ANAND CLASSES study material and notes explains the concepts of Rest and Motion in Physics with detailed explanations, FAQs, MCQs, conceptual questions, worksheets, and test papers for JEE, NEET, and CBSE Class 11 exams. Learn about the frame of reference and the relative nature of motion with examples and important revision points.
Introduction
Understanding rest and motion is fundamental in physics. Every object is either at rest or in motion relative to some reference frame. The concept of rest and motion is relative and depends on the observer’s point of view.
Definition of Rest and Motion
- Rest : If a body does not change its position as time passes with respect to a frame of reference, it is said to be at rest.
- Motion : If a body changes its position as time passes with respect to a frame of reference, it is said to be in motion.
Frame of Reference
- It is a system to which a set of coordinates are attached and with reference to which an observer describes any event.
- Example:
- A passenger standing on a platform observes a tree as stationary (at rest).
- The same passenger, when moving in a train, observes the tree as moving.
- In both conditions, the observer is correct because the reference frame is different.
- Conclusion: Rest and motion are relative terms, depending upon the chosen frame of reference.
A tree is fixed at a place, so we say that it is stationary. Similarly, a house, a school, a factory, electric poles and telephone poles are all stationary objects which remain fixed at a place. On the other hand, a man, animals, birds, cars, buses, trains, ships and aeroplanes, etc., do not remain stationary all the time. They can move from one place to another.
For example, a man moves when he walks along a road, a bird moves when it flies in the sky, a cheetah moves when it runs in the jungle, and a fish moves when it swims in water. Similarly, a car or bus moves on a road, a train moves on the track, a ship moves in water and an aeroplane moves when it flies in air from one place to another (see Figure). The movement of a body (or object) is called motion.
A common characteristic of all the moving bodies is that they change their position with time. We can now define motion as follows :
Define Motion
A body is said to be in motion (or moving) when its position changes continuously with respect to a stationary object taken as reference point.
For example, when the position of a car changes continuously with respect to stationary objects like houses and trees, etc., we say that the car is moving or that the car is in motion.
Let us take an example to understand the meaning of motion more clearly. In Figure we see a car at position A in front of a house and a tree at a particular time (In this case, the house and tree are the stationary objects which are taken as a reference point). Now, after 5 seconds, we see the same car at position B which is quite far away from the house and the tree (see Figure). This means that the position of this car is changing continuously with respect to a stationary object, house or tree. So, we say that this car is moving or that this car is in motion. Some other bodies (or objects) around us which show different kinds of motion are : swing (jhoola), merry-go-round, pendulum of a clock, and hands of a watch.
Exam Oriented Frequently Asked Questions (FAQs)
What is the definition of rest and motion?
Ans: Rest is when a body does not change its position with respect to a chosen frame of reference, whereas motion is when it does.
What is meant by the frame of reference?
Ans: A frame of reference is a coordinate system or set of points with respect to which an observer measures the position of an object.
Can an object be at rest and in motion at the same time?
Ans: Yes, rest and motion are relative. A book on a table is at rest with respect to the table but in motion with respect to the Earth’s rotation.
Give an example where motion is relative.
Ans: A person sitting in a moving bus is at rest with respect to the bus but in motion with respect to an observer on the road.
Multiple-Choice Questions (MCQs)
Question 1:
Which of the following statements is correct about rest and motion?
A) They are absolute concepts.
B) They depend on the observer’s frame of reference.
C) An object can never be at rest.
D) Motion is independent of the observer.
Answer: B
Explanation: Rest and motion depend on the chosen frame of reference, making them relative.
Question 2:
A person sitting inside a moving bus sees a stationary bag on a seat. With respect to the ground, the bag is:
A) At rest
B) In motion
C) Both at rest and in motion
D) None of the above
Answer: B
Explanation: The bag is at rest relative to the bus but in motion relative to the ground.
Conceptual Questions and Answers
Can we say that an object is always in motion?
Answer: No, motion depends on the frame of reference. If observed from a moving train, a tree outside appears to be moving, but from the ground, it is at rest.
How does the concept of relative motion help in physics?
Answer: It helps understand different observations by different observers and is crucial in areas like mechanics and Einstein’s theory of relativity.
Do You Know?
- Motion and rest are not absolute but depend on the observer’s point of view.
- The concept of relative motion is used in astronomy to understand planetary motion.
- Einstein’s Theory of Relativity is based on the idea that motion is relative.
Worksheet
- Define rest and motion with examples.
- What is a frame of reference? Explain with an example.
- Explain why motion is a relative concept.
- A passenger sitting inside a moving train throws a ball upwards. Describe its motion relative to:
- The passenger
- A person standing on the platform
Test Paper (10 Marks)
Section A: Conceptual Questions (5 Marks)
- Define rest and motion with an example. (2 Marks)
- Explain the frame of reference with a real-life example. (3 Marks)
Section B: Multiple-Choice Questions (5 Marks)
Q.1 Rest and motion are:
A) Absolute concepts
B) Dependent on an observer’s frame of reference
C) The same for all observers
D) None of the above (1 Mark)
Q.2 : Which of the following is in motion relative to a person in a moving car?
A) The dashboard of the car
B) The trees outside
C) The person sitting next to them
D) The seat belt (1 Mark)
Q3. : A book lying on a table is:
A) Always in motion
B) Always at rest
C) At rest relative to the table but in motion relative to the Earth
D) None of the above (1 Mark)
Q4: A person in a train observes a pole outside moving. This is an example of:
A) Absolute motion
B) Relative motion
C) Circular motion
D) None of the above (1 Mark)
Q.5 : Which of the following is an example of relative motion?
A) A bird flying in the sky
B) A car moving on the road
C) A person sitting in a moving bus observing another moving bus
D) All of the above (1 Mark)
Quick Revision Points
- Rest: No change in position w.r.t. a frame of reference.
- Motion: Change in position w.r.t. a frame of reference.
- Frame of Reference: A system with coordinates used to describe motion.
- Relative Nature of Motion: An object may be at rest in one frame and in motion in another.
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