1. Rest and Motion:

(i) Reference object is the object with respect to which the state of motion or state of rest of another object is described.

(ii) When a body does not change its position with respect to time and a reference point, it is said to be at rest with respect to that reference point.

(iii) When the position of a body continuously changes with respect to the time and a reference point it is said to be in a state of motion.

2. Distance and Displacement:

(i) Distance is the actual length of the path travelled by a moving body, irrespective of the direction in which the body moves. It is a scalar quantity.

(ii) The straight line distance between initial and final position. It has both magnitude and direction. It is a vector quantity.

(iii) The SI unit of both distance and displacement is metre.

3. Speed and Velocity:

(i) Speed is the distance travelled by a body in unit time. It is a scalar quantity and its SI unit is $m / s$ .

(ii) Velocity is the displacement covered by a body in a unit time in a specified direction. It is a vector quantity and its SI unit is $m / s$ .

(iii) If a body in motion covers equal distances in equal intervals of time, the body is said to have uniform speed. If it covers unequal distances in equal intervals of time, the body is said to have non-uniform speed.

(iv) Average Speed $= \frac{total distance travelled}{time taken to travel the distance}$

(v) A body has uniform velocity if it covers equal displacements in the same direction in equal intervals of time.

4. Acceleration:

(i) Acceleration is the rate of change of velocity. It has the unit $m / s^{2}$ .

(ii) Acceleration can be calculated as $a = \frac{change in velocity}{time} = \frac{(v - u)}{t}$ , where $v$ is the final velocity, $u$ is the initial velocity and $t$ is the time. It is a vector quantity.

(iii) Negative acceleration is known as retardation. The velocity of a retarding body decreases with time.

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Important Questions on Motion

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