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CAPF AC Motion & Laws

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This page covers CAPF AC Motion & Laws with complete concept notes, 4 graded practice MCQs, key points and exam-specific tips. Free to study.

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Concept Notes

Motion & Laws— Rules & Concept

Core ConceptRead this first — the foundation of the topic

Motion and Laws are the backbone of physics. This topic covers how objects move and what forces cause that movement. Newton's three laws of motion form the foundation. CORE CONCEPT:

Motion is the change in position of an object with time. Laws of motion explain why objects start moving, stop moving, or change their speed and direction.

Key RulesCore rules you must know cold

Newton's First Law (Law of Inertia): An object at rest stays at rest, and an object in motion stays in motion at constant velocity, unless acted upon by an external force. Newton's Second Law: Force equals mass times acceleration. F = ma Newton's Third Law: For every action, there is an equal and opposite reaction.

Formula BlockMemorise — at least one formula appears in every paper
F = ma (Force = mass × acceleration)
v = u + at (Final velocity = Initial velocity + acceleration × time)
s = ut + (1/2)at² (Distance = initial velocity × time + half × acceleration × time squared)
v² = u² + 2as (Final velocity squared = Initial velocity squared + 2 × acceleration × distance)
Momentum = mass × velocity
Exam PatternsWhat examiners ask — read before attempting PYQs

RRB Group D frequently asks direct formula applications, especially F = ma problems. Numerical problems on equations of motion are very common. Questions often involve trains, cars, or falling objects. Third law applications like rocket propulsion appear regularly.

ShortcutsUse these to save 30–60 seconds per question
UVAT Method

Remember u, v, a, t, s. Any three given, find remaining two using equations. 2. For free fall problems: Always use g = 10 m/s² for quick calculations instead of 9.8 m/s² 3

Action-Reaction Pairs

Walking, swimming, rocket propulsion - all follow third law

Worked ExampleSolve this step-by-step before moving on

A train accelerates from rest at 2 m/s² for 10 seconds. Find the distance covered. Given: u = 0 m/s (starts from rest), a = 2 m/s², t = 10 s Required: s = ? Solution: Using s = ut + (1/2)at² s = (0)(10) + (1/2)(2)(10)² s = 0 + (1)(100) s = 100 meters WORKED EXAMPLE 2: A 5 kg box is pushed with 20 N force. Calculate acceleration. Given: m = 5 kg, F = 20 N Required: a = ? Solution: Using F = ma 20 = 5 × a a = 20/5 = 4 m/s²

Exam TrapsCommon mistakes students make — avoid these

The #1 trap students fall into is confusing mass and weight. Mass is measured in kg and remains constant everywhere. Weight is force (mg) measured in Newtons and changes with gravity.

Always check if the question asks for mass or weight. Many students write kg when answer should be in Newtons or vice versa. Another frequent error is mixing up action-reaction pairs. The forces act on DIFFERENT objects, not the same object.

When you push a wall, you feel the wall pushing back on you - these are action-reaction forces. For numerical problems, always write given data first, identify what to find, then choose the correct equation. This systematic approach prevents formula confusion and silly mistakes.

Key Points to Remember

  • Newton's First Law: Objects resist change in motion due to inertia
  • Newton's Second Law: F = ma is the most important formula for calculations
  • Newton's Third Law: Action and reaction forces act on different objects
  • Quick formula: v² = u² + 2as for problems without time
  • Free fall shortcut: Use g = 10 m/s² for fast calculations
  • Momentum = mass × velocity, measured in kg⋅m/s
  • UVAT method: Know any 3 of u,v,a,t,s to find other 2
  • Mass is in kg and constant, weight is mg in Newtons
  • Distance formula: s = ut + (1/2)at² for uniform acceleration
  • Acceleration = (final velocity - initial velocity) ÷ time

Exam-Specific Tips

  • Standard gravity g = 9.8 m/s² or approximately 10 m/s²
  • Force is measured in Newtons (N), acceleration in m/s²
  • 1 Newton = 1 kg⋅m/s² (unit of force)
  • Inertia depends only on mass, not on velocity or force
  • Rocket propulsion works on Newton's third law principle
  • Momentum is a vector quantity with direction
  • Uniform motion means zero acceleration
  • Action-reaction forces are always equal in magnitude and opposite in direction
Practice MCQs

Motion & Laws — Practice Questions

4graded MCQs · easy to hard · full solution & trap analysis

All MCQs →
Practice 1easy

A car accelerates uniformly from rest and reaches a velocity of 20 m/s in 5 seconds. What is the acceleration of the car?

Practice 2medium

A car starts from rest and accelerates uniformly at 2 m/s² for 10 seconds. What distance does the car travel during this time?

Practice 3medium

A car accelerates uniformly from rest and covers a distance of 80 m in 4 seconds. What is the acceleration of the car?

Practice 4hard

A car accelerates uniformly from rest. It travels 24 m in the first 4 seconds. What is the magnitude of its acceleration?

60-Second Revision — Motion & Laws

  • Remember: F = ma, v = u + at, s = ut + (1/2)at², v² = u² + 2as
  • Formula: Momentum = mass × velocity, Force = Change in momentum ÷ time
  • Trap: Mass is in kg, Weight is in Newtons (mg)
  • Shortcut: Use g = 10 m/s² for quick free fall calculations
  • Key point: Action-reaction forces act on different objects, not same object
  • Method: UVAT technique - identify given values first, then choose equation
  • Common error: Don't confuse distance (s) with displacement in problems
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