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# Physics Theory : Motion

 Preparation Just what you need to know !

Motion is the term used for a body that changes its position with respect to its surroundings.
Rest is the term used for a body that does not change its position with respect to its surroundings.
Motion and rest are relative terms.
For example, a passenger sitting in a train is in motion with respect to the surroundings outside the train (houses, trees, fields, etc.) but is at rest with respect to the surroundings inside the train (other sitting passengers, floor and roof of the train, etc.)

Speed is defined as the rate of change of position in terms of distance.
∴ Speed = Distance / Time
Thus, speed is simply the distance covered per unit time.
A body moving through equal distances in equal time intervals (whatsoever may be the length of the time intervals) is moving at a constant speed.

Velocity is defined as the rate of change of position in terms of displacement.
∴ Velocity = Displacement / Time
Thus, velocity is simply the displacement per unit time.
Velocity and speed both have the unit of length/time. Their SI unit is m/s (meter/second).

Acceleration is defined as the rate of change of velocity.
∴ Acceleration = Change in velocity / Time taken for that change
Thus, acceleration is simply the velocity change per unit time.
Acceleration has the units of length/time2. The SI unit of acceleration is m/s2.
The acceleration due to gravity on the Earth's surface is 9.81 m/s2, and decreases with increasing distance from the surface of the Earth.
Negative acceleration is called deceleration or retardation.

Scalar is a quantity with magnitude but no direction, e.g., distance, time, speed.

Vector is a quantity with magnitude as well as direction, e.g., displacement, velocity, acceleration.
If a body moves from point A to point B and returns to point A, then the distance covered is twice the gap between points A and B, but its displacement is zero.

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