01 · Explore
Describing Position and Motion
Motion is a change in the position of an object over time. To describe where an object is, we must use a fixed reference point.
In physics, we describe the position of an object by specifying its distance and direction from a fixed reference point called the origin (O). For example, if we say a school is 2 km North of a railway station, the station is our reference point.
As detailed on page 2, an object is said to be in motion if its position changes relative to the reference point as time passes. Conversely, the object is said to be at rest if its position relative to the reference point does not change with time. Motion is relative; an object might be moving relative to one reference point while being at rest relative to another.
For motion along a straight line (linear motion), we use a number line. Positions to the right of the origin are positive (+), and positions to the left are negative (–). We also distinguish between an 'instant of time' (a single clock reading) and a 'time interval' (the duration between two readings).
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What is the origin in the context of describing motion?
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When is an object considered to be in motion?
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Sign inNew here? Sign up freeNCERT reference: chapter PDF pages 1, 2, 3.
02 · Explore
Distance Travelled and Displacement
While both measure length, distance and displacement describe different aspects of an object's journey.
Distance is the total path length covered by an object. It is a scalar quantity, meaning it has only magnitude (numerical value) and no direction. For instance, if you walk 5 m forward and 3 m back, your total distance is 8 m.
Displacement is the net change in position, representing the shortest straight-line path between the initial and final points. It is a vector quantity, requiring both magnitude and direction. In the previous example, your displacement would be only 2 m in the forward direction.
The SI unit for both is the metre (m). The magnitude of displacement is always less than or equal to the distance travelled. They are equal only if the object moves in a single direction without turning back.
| Feature | Distance | Displacement |
|---|---|---|
| Definition | Total path length covered | Shortest path between start and end |
| Type | Scalar (Magnitude only) | Vector (Magnitude and Direction) |
| Can it be zero? | No (if motion occurs) | Yes (if object returns to start) |
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An athlete runs one complete lap around a 400 m circular track. What is the distance and displacement?
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Sign inNew here? Sign up freeNCERT reference: chapter PDF pages 3, 4.
03 · Explore
Average Speed and Average Velocity
To understand how fast an object moves, we calculate its rate of change of position.
Average speed is the total distance travelled divided by the time interval. It is a scalar quantity. If an object covers equal distances in equal time intervals, it is in uniform motion; otherwise, it is in non-uniform motion.
Average velocity (vav) is the displacement divided by the time interval. It is a vector quantity and indicates both how fast and in what direction an object moves. The formula is vav = s / t, where 's' is displacement.
The SI unit for both speed and velocity is metre per second (m s⁻¹ or m/s). For motion in a straight line in one direction, the magnitude of average velocity equals the average speed.
Calculating Speed and Velocity in a Pool
Average Speed = 50 m / 50 s = 1 m s⁻¹; Average Velocity = 0 m / 50 s = 0 m s⁻¹
Sarang swims 25 m to one end and 25 m back in 50 s. Total distance = 50 m. Displacement = 0 m (returned to start). Speed is exactly 1 m/s, but velocity is 0 m/s because there is no net change in position.
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Under what condition is average velocity equal to average speed?
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Sign inNew here? Sign up freeNCERT reference: chapter PDF pages 5, 6.
04 · Explore
Average Acceleration
Acceleration describes how quickly the velocity of an object changes over time.
Average acceleration (a) is defined as the change in velocity divided by the time interval. If an object's velocity changes from initial value 'u' to final value 'v' in time 't', then a = (v – u) / t. The SI unit is m s⁻².
Acceleration is a vector. If velocity increases, acceleration is in the direction of velocity. If velocity decreases (retardation or deceleration), acceleration is opposite to the direction of velocity.
An object can have zero acceleration even if it is moving very fast, provided its velocity is constant. Acceleration only occurs when there is a change in speed, direction, or both.
Bus Acceleration and Braking
(i) u = 36 km/h = 10 m/s, v = 54 km/h = 15 m/s; a = (15 - 10) / 10 = 0.5 m s⁻²; (ii) a = (0 - 15) / 5 = -3 m s⁻²
A bus speeds up from 36 km/h (10 m/s) to 54 km/h (15 m/s) in 10 s, giving an acceleration of 0.5 m/s². When it brakes from 15 m/s to a stop (0 m/s) in 5 s, the acceleration is -3 m/s², where the minus sign indicates deceleration.
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What is the acceleration of a car moving at a constant 80 km/h on a straight road?
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Sign inNew here? Sign up freeNCERT reference: chapter PDF pages 7, 8, 9.