When Can a Body Be Considered a Point Object? CBSE Class 11 Solution
Q1. In which of the following examples of motion can the body be considered approximately a point object?
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| Examples showing when a body can and cannot be considered a point object in Class 11 Physics. |
Principle: A body can be considered a point object if the distance travelled by it is much greater than its own size (dimensions).
(a) A railway carriage moving without jerks between two stations.
Answer: Yes, the railway carriage can be considered a point object.
Explanation: The distance between two stations is very large compared to the length of the carriage. Hence, the size of the carriage is negligible in comparison to the distance travelled. Therefore, it can be treated as a point object.
(b) A cap on top of a man cycling smoothly on a circular track.
Answer: Yes, the cap can be considered a point object.
Explanation: The size of the cap is very small compared to the circumference of the circular track covered during motion. Therefore, its dimensions can be neglected and it may be treated as a point object.
(c) A spinning cricket ball that turns sharply on hitting the ground.
Answer: No, the cricket ball cannot be considered a point object.
Explanation: The sharp turn of the spinning ball involves rotational motion, and the distance over which the change in direction occurs is comparable to the size of the ball. Hence, its dimensions cannot be neglected.
(d) A tumbling beaker that has slipped off the edge of a table.
Answer: No, the beaker cannot be considered a point object.
Explanation: The size of the beaker is comparable to the height through which it falls. Moreover, tumbling involves rotational motion. Therefore, the dimensions of the beaker are important and it cannot be treated as a point object.
Conclusion
Hence, the body can be considered approximately a point object in cases (a) and (b) only.
Final Answer: (a) and (b)
Topper's Tip for the Exam:
The Core Principle: Always state the rule first. An object can be considered a point object if the distance it travels during its motion is much greater than its own linear dimensions.
बहुत अच्छा विचार है। NEET/CBSE Physics के किसी भी टॉपिक को याद करने और ट्रिकी प्रश्न हल करने के लिए "4-Layer Topper Memory Structure" सबसे प्रभावी रहता है। आपके दिए हुए "Point Object" वाले प्रश्न को उदाहरण बनाकर यह संरचना तैयार की जा सकती है।
NEET में इस प्रकार उत्तर Explain करें:
- पहले Rule लिखें।
- फिर Distance vs Size की तुलना करें।
- अंत में Yes/No लिखें।
- यदि rotation या tumbling हो तो उल्लेख करें कि आकार और orientation महत्वपूर्ण हो जाते हैं।
Practice Questions: Point Object (CBSE Class 11 Physics)
Key Rule
A body can be considered a point object if the distance travelled by it is much greater than its size.
Question 1
Can an airplane flying from Delhi to Mumbai be considered a point object?
Answer
Yes.
The distance between Delhi and Mumbai is much greater than the size of the airplane. Therefore, the airplane can be treated as a point object while studying its motion.
Question 2
Can a football rolling across a playground be considered a point object?
Answer
Yes.
The distance covered by the football across the playground is much larger than its diameter. Hence, it can be considered a point object.
Question 3
Can a spinning top rotating at one place be considered a point object?
Answer
No.
The top is mainly rotating about its own axis. Its size and rotational motion are important. Therefore, it cannot be treated as a point object.
Question 4
Can a student walking around a 400 m circular track be considered a point object?
Answer
Yes.
The size of the student is very small compared to the distance covered on the track. Hence, the student can be considered a point object.
Question 5
Can a coin spinning on a table be considered a point object?
Answer
No.
The motion depends on the coin's size and rotation. Therefore, it cannot be treated as a point object.
Multiple Choice Questions (MCQs)
Question 6
Which of the following can be treated as a point object?
(a) A train travelling between two cities
(b) A rotating ceiling fan
(c) A spinning coin
(d) A tumbling box
Answer
(a) A train travelling between two cities
Reason: Distance travelled is much greater than the train's size.
Question 7
A body can be treated as a point object when:
(a) Its mass is very small
(b) Its shape is circular
(c) Its size is negligible compared to the distance travelled
(d) It is at rest
Answer
(c) Its size is negligible compared to the distance travelled
Question 8
A cricket ball moving from one boundary to another on a cricket field can be treated as:
(a) A point object
(b) A rigid body only
(c) A fluid
(d) None of these
Answer
(a) A point object
Reason: The distance covered is much larger than the size of the ball.
Assertion-Reason Questions
Question 9
Assertion (A): A train moving between two stations can be treated as a point object.
Reason (R): The distance travelled is much greater than the size of the train.
Answer
Both A and R are true, and R is the correct explanation of A.
Question 10
Assertion (A): A spinning beaker falling from a table can be treated as a point object.
Reason (R): The size of the beaker is important in describing its motion.
Answer
Assertion is false, but Reason is true.
Exam Tip
Remember:
Distance ≫ Size → Point Object ✅
Distance ≈ Size → Not a Point Object ❌
Easy Trick:
"Long Journey = Point Object"
"Rotation Important = Not Point Object"
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