Download PDF – Some important definitions related to motion | Scalar and Vector Quantities

Let us discuss some important definitions related to motion.

• Reference point:
It is the point around which the location of an object is measured. It is also known as the origin.

• Scalar quantity:
This physical quantity has its own magnitude but not specific direction. Example- distance, speed etc.

• Vector quantity :
This physical quantity has both magnitudes as well as specific direction. Example – displacement, velocity, etc.

• Distance :
1. The actual path or length covered by a body during its journey from the initial position to the final position.
2. It’s a scalar quantity
3. Ex – Rohan travelled 37 km.

• Displacement :
1. The shortest possible distance covered by a body during it’s journey from the initial position to the final position.
2. It’s a vector quantity.
3. Example – Rohan travelled 37 km in South-West direction.
4. It can be zero if the initial position and final position are the same. Ex – circular motion.

• Motion can be one dimensional, two dimensional and three dimensional on the basis of direction.

One dimensional – It is also termed as motion in a straight line. When an object moves in a straight line with respect to the observer via using only one coordinate out of three.
Example: the motion of the lift, walking in Narrow Street etc.

Two dimensional – When an object moves via using two out of the three coordinates is termed as two dimensional. For example the motion of a car, the motion of rolling ball etc.

Three dimensional – When a body moves via using all the three coordinates and even movement in any direction is termed as three-dimensional motion. For example a flying kite, a moving aeroplane etc.

• Uniform motion:

When a body travels equal distance in equal interval of time .

• Non uniform motion:

When a body travels unequal distance in equal interval of time.

• Speed :

1. Measurements of distance travelled per unit time .
2. Speed – distance travelled / time taken.
3. V – S / T
4. I unit – meter / sec i.e., m/s

• Average speed :

Total distance travelled / total time taken.

• Velocity :

1. It’s speed of the body in given direction.
2. Velocity – displacement / time
3. Its a vector quantity.
4. V_avg ( for non uniform motion ) – total displacement / total time
5. V_avg ( for uniform motion ) – initial distance (u) + final distance (v) / 2 or u + v / 2
6. S I unit of velocity – ms ^-1
7. It can be positive, negative or zero.

• Acceleration :

1. It is seen in non uniform motion and it can be defined as the rate of change of velocity per unit time.
2. Acceleration (a) = change in velocity (v – u) / t
3. a = v – u / t
4. If v > u then (a) will be positive.
5. If u > v then (a) will be negative.

• Deaceleration : a = v – u / t
• Equation of motion for uniformly accelerated motion:

1. First equation :

v – u + at

Final velocity = initial velocity + acceleration × time

2. Second equation :

S = ut + ½ at²

3. Third equation :

V² = u² + 2as