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Motion, Force & Laws

General Science · Physics

Useful for: Railway RRB

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Overview

Motion is described by speed, velocity and acceleration; Newton’s three laws explain how forces change motion.

Key points

Speed is scalar; velocity is speed with direction; acceleration = change in velocity ÷ time.
Equations: v = u + at; s = ut + ½at²; v² = u² + 2as.
First law: a body stays at rest or in uniform motion unless a net force acts (inertia).
Second law: F = ma; unit newton.
Third law: every action has an equal and opposite reaction.

Explanation

Momentum p = mv; total momentum is conserved when no external force acts.
Impulse = force × time = change in momentum.
Free fall: a = g ≈ 9.8 m/s².
Friction opposes relative motion.

Step by step

Identify the concept or law involved.
Recall the definition, unit or formula.
Check units and the direction of the effect before choosing.

Examples

A car starts from rest with a = 2 m/s² for 5 s: v = 10 m/s, s = ½ × 2 × 25 = 25 m.
F = ma: 5 kg × 4 m/s² = 20 N.
The recoil of a gun is explained by Newton’s third law.

Traps and advanced points

Trap: Treating velocity as scalar.
Trap: Thinking a moving body needs a force to keep moving.
Trap: Forgetting that action and reaction act on different bodies.

Quick revision

Three equations of motion.
F = ma.
p = mv conserved.

Practice MCQs

Concept check: Vitamin A deficiency is classically associated with what?
1. Anaemia only
2. Scurvy
3. Night blindness
4. Rickets
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Answer: (c) Night blindness. Vitamin A deficiency is classically associated with Night blindness.
Concept check: What is the common chemical name of baking soda?
1. Sodium hydroxide
2. Sodium chloride
3. Sodium hydrogen carbonate
4. Calcium carbonate
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Answer: (c) Sodium hydrogen carbonate. Sodium hydrogen carbonate is the common chemical name of baking soda.
Concept check: Which is the largest organ of the human body?
1. Lung
2. Liver
3. Heart
4. Skin
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Answer: (d) Skin. Skin is the largest organ of the human body.
Elements in Group 17 are commonly called what?
1. Alkaline earth metals
2. Alkali metals
3. Halogens
4. Noble gases
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Answer: (c) Halogens. Elements in Group 17 are commonly called Halogens.
Exam practice: Dengue is mainly transmitted by which mosquito?
1. Aedes
2. Anopheles
3. Housefly
4. Culex only
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Answer: (a) Aedes. Dengue is mainly transmitted by Aedes.
Which option correctly answers this General Science check: potential energy near Earth for height h?
1. mgh
2. newton
3. concave lens
4. rate of reaction
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Answer: (a) mgh. Near Earth, gravitational potential energy change is mgh.
Which gas is known as laughing gas?
1. Nitric oxide
2. Ammonia
3. Nitrogen dioxide
4. Nitrous oxide
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Answer: (d) Nitrous oxide. Nitrous oxide (N₂O) is laughing gas, used as a mild anaesthetic.
Which option correctly answers this General Science check: kinetic energy of a mass m moving at speed v?
1. ½mv²
2. watt
3. ohm
4. cos 90° = 0
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Answer: (a) ½mv². This is the classical translational kinetic-energy formula.
The SI unit of power is —
1. volt
2. joule
3. newton
4. watt
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Answer: (d) watt. Power = work/time, measured in watts.
A car accelerates from rest to 20 m/s in 5 s. Its acceleration is —
1. 5 m/s²
2. 100 m/s²
3. 2 m/s²
4. 4 m/s²
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Answer: (d) 4 m/s². a = (20 − 0)/5 = 4 m/s².