. Motion of flying kite, the random motion of gas-particle, a flying bird, a flying aeroplane etc.
2. On the basis of nature of motion of the particle:
(a) Translatory motion: Translatory motion is the motion by which a body shifts from one point in space to another. One example of translatory motion is motion of a bullet fired from a gun. Another example: horizontal motion of a body, a car moving in a straight line or a train moving along a straight line are some examples of translatory motion.
(b) Rotational motion: When a body rotates around a fixed axis, then it is called rotational motion. Rotational motion deals only with the rigid bodies. Rigid body is an object which can not change the position, shape and size under the influence of external force.
For example Motion of ceiling fan, a string whirled in a circular loop, the motion of a wheel about its axis are the examples of rotatory motion.
(c) Oscillatory or vibrational motion:
The motion in which a particle moves to and fro motion about a given point is known as oscillatory or vibrational motion. For example: Motion of a simple pendulum, motion of a mass attached to a spring, etc.