Chapter 13: Motion and Time
गति एवं समय (सरल लोलक, आवर्तकाल, चाल का मात्रक एवं दूरी-समय ग्राफ)
Don't Let a 20-Minute Bubbling Delay Cost You CTET Qualification
In offline CTET exams, bubbling 150 bubbles takes 18 to 22 minutes of mechanical time. A single misaligned row can trigger an 8-question domino error. Practice bubble-filling this chapter's solved MCQs inside Mentora's official OMR simulator.
- Speed Formula & Standard Units: Speed = Total Distance Covered / Total Time Taken. Standard SI unit is meters per second (m/s); vehicular speeds measured in km/h (multiply by 5/18 to convert km/h to m/s).
- Periodic Motion & The Simple Pendulum: A metallic bob suspended by a thread from a rigid stand. One complete to-and-fro motion is called an Oscillation; time taken for one oscillation is the Time Period (T).
- Galileo's Pendulum Law: The time period of a simple pendulum depends SOLELY on the length of the string; it does NOT depend on the mass of the bob or the extent of displacement.
- Distance-Time Graphs: 1. Straight diagonal line -> Uniform constant speed; 2. Horizontal line parallel to time axis -> Object is stationary (at rest); 3. Curved line -> Non-uniform speed.
- Trap: Does doubling the mass of the pendulum bob change its time period? NO. The time period depends **only on the length of the string**.
- Trap: What does a horizontal line parallel to the x-axis represent on a distance-time graph? Correct: **The object is at rest (stationary)**.
NCERT Core Facts: Pendulum Physics, Kinematics & Graph Interpretation
Class 7 Chapter 13 Motion and Time tests core mechanics:
1. The Physics of the Simple Pendulum
- Bob: Small metallic sphere suspended by a light thread.
- Mean Position: Center position when at rest.
- Extreme Positions: Points of maximum displacement on either side.
- Oscillation: Movement from mean position to extreme A, then to extreme B, and back to mean position.
- Galileo Galilei's Discovery: A pendulum of a given length always takes the exact same time to complete one oscillation. Changing the weight of the bob or displacing it further makes zero difference to its time period.
2. Distance-Time Graph Interpretation
| Graph Trajectory Shape | Motion State Represented | Formula Indicator |
|---|---|---|
| Straight diagonal line rising upwards | Uniform Speed (एकसमान चाल) | Slope = (Δ d)/(Δ t) = Speed (constant) |
| Horizontal line parallel to Time axis | Object at Rest (विराम अवस्था) | Slope = 0 implies Speed = 0 |
| Curved upwards line | Accelerated Motion (असमान चाल) | Speed is increasing over time. |
Test Your Mastery: Motion and Time
Authentic CBSE exam questions tested directly from this NCERT chapter.
A simple pendulum takes 32 seconds to complete 20 oscillations. What is the time period of the pendulum?
एक सरल लोलक 20 दोलन पूरे करने में 32 सेकंड का समय लेता है। लोलक का आवर्तकाल (Time Period) क्या है?
What does a horizontal straight line parallel to the time axis on a distance-time graph indicate?
दूरी-समय ग्राफ पर समय अक्ष के समानांतर एक सीधी क्षैतिज रेखा क्या दर्शाती है?
On what physical factor does the time period of a simple pendulum depend?
Solely on the length of the string, not on the mass of the bob.
How do you convert speed from km/h to m/s?
Multiply by 5/18 (e.g. 72 km/h × 5/18 = 20 m/s).