In a frame of reference, a body is said to be in motion if, with respect to another body in rest, the position of the first body changes with respect to time. In mathematics and relative physics, motion is expressed in terms of displacement, distance, velocity, acceleration, speed, and time. By assigning a frame of reference to a stationary observer, and measuring the change in position of an object, the motion of that object can be expressed in both magnitude and direction considering the time taken in that frame of reference. An appropriate frame of reference is important because if the observer to which the frame of reference is assigned is also in motion in the same direction, the relative motion with respect to the original body can be mistaken to be zero or lead to inaccurate observations. The opposite of motion, rest, is when an object does not change its position with respect to time or remains in its position of equilibrium in the frame of reference. There is no absolute frame of reference, owing to the reason that the universe is in constant motion, and therefore, theoretically, an object can never be at rest.
The motion of sub-atomic particles like electrons, protons, etc., is given by quantum mechanics, whereas the motion of everything else is governed by classical mechanics. Sir Isaac Newton, in 1686, presented the three laws of motion which formed the foundation of classical mechanics and has allowed for major strides in the field.
Uniform motion is observed when an object covers a particular distance with a constant velocity throughout the duration of its motion. This means that if the velocity value of the body is measured in one instant of time, and another value of velocity is taken at another instant, the values of velocity will always be equal. Because the body has constant velocity, the acceleration of the body is zero. Let us understand uniform motion using an example. If a car covers 10 km over a duration of 1 hour and it is said to be in uniform motion, this would mean that at any given instant, irrespective of how long the measured instant is, the speed will always be equal to 10 km/hr. In the case of uniform motion, the average speed and instantaneous speed is equal in value.
Non-uniform motion is when an object in motion covers different lengths of distances in equal intervals of time, meaning that the value of velocity measured at one instant of time is highly unlikely to be the same as the value of velocity measured at another instant of time. Since, in this case, the velocity differs with time, the body is said to possess acceleration. This acceleration can either be positive or negative because the body can speed up or slow down irrespective of the time. So the acceleration is said to be non-zero. Non-uniform motion is also called accelerated motion. However, this is not the same as uniform acceleration. For example, in the game of cricket, when a bowler throws a ball, from the moment the ball is released from the hand of the bowler, the ball is accelerated throughout its motion until it is stopped momentarily and accelerated in the other direction by the batsman. Here, the velocity of the ball is not the same at any two measured instants.
Let us distinguish uniform and non-uniform motion to have a much clearer understanding.
Uniform motion | Non-uniform motion |
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Acceleration is zero. | Acceleration is non-zero. |
Average speed and instantaneous speed values are the same. | Average speed and instantaneous speed have different values. |
The distance-speed graph is a straight line. | The distance-speed graph is a curved line. |
E.g., Hands of a clock. | E.g., An object in free fall. |