Chapter 4: Laws of Motion – Introduction (NEET Level Easy Notes)
1. What was studied in the previous chapter?
- In the previous chapter, we studied motion of particles.
- We learned how to describe motion using:
- Velocity → tells how fast and in which direction an object moves.
- Acceleration → tells how velocity changes with time.
- We could describe motion mathematically.
- But one important question remained:
Question:
What causes a body to move or change its motion?
This question is answered in the chapter Laws of Motion.
2. Need for Force in Daily Life
- From our everyday experience, we observe that an object at rest does not start moving on its own.
- Some external action is needed.
Examples:
- A football starts moving only when someone kicks it.
- A stone moves upward only when someone throws it.
- Tree branches swing because of wind.
- A boat moves because of the flowing river.
Conclusion:
➡️ To move an object from rest, an external force is required.
3. Force Can Also Stop Motion
- Force is not only used to start motion.
- It can also slow down or stop a moving object.
Example:
- A ball rolling down a slope can be stopped by applying force in the opposite direction.
Conclusion:
➡️ Force can:
- Start motion.
- Stop motion.
- Change speed.
- Change direction.
4. Contact Forces
- In many cases, force is applied through physical contact.
Examples:
- Hand kicking a football.
- Hand throwing a stone.
- Wind pushing tree branches.
- Water pushing a boat.
Definition:
Contact Force: A force that acts when two objects touch each other.
Examples:
- Muscular force
- Friction force
- Normal reaction force
5. Non-Contact Forces
- Force can also act without physical contact.
Example 1: Gravitational Force
- A stone dropped from a building falls toward Earth.
- Earth attracts the stone due to gravity.
Example 2: Magnetic Force
- A magnet attracts an iron nail from a distance.
Conclusion:
➡️ Physical contact is not always necessary for force to act.
Definition:
Non-Contact Force: A force that acts without direct contact.
Examples:
- Gravitational force
- Magnetic force
- Electrostatic force
6. Important Definition of Force
Force
A force is a push or pull that can:
- Start motion,
- Stop motion,
- Change speed,
- Change direction,
- Change shape of an object.
SI Unit:
Newton (N)
7. External Agency
- The source that applies force is called an external agency.
Examples:
| External Agency | Force Produced |
|---|---|
| Hand | Muscular force |
| Wind | Air force |
| Flowing water | Water force |
| Earth | Gravitational force |
| Magnet | Magnetic force |
Key Point:
- An external agency may be:
- In contact with the object.
- Away from the object.
8. Important Question Raised in the Chapter
Consider a skater moving on a smooth ice surface with constant speed.
Question:
Does he need a continuous force to keep moving?
At first glance, the answer seems "Yes."
But this idea is actually incorrect.
This misconception was given by Aristotle, and later corrected by Galileo and Newton.
This leads to:
- Aristotle's Fallacy
- Law of Inertia
- Newton's First Law of Motion
which are discussed in the next sections.
NEET Quick Facts
Force is needed to:
✅ Start motion
✅ Stop motion
✅ Change speed
✅ Change direction
✅ Change shape
Types of Forces
1. Contact Forces
- Muscular force
- Friction
- Normal force
2. Non-Contact Forces
- Gravitational force
- Magnetic force
- Electrostatic force
Most Important NEET Concept
A force is required to change the state of motion, but not necessarily to maintain uniform motion.
This idea forms the basis of Newton's First Law of Motion (Law of Inertia).
One-Line Revision
- Velocity describes motion.
- Acceleration describes change in motion.
- Force explains why motion changes.
- Force can start, stop, speed up, slow down, or change direction.
- Force may be contact or non-contact.
- Earth attracts objects through gravity.
- Magnets attract iron without contact.
- The major question of this chapter is: "Is force required to keep a body moving uniformly?"
- The answer leads to Newton's Laws of Motion. ✅
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| Class 11 Physics Laws of Motion Introduction – Easy Notes, MCQs, and Exam Questions for CBSE and NEET Preparation. |
Class 11 Physics – Laws of Motion (Introduction) Question Bank with Answers
(CBSE + NEET Foundation Level)
A. Multiple Choice Questions (MCQs)
1. Which quantity is sufficient to describe uniform motion?
a) Acceleration b) Velocity c) Force d) Momentum
Answer: (b) Velocity
2. Which quantity is additionally required to describe non-uniform motion?
a) Mass b) Velocity c) Acceleration d) Density
Answer: (c) Acceleration
3. A football at rest starts moving when:
a) Gravity acts b) It is kicked c) Air acts d) It moves itself
Answer: (b) It is kicked
4. Which of the following is a non-contact force?
a) Friction b) Muscular force c) Magnetic force d) Tension
Answer: (c) Magnetic force
5. A stone falls toward Earth due to:
a) Friction b) Magnetic force c) Gravitational force d) Tension
Answer: (c) Gravitational force
6. The SI unit of force is:
a) Joule b) Newton c) Watt d) Pascal
Answer: (b) Newton
7. Which of the following is a contact force?
a) Gravity b) Magnetism c) Friction d) Electrostatic force
Answer: (c) Friction
8. Force can change:
a) Speed b) Direction c) State of motion d) All of these
Answer: (d) All of these
B. Very Short Answer Questions (1 Mark)
1. What is force?
Answer: Force is a push or pull that can change the state of motion of a body.
2. Name one contact force.
Answer: Frictional force.
3. Name one non-contact force.
Answer: Gravitational force.
4. What is the SI unit of force?
Answer: Newton (N).
5. Give one example of gravitational force.
Answer: A stone falling toward Earth.
6. Give one example of magnetic force.
Answer: A magnet attracting an iron nail.
C. Short Answer Questions (2–3 Marks)
1. Why is force required to move a stationary object?
Answer: A stationary object remains at rest unless an external force acts on it. Force changes the state of rest and starts motion.
2. Differentiate between contact and non-contact forces.
| Contact Force | Non-Contact Force |
|---|---|
| Requires physical contact | Does not require contact |
| Example: Friction | Example: Gravity |
3. Give three examples where force starts motion.
Answer:
- Kicking a football.
- Throwing a stone.
- Wind moving tree branches.
4. Give two examples where force acts from a distance.
Answer:
- Earth attracting a stone.
- Magnet attracting an iron nail.
D. Long Answer Questions (5 Marks)
1. Explain the need for force with suitable examples.
Answer:
Force is a push or pull that changes the state of motion of a body.
Force is required to:
-
Start motion
- Example: Kicking a football.
-
Stop motion
- Example: Stopping a rolling ball.
-
Change speed
- Example: Accelerating a bicycle.
-
Change direction
- Example: Hitting a moving cricket ball.
Force may act through contact or from a distance. Examples include gravitational and magnetic forces.
Thus, force is responsible for changing the state of motion of objects.
2. Explain contact and non-contact forces with examples.
Answer:
Contact Forces
These forces act only when objects are in physical contact.
Examples:
- Friction
- Muscular force
- Tension
Non-Contact Forces
These forces act without physical contact.
Examples:
- Gravitational force
- Magnetic force
- Electrostatic force
Thus, forces can act both through contact and at a distance.
E. Assertion and Reason Questions
1.
Assertion (A): A magnet can attract an iron nail from a distance.
Reason (R): Magnetic force is a non-contact force.
Answer: ✔ Both A and R are true and R is the correct explanation of A.
2.
Assertion (A): A football at rest starts moving when kicked.
Reason (R): Force changes the state of motion.
Answer: ✔ Both A and R are true and R correctly explains A.
3.
Assertion (A): A falling stone accelerates toward Earth.
Reason (R): Earth exerts gravitational force on the stone.
Answer: ✔ Both A and R are true and R correctly explains A.
F. Fill in the Blanks
1.
A ______ is needed to change the state of motion of a body.
Answer: force
2.
A force acting without contact is called a ______ force.
Answer: non-contact
3.
The SI unit of force is ______.
Answer: Newton
4.
A magnet attracts an iron nail due to ______ force.
Answer: magnetic
5.
A stone falls toward Earth due to ______ force.
Answer: gravitational
G. True/False Questions
1.
Force is required to change the state of motion.
Answer: True
2.
Gravitational force is a contact force.
Answer: False
3.
A magnet can attract iron without touching it.
Answer: True
4.
Velocity alone describes non-uniform motion.
Answer: False
5.
Wind can exert force on tree branches.
Answer: True
H. Statement-Based Questions
Statement I:
A force can act without physical contact.
Statement II:
Gravity and magnetic force are examples of non-contact forces.
a) Both statements are true. b) Both statements are false. c) Statement I is true but II is false. d) Statement I is false but II is true.
Answer: (a) Both statements are true.
Statement I:
Force is needed to stop a moving object.
Statement II:
Force cannot change speed.
Answer: Statement I is true but Statement II is false.
I. Match the Columns
| Column A | Column B |
|---|---|
| A. Gravity | 1. Contact force |
| B. Friction | 2. Iron nail attraction |
| C. Magnet | 3. Earth pulls stone |
| D. Hand push | 4. Muscular force |
Answers
A → 3
B → 1
C → 2
D → 4
J. Case Study Questions
Case Study
A student drops a stone from the roof of a building. The stone starts moving downward and gains speed. Nearby, a magnet attracts an iron nail placed on a table.
Questions
1. Why does the stone fall?
Answer: Due to gravitational force.
2. Is gravity a contact force?
Answer: No, it is a non-contact force.
3. Why does the nail move toward the magnet?
Answer: Due to magnetic force.
4. What type of force is magnetic force?
Answer: Non-contact force.
5. Name two non-contact forces involved in this case.
Answer:
- Gravitational force
- Magnetic force
Important CBSE Exam Questions
1 Mark
- Define force.
- State SI unit of force.
- Give one example of non-contact force.
2 Marks
- Differentiate between contact and non-contact forces.
- Explain why force is required to stop a moving body.
3 Marks
- Explain three effects of force.
- Give examples of contact and non-contact forces.
5 Marks
- Explain the need for force in daily life with examples.
- Discuss contact and non-contact forces with suitable examples.
HOTS Question
A skater moves on a smooth ice surface with constant speed. Is force necessary to keep him moving? Explain.
Answer: No. A force is required only to change the state of motion. Uniform motion can continue without an external force if no opposing force acts.
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