Rest and Motion are two important terms of physics usually taught in class IX but drills down to several important concepts core to graduation and engineering.
Define rest and motion in terms of physics
If a body does not change its position with respect to time, the body is said to be at rest.
On the other hand, if a body changes its position with respect to time, it is said to be in motion.
Rest and motion are relative term
Rest and motion depend upon the position of the observer and the time elapsed. To an observer sitting inside the compartment of a running train, the doors, windows, benches and other passenger sitting next to the observers, appears to be at rest. However when the person looks outside of the window, trees, bridges and houses appear to be in motion. This observer is also known as reference body.
However, to any person standing on a station platform, it is just the opposite. The train along with benches, windows, passenger appears to be in motion while trees and everything fixed to the platform is at rest.
Now imagine two trains running at same speed in the same direction on two parallel tracks, side by side, and two persons are sitting in each of the train just beside the adjacent window. To these persons, the other train appears to be at rest including whoever is inside.
To an observer in space, the earth along with everything including house, trees are in motion. So motion is relative. Also we can identify a motion only when there is a time gap however small it may be. But if we take a particular instance, and compare it with same instance, then there is no change in position. So everything appears to be at rest.
Motion has both magnitude and direction and hence it is a vector quantity.
What is reference frame?
To locate the position of a body relative to the reference body, a system of co-ordinates fixed on the reference body is constructed. This is known as reference frame.
What is displacement?
The change of position of a moving body in a particular direction is called the displacement of the body and is measured by the linear distance between the initial and final positions of the body.
Let a particle moves from a position A to another position B. The particle can travel along a straight line AB (dotted) or any other curved path (solid line). Whatever path it may take, displacement is always the linear shortest path AB (dotted line in the diagram) with direction from A to B.
If we start from A go to B and come back, then even through have travelled twice the distance, our displacement is zero. The unit of displacement in CGS system is centimeter and in SI system is meter. In FPS system, it is foot.
Displacement has both magnitude and direction and hence it is a vector quantity.
What is speed?
The rate of change of position of a body with respect to time is called speed.
In other word, the distance travelled by a body in unit time is called its speed. In case of speed, we are not bothered about direction. Speed remains same irrespective of whether the body is travelling from A to B or B to A.
In C.G.S. and S.I., the units of speed are respectively expressed as cm/s and m/s.
Now if we consider 2 trains travelling in opposite direction, covering equal distance in same interval of time, then both of them have same speed.
Speed has magnitude but no direction. Hence it is a scalar quantity.
What is uniform speed?
When a particle in motion, moves equal distance in any equal interval of time, then the speed of the particle is called uniform speed.
What is variable speed?
When a particle in motion moves unequal distance in any equal interval of time, then the speed of the particle is called variable speed.
What is average speed?
Average speed of a body is obtained by dividing the total distance traversed by the corresponding total time taken by the body. It is applicable for both uniform and variable speed.
If a car travels a total distance s in total time t, in whatever direction it may be, then
Average Speed = Distance traversed/Time taken=s/t
Or, Distance traversed = Average Speed x Time taken
What is velocity?
The rate of change of position of a body in a particular direction with respect to time is called the velocity of the body.
Contrary to speed, this is not considering the distance travelled, but considers the displacement.
Hence velocity has both magnitude and direction and is a vector quantity.
The unit of velocity is same as the unit of speed, however the value may differ and direction must be mentioned in case of velocity. Like speed, velocity can also be uniform or variable.
What is uniform velocity?
When a particle in motion, moves equal distance in any equal interval of time in same direction, then the velocity of the particle is called uniform velocity.
What is variable velocity?
When in any equal interval of time, a particle moves unequal distance or in different direction or both distance and direction varies, then the velocity of the particle is called variable velocity.
In other words, if magnitude of displacement or direction or both varies then velocity is variable.