1. What is Motion?
An object is said to be in motion if it changes its position with time.
Examples: moving car, flying bird, flowing river, rotating fan.
If an object does not change its position → it is at rest.
If an object does not change its position → it is at rest.
हिंदी: किसी वस्तु की स्थिति समय के साथ बदलती है → गति (Motion).
स्थिति नहीं बदलती → विश्राम (Rest).
2. Distance vs Displacement
Distance: Total path covered. (Scalar)
Displacement: Shortest straight-line distance from initial to final position. (Vector)
Examples:
Examples:
- Walking 10 m east + 10 m west → distance = 20 m, displacement = 0
हिंदी:
Distance = पूरा रास्ता
Displacement = सीधी रेखा वाला न्यूनतम अंतर
→ NDA में बार-बार पूछा जाता है।
3. Speed vs Velocity
Speed = Distance / Time
Scalar → only magnitude.
Velocity = Displacement / Time Vector → magnitude + direction.
If direction changes, velocity changes even if speed is same.
Velocity = Displacement / Time Vector → magnitude + direction.
If direction changes, velocity changes even if speed is same.
हिंदी: Speed में दिशा की ज़रूरत नहीं।
Velocity में दिशा ज़रूरी — East, West, North etc.
4. Uniform & Non-uniform Motion
Uniform Motion: Equal distances in equal intervals of time.
Non-uniform Motion: Unequal distances in equal intervals.
Examples:
Examples:
- Train moving at constant speed → uniform.
- Car in city traffic → non-uniform.
5. Acceleration
Acceleration = Change in velocity / Time
Types:
Types:
- Positive (speed increases)
- Negative (retardation)
- Zero (constant velocity)
हिंदी: Velocity बढ़े → Positive acceleration
Velocity घटे → Negative acceleration
Velocity same → Zero acceleration
6. Force – Basic NDA Concept
A force is a push or pull acting on a body.
Effects of Force:
Effects of Force:
- Can change the speed
- Can change the direction
- Can change the shape (e.g., stretching a rubber band)
हिंदी: Force = धक्का या खींच — इससे वस्तु की गति, दिशा या आकार बदल सकता है।
7. Newton’s Three Laws (Exam Focus)
7.1 First Law – Law of Inertia
A body at rest stays at rest, and a body in motion stays in motion unless an external force acts.
Inertia = resistance to change of state.
हिंदी: कोई वस्तु अपनी अवस्था नहीं बदलती जब तक कोई बाहरी बल न लगे — यही inertia है।
7.2 Second Law – F = m × a
Force = mass × acceleration
More force → more acceleration.
7.3 Third Law – Action & Reaction
For every action, there is an equal and opposite reaction.
Example: gun recoil.
हिंदी: Action → Reaction बराबर और विपरीत होता है।
Gun चलाते समय पीछे धक्का लगना इसी का उदाहरण।
8. Circular Motion
Movement in a circular path with constant speed but changing velocity.
Reason: direction keeps changing.
Centripetal Force: Force acting towards the center (very important).
Centripetal Force: Force acting towards the center (very important).
हिंदी:
समान speed पर भी circular motion में velocity बदलती रहती है → क्योंकि दिशा बदलती है।
Centripetal force = केंद्र की ओर लगने वाला बल।
📘 Practice Questions (NDA Style)
1. Velocity is:
a) Distance/time
b) Displacement/time
c) Force/area
d) Mass × acceleration
2. Force =
a) m/v
b) m × a
c) a/m
d) v × t
3. Which law explains inertia?
a) Newton’s 1st law
b) Newton’s 2nd law
c) Newton’s 3rd law
d) Law of gravitation
4. A gun recoils due to:
a) Inertia
b) Conservation of mass
c) Action–reaction
d) Centripetal force
5. Centripetal force acts:
a) Away from center
b) Towards center
c) Upwards only
d) Downwards only
a) Distance/time
b) Displacement/time
c) Force/area
d) Mass × acceleration
2. Force =
a) m/v
b) m × a
c) a/m
d) v × t
3. Which law explains inertia?
a) Newton’s 1st law
b) Newton’s 2nd law
c) Newton’s 3rd law
d) Law of gravitation
4. A gun recoils due to:
a) Inertia
b) Conservation of mass
c) Action–reaction
d) Centripetal force
5. Centripetal force acts:
a) Away from center
b) Towards center
c) Upwards only
d) Downwards only
Answers:
1 – b) Displacement/time
2 – b) m × a
3 – a) Newton’s 1st law
4 – c) Action–reaction
5 – b) Towards center
1 – b) Displacement/time
2 – b) m × a
3 – a) Newton’s 1st law
4 – c) Action–reaction
5 – b) Towards center
Quick Recap | सार
Now you should be able to:
- Recall the most important ideas of this topic.
- Solve typical NDA-style questions from this chapter.
- Spot and avoid common traps used in competitive exams.
Self-check: 3–4 random questions उठा कर देखिए – अगर आसानी से हो रहे हैं, तो chapter strong है।
Common Exam Mistakes | आम गलतियाँ
- Concept ठीक से clear न करना और सीधे रटने की कोशिश करना।
- Question पूरी तरह पढ़े बिना answer mark कर देना।
- Figures, dates, या formulas को last step में verify न करना।
NDA में smart strategy यह है कि आसान वाले सवाल पहले, doubtful बाद में – और silly mistakes से बचना सबसे ज़रूरी है।