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Laws of Motion Basics

Introduction

The Laws of Motion are fundamental principles in physics that describe the relationship between the motion of an object and the forces acting on it. These laws are frequently asked in exams like SSC CGL, RRB NTPC, and IBPS PO to test understanding of basic mechanics concepts.

Pattern: Laws of Motion Basics

Pattern

This pattern tests knowledge of Newton's three laws of motion, their definitions, and applications in daily life and competitive exams.

Key Concept:

Newton's Laws of Motion describe how objects move under the influence of forces.

Important Points:

  • First Law (Law of Inertia) = A body remains at rest or in uniform motion unless acted upon by an external force.
  • Second Law = Force (F) = mass (m) × acceleration (a); unit of force is Newton (N).
  • Third Law = For every action, there is an equal and opposite reaction.

Related Topics:

  • Work, Energy, and Power
  • Friction and its effects

Step-by-Step Example

Question

Which of the following correctly states Newton's Second Law of Motion?

Options:

  • A. A body continues in its state of rest or uniform motion unless acted upon by an external force.
  • B. Force is equal to mass multiplied by acceleration.
  • C. For every action, there is an equal and opposite reaction.
  • D. Energy can neither be created nor destroyed.

Solution

  1. Step 1: Identify the law described

    The question asks for Newton's Second Law, which relates force, mass, and acceleration.

  2. Step 2: Analyze each option

    "A body continues in its state of rest or uniform motion unless acted upon by an external force." describes the First Law (Law of Inertia). "Force is equal to mass multiplied by acceleration." states the formula F = ma, which is the Second Law. "For every action, there is an equal and opposite reaction." is the Third Law. "Energy can neither be created nor destroyed." is the law of conservation of energy, unrelated here.

  3. Step 3: Select the correct statement

    Force is equal to mass multiplied by acceleration correctly states Newton's Second Law of Motion.

  4. Final Answer:

    Force is equal to mass multiplied by acceleration. → Option B

  5. Quick Check:

    Newton's Second Law = F = ma ✅

Quick Variations

This pattern may appear as:

  • 1. Identification of laws based on given statements or scenarios.
  • 2. Numerical problems using F = ma to calculate force, mass, or acceleration.
  • 3. Application-based questions involving action-reaction pairs.

Trick to Always Use

  • Remember the mnemonic "In-Ma-Re" for the three laws: Inertia, Mass×Acceleration, Reaction.
  • For quick recall, associate Newton's Second Law with the formula F = ma and unit Newton (N).

Summary

Summary

  • First Law: Law of Inertia - object resists change in motion.
  • Second Law: Force equals mass times acceleration (F = ma).
  • Third Law: Every action has an equal and opposite reaction.

Remember:
In-Ma-Re = Inertia, Mass×Acceleration, Reaction

Practice

(1/5)
1. Which of the following is known as the Law of Inertia?
easy
A. A body remains at rest or in uniform motion unless acted upon by an external force.
B. Force is equal to mass multiplied by acceleration.
C. For every action, there is an equal and opposite reaction.
D. Energy can neither be created nor destroyed.

Solution

  1. Step 1: Identify the concept

    The question asks about the Law of Inertia, which is Newton's First Law of Motion.

  2. Step 2: Apply the concept

    The Law of Inertia states that a body remains at rest or moves uniformly unless an external force acts on it. This matches A body remains at rest or in uniform motion unless acted upon by an external force..

  3. Final Answer:

    A body remains at rest or in uniform motion unless acted upon by an external force. → Option A

  4. Quick Check:

    Newton's First Law = Law of Inertia ✅
Hint: Remember inertia means resistance to change in motion.
Common Mistakes: Confusing First Law with Third Law or conservation of energy.
2. The SI unit of force is:
easy
A. Joule
B. Watt
C. Newton
D. Pascal

Solution

  1. Step 1: Identify the concept

    The question tests knowledge of SI units related to force.

  2. Step 2: Apply the concept

    The SI unit of force is Newton, defined as kg·m/s². Joule is energy, Watt is power, Pascal is pressure.

  3. Final Answer:

    Newton → Option C

  4. Quick Check:

    SI unit of force = Newton ✅
Hint: Recall force unit Newton = kg·m/s².
Common Mistakes: Mistaking Joule or Pascal as force units.
3. According to Newton's Third Law of Motion, when you push a wall, the wall:
easy
A. Does not exert any force back on you.
B. Exerts a force only if it moves.
C. Exerts a force greater than the force you apply.
D. Exerts a force equal in magnitude and opposite in direction on you.

Solution

  1. Step 1: Identify the concept

    The question tests understanding of Newton's Third Law of Motion.

  2. Step 2: Apply the concept

    Newton's Third Law states that for every action, there is an equal and opposite reaction. So, the wall exerts an equal and opposite force on you.

  3. Final Answer:

    Exerts a force equal in magnitude and opposite in direction on you. → Option D

  4. Quick Check:

    Newton's Third Law = Action equals reaction ✅
Hint: Action and reaction forces are always equal and opposite.
Common Mistakes: Thinking the wall exerts no force or only if it moves.
4. If a force of 20 N produces an acceleration of 4 m/s² on an object, what is the mass of the object?
medium
A. 5 kg
B. 80 kg
C. 16 kg
D. 24 kg

Solution

  1. Step 1: Identify the formula

    Newton's Second Law states Force = mass × acceleration (F = ma).

  2. Step 2: Calculate mass

    Rearranging, mass = Force / acceleration = 20 N / 4 m/s² = 5 kg.

  3. Final Answer:

    5 kg → Option A

  4. Quick Check:

    Mass = Force ÷ Acceleration = 5 kg ✅
Hint: Use m = F/a to quickly find mass.
Common Mistakes: Confusing mass with force or acceleration values.
5. Which of the following statements is TRUE according to Newton's Laws of Motion?
medium
A. A moving object will stop immediately if no force acts on it.
B. Action and reaction forces act on different bodies.
C. Force and acceleration are inversely proportional.
D. Mass and acceleration are unrelated.

Solution

  1. Step 1: Analyze each statement

    Check each statement against Newton's Laws.

  2. Step 2: Evaluate correctness

    Statement A is false; without force, object continues moving (Newton's First Law). Statement B is true; action and reaction forces act on different bodies (Newton's Third Law). Statement C is false; force and acceleration are directly proportional (Newton's Second Law). Statement D is false; mass and acceleration are related via F=ma.

  3. Final Answer:

    Action and reaction forces act on different bodies. → Option B

  4. Quick Check:

    Newton's Third Law = Action-reaction on different bodies ✅
Hint: Remember action-reaction forces act on separate objects.
Common Mistakes: Assuming action and reaction act on the same object.

Mock Test

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