Describing Motion
NCERT Class 9 • Science • Chapter "Motion"
Describing Motion
An object is said to be in motion when its position changes with time with respect to a chosen reference point. Motion is always relative, since a passenger sitting in a moving train is at rest with respect to the train but in motion with respect to the platform.
Distance, Displacement, Speed and Velocity
- Distance is the total path length covered; it is a scalar quantity and is never negative.
- Displacement is the shortest distance from the initial to the final position with direction; it is a vector and can be zero.
- If an object returns to its starting point, displacement is zero while distance is not.
- Speed is distance divided by time and is a scalar; its unit is metre per second.
- Velocity is displacement divided by time and is a vector, so it has both magnitude and direction.
- Uniform motion means equal distances in equal intervals of time; non uniform motion means unequal distances.
- Acceleration is the rate of change of velocity, and it is negative when velocity decreases, which is called retardation.
Equations and Graphs
- For uniformly accelerated motion in a straight line, v = u + at relates final velocity to initial velocity, acceleration and time.
- s = ut + ½at² gives the distance covered in that time.
- v² = u² + 2as relates the velocities to the distance without using time.
- On a distance time graph, the slope gives the speed; a straight line means uniform speed.
- On a velocity time graph, the slope gives acceleration and the area under the graph gives displacement.
- In uniform circular motion the speed stays constant but the direction keeps changing, so the velocity changes and the motion is accelerated.
- Average speed is total distance divided by total time, which differs from instantaneous speed at any moment.
| Quantity | Type | Unit |
|---|---|---|
| Distance | Scalar | metre |
| Displacement | Vector | metre |
| Speed | Scalar | metre per second |
| Velocity | Vector | metre per second |
| Acceleration | Vector | metre per second squared |
Displacement — the shortest distance between the initial and final positions of an object, measured with direction.
Hindi me samjhein
दूरी कुल चला गया पथ है, जबकि विस्थापन आरंभ से अंत तक की सबसे छोटी दूरी है। यदि वस्तु वापस उसी स्थान पर आ जाए तो विस्थापन शून्य होता है पर दूरी शून्य नहीं।
Exam me kaise aata hai
- Which quantity can be zero even after moving — displacement
- The slope of a velocity time graph gives — acceleration
- Area under a velocity time graph gives — displacement
- Uniform circular motion is accelerated because of change in — direction
UPSC/State PSC ke liye
- Uniform circular motion is accelerated even at constant speed, because velocity is a vector and its direction changes continuously.
- Displacement can never be greater than distance, and the two are equal only for motion in a straight line without reversal.
Yahan confuse hote hain
✗ Distance and displacement are the same | Distance is total path; displacement is shortest with direction | ✓
✗ Speed and velocity mean the same | Velocity has direction; speed does not | ✓
✗ Constant speed means no acceleration | In circular motion direction changes, so there is acceleration | ✓
Ek nazar me
- Motion is relative to a reference point.
- Distance and speed are scalars; displacement and velocity are vectors.
- Three equations relate u, v, a, s and t for uniform acceleration.
- Circular motion at constant speed is still accelerated.