Newton's Second Law Explained: Projectile Motion and Force Components
Easy Notes (NEET Level) – Important Points from Newton’s Second Law
1. Component of Velocity Normal (Perpendicular) to the Force Remains Unchanged
- A force can only change the velocity component in its own direction.
- The velocity component perpendicular (normal) to the force does not change.
- This is because there is no acceleration in the perpendicular direction.
Example: Projectile Motion
- In projectile motion, only gravitational force acts on the particle.
- Gravity acts vertically downward.
- Therefore, acceleration is only in the vertical direction.
- There is no horizontal force acting on the projectile (neglecting air resistance).
- Hence, the horizontal component of velocity remains constant throughout the motion.
- Only the vertical component of velocity changes due to gravity.
Key NEET Point:
Force changes velocity only along its direction; perpendicular velocity remains unchanged.
2. Newton's Second Law for a Single Particle
- Newton's second law is written as:
- Here, F is the net external force acting on the particle.
- a is the acceleration produced in the particle.
- This form is directly applicable to a single point particle.
Key NEET Point:
Always use the net external force, not individual forces separately.
3. Newton's Second Law for a System of Particles
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The same law can also be applied to:
- A rigid body
- A group of particles (system)
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In such cases:
- F = total external force on the system.
- a = acceleration of the centre of mass of the system.
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Internal forces between particles of the system are not included.
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Internal forces cancel each other and do not affect the motion of the whole system.
Example
- Two blocks connected by a string.
- Tension between the blocks is an internal force.
- Only external forces like gravity or an applied pull are considered.
Key NEET Point:
For a system, consider only external forces; ignore internal forces.
4. Acceleration Depends on Present Force Only
- Acceleration at any instant is determined by the force acting at that same instant.
- Past motion does not affect present acceleration.
- A particle has no "memory" of its previous motion.
Example: Stone Dropped from an Accelerated Train
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A train is moving with acceleration.
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A stone is dropped from the train.
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Immediately after release:
- The stone is no longer connected to the train.
- No horizontal force acts on the stone (ignoring air resistance).
- Therefore, horizontal acceleration becomes zero.
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The stone keeps its horizontal velocity but does not keep the train's acceleration.
Key NEET Point:
Velocity may continue, but acceleration changes instantly according to the force acting at that moment.
5. Newton's Second Law is a Local Relation
- Newton's second law is called a local relation.
- It relates force and acceleration at the same place and same time.
Meaning
- Force acting here and now determines acceleration here and now.
- Previous positions, velocities, or accelerations do not directly determine present acceleration.
- Only the current force matters.
Key NEET Point:
Present acceleration depends only on present force, not on the history of motion.
Quick Revision for NEET
- Force changes velocity only in its own direction.
- Velocity perpendicular to force remains unchanged.
- In projectile motion, horizontal velocity remains constant.
- Newton's second law: .
- For a system, use total external force only.
- Internal forces are ignored.
- Acceleration depends on the force acting at that instant.
- A body has no memory of past acceleration.
- Newton's second law is a local relation.
- Present force determines present acceleration.
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| Newton's Second Law explains how force produces acceleration and why horizontal velocity remains constant in projectile motion. |
CBSE Class 11 Physics – Newton's Second Law (Important Questions with Answers)
A. MCQs (1 Mark Each)
1. In projectile motion, the horizontal component of velocity remains constant because:
a) Gravity acts horizontally
b) No horizontal force acts on the projectile
c) Air resistance is maximum
d) Vertical velocity is constant
Answer: (b) No horizontal force acts on the projectile
2. The force in Newton's second law represents:
a) Internal force
b) External force
c) Gravitational force only
d) Friction only
Answer: (b) External force
3. For a system of particles, acceleration refers to the acceleration of:
a) Any particle
b) Largest particle
c) Centre of mass
d) Geometric centre
Answer: (c) Centre of mass
4. Internal forces are:
a) Included in net external force
b) Ignored while applying Newton's second law to a system
c) Greater than external forces
d) Always zero
Answer: (b)
5. A body remembers:
a) Past acceleration
b) Past force
c) Present force only affects acceleration
d) Future force
Answer: (c)
B. Very Short Answer Questions (1 Mark)
1. What is meant by the net force on a particle?
Answer: The vector sum of all external forces acting on the particle.
2. Which velocity component remains unchanged in projectile motion?
Answer: Horizontal component of velocity.
3. What is the SI unit of force?
Answer: Newton (N).
4. Which force is not included while applying Newton's second law to a system?
Answer: Internal force.
5. What is meant by a local relation?
Answer: A relation that connects physical quantities at the same place and same instant.
C. Short Answer Questions (2–3 Marks)
1. Why does the horizontal velocity of a projectile remain constant?
Answer:
- Gravity acts vertically downward.
- No horizontal force acts on the projectile.
- Therefore, horizontal acceleration is zero.
- Hence, horizontal velocity remains constant.
2. What is the role of internal forces in a system?
Answer:
- Internal forces act between particles of the same system.
- They cancel each other in pairs.
- Therefore, they do not affect the motion of the system as a whole.
3. Explain why a stone dropped from an accelerating train has no horizontal acceleration.
Answer:
- After release, the stone is no longer connected to the train.
- No horizontal force acts on it.
- According to Newton's second law, acceleration depends on force.
- Hence horizontal acceleration becomes zero.
D. Long Answer Questions (3–5 Marks)
1. Explain Newton's second law for a system of particles.
Answer: Newton's second law states that the net external force acting on a body equals the product of its mass and acceleration.
For a system of particles:
- F represents the total external force on the system.
- a represents the acceleration of the centre of mass.
- Internal forces are not included because they cancel each other.
- The motion of the entire system depends only on external forces.
2. Explain why Newton's second law is called a local relation.
Answer:
- Force and acceleration are related at the same place and same instant.
- Present acceleration depends only on present force.
- Past motion does not influence present acceleration directly.
- A body has no memory of previous forces or accelerations.
- Therefore Newton's second law is called a local relation.
E. Assertion and Reason Questions
1.
Assertion (A): Horizontal velocity remains constant in projectile motion.
Reason (R): No horizontal force acts on the projectile.
Answer: Both A and R are true, and R is the correct explanation of A.
2.
Assertion (A): Internal forces are included in the net force acting on a system.
Reason (R): Internal forces cancel each other.
Answer: Assertion is false, Reason is true.
3.
Assertion (A): A body remembers its previous acceleration.
Reason (R): Present acceleration depends only on present force.
Answer: Assertion is false, Reason is true.
F. Fill in the Blanks
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In projectile motion, the ________ component of velocity remains constant. Answer: horizontal
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Newton's second law relates force and ________. Answer: acceleration
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For a system, acceleration refers to the acceleration of the ________. Answer: centre of mass
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Internal forces are ________ while applying Newton's second law to a system. Answer: ignored
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Present acceleration depends on ________ force. Answer: present
G. True or False
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Gravity changes the horizontal velocity of a projectile. Answer: False
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Internal forces affect the motion of the centre of mass. Answer: False
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External force determines acceleration. Answer: True
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Newton's second law is a local relation. Answer: True
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A body remembers its past acceleration. Answer: False
H. Match the Columns
| Column A | Column B |
|---|---|
| 1. Projectile motion | (a) Centre of mass |
| 2. System acceleration | (b) Horizontal velocity constant |
| 3. Internal forces | (c) Present force |
| 4. Local relation | (d) Ignored |
| 5. Acceleration depends on | (e) Same place and time |
Answers
1 → (b)
2 → (a)
3 → (d)
4 → (e)
5 → (c)
I. Statement-Based Questions
Statement 1:
A projectile is moving in air with negligible air resistance.
Statement 2:
Its horizontal velocity remains constant.
a) Both statements are true and Statement 2 explains Statement 1.
b) Both statements are true but Statement 2 does not explain Statement 1.
c) Statement 1 is true, Statement 2 is false.
d) Statement 1 is false, Statement 2 is true.
Answer: (a)
J. Case Study Questions (4 Marks)
Case Study
A train is moving with acceleration. A stone is dropped from the train. Immediately after release, the stone continues moving forward but experiences only gravitational force.
Questions
1. Which force acts on the stone after release?
Answer: Gravitational force.
2. What is the horizontal acceleration of the stone?
Answer: Zero.
3. Why does the stone continue moving horizontally?
Answer: Due to its existing horizontal velocity.
4. What does this example prove about Newton's second law?
Answer: Present acceleration depends only on present force and not on past motion.
Exam-Oriented One-Line Revision
- Force changes velocity only in its own direction.
- Velocity perpendicular to force remains unchanged.
- Horizontal velocity remains constant in projectile motion.
- Only external forces are considered for a system.
- Internal forces cancel each other.
- Acceleration of a system is acceleration of its centre of mass.
- Present force determines present acceleration.
- Newton's second law is a local relation.
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