Critical Evaluation of Rest and Motion
Key Concepts
- Rest: A body is said to be at rest if it does not change its position with respect to a reference point over time.
- Motion: A body is said to be in motion if it changes its position with respect to a reference point over time.
Types of Motion
- Translational Motion: The whole body moves in the same direction.
- Rotational Motion: The body rotates around a fixed axis.
- Oscillatory Motion: The body moves back and forth around a mean position.
Frame of Reference
- Motion and rest are relative. A passenger inside a moving train is at rest relative to another passenger but in motion relative to an observer outside the train.
Critical Evaluation
Challenges in Understanding Rest and Motion
- Concept of Relativity
- Students often find it difficult to grasp that rest and motion depend on the observer’s frame of reference.
- Example: A person sitting inside a moving bus is at rest with respect to other passengers but in motion relative to a person standing on the road.
- Types of Motion Confusion
- Differentiating between translational, rotational, and oscillatory motion can be challenging.
- Example: The motion of a spinning top includes both rotational and translational motion.
- Uniform vs. Non-Uniform Motion
- Many students struggle with distinguishing between uniform motion (constant velocity) and non-uniform motion (changing velocity).
- Example: A freely falling object undergoes non-uniform motion due to acceleration due to gravity.
- Graphical Representation
- Motion is often represented graphically (distance-time and velocity-time graphs), which students may struggle to interpret.
- Example: A straight-line distance-time graph represents uniform motion, while a curved line represents non-uniform motion.
CBSE Previous Years’ Exam Questions (Last 10 Years) with Solutions
1. Definition-Based Questions
Question (2023, 2019, 2016)
- Define rest and motion. Give an example where an object is in motion concerning one observer and at rest concerning another.
Solution
- Rest: A body is at rest if its position does not change with time concerning a reference point.
- Motion: A body is in motion if its position changes with time concerning a reference point.
- Example: A passenger sitting inside a moving train is at rest concerning other passengers but in motion concerning an observer outside the train.
2. Conceptual Questions
Question (2022, 2018, 2015)
- A man sitting inside a moving car throws a ball vertically upwards. Describe the motion of the ball as seen by (a) the man inside the car and (b) an observer standing on the roadside.
Solution
- (a) For the man inside the car: The ball moves straight up and down. It appears to follow a vertical path.
- (b) For an observer on the roadside: The ball follows a parabolic path because, in addition to the upward motion, it also has horizontal velocity equal to that of the car.
3. Numericals on Motion
Question (2021, 2017, 2014)
- A car travels 100 meters in 5 seconds. What is its speed? If the same car covers 200 meters in the next 10 seconds, is its motion uniform or non-uniform?
Solution
- First 100 m: Speed = Distance / Time = 100/5=20100/5 = 20 m/s
- Next 200 m: Speed = 200/10=20200/10 = 20 m/s
- Since the speed remains constant, the motion is uniform.
4. Graph-Based Questions
Question (2020, 2016, 2013)
- The distance-time graph of a body is a straight line inclined to the time axis. What type of motion does this represent?
Solution
- A straight-line distance-time graph represents uniform motion because the object covers equal distances in equal intervals of time.
5. Application-Based Questions
Question (2019, 2015, 2012)
- A satellite moving around the Earth in a circular orbit is said to be in motion. But why is it also considered to be at rest?
Solution
- The satellite is in motion relative to an observer on Earth because it continuously changes its position.
- However, it is at rest concerning other objects inside it (like astronauts) because they all move together at the same velocity.
Conclusion & Tips for CBSE Exams
- Understand Motion Relativity: Rest and motion depend on the observer’s frame of reference.
- Know Different Types of Motion: Understand and differentiate between translational, rotational, and oscillatory motion.
- Focus on Graphs: Practice interpreting distance-time and velocity-time graphs.
- Solve Numericals: Work on speed, velocity, and acceleration-based problems.
Would you like more solved numerical examples or diagrams for better visualization? 😊
