Science — Force & Newton's Laws

विज्ञान — बल एवं न्यूटन के गति नियम

Force / बल

A force is a push or pull that can change or tend to change the state of motion, direction, or shape of an object.

बल एक धक्का या खिंचाव है जो किसी वस्तु की गति की अवस्था, दिशा या आकार को बदल सकता है।

SI unit:

Newton (N)

Effects of Force

Force can:

1. Start a stationary object moving.
2. Stop a moving object.
3. Change speed.
4. Change direction.
5. Change shape.

बल किसी वस्तु को गति दे सकता है, रोक सकता है, उसकी चाल या दिशा बदल सकता है अथवा उसका आकार बदल सकता है।

Balanced Forces

When net force is zero:

Resultant Force = 0

These are balanced forces.

Unbalanced Force

If resultant force is non-zero, the object's state of motion can change.

Newton's First Law

An object remains at rest or continues moving with uniform velocity in a straight line unless acted upon by an external unbalanced force.

कोई वस्तु विराम में या समान वेग से सीधी रेखा में चलती रहती है जब तक उस पर कोई बाहरी असंतुलित बल कार्य न करे।

This is also called the Law of Inertia.

Inertia / जड़त्व

Inertia is the tendency of a body to resist a change in its state of rest or motion.

किसी वस्तु की अपनी वर्तमान अवस्था में परिवर्तन का विरोध करने की प्रवृत्ति जड़त्व कहलाती है।

Mass is a measure of inertia.

Greater mass → greater inertia

Newton's Second Law

Force is related to the rate of change of momentum.

For constant mass:

F = ma

where:

F = force
m = mass
a = acceleration

Momentum / संवेग

Momentum = Mass × Velocity

p = mv

SI unit:

kg·m/s

Momentum is a vector quantity.

Newton's Third Law

For every action, there is an equal and opposite reaction.

प्रत्येक क्रिया के बराबर और विपरीत दिशा में प्रतिक्रिया होती है।

Important:

Action and reaction act on different bodies.

Examples

Walking

Your foot pushes the ground backward; the ground exerts a forward force on you.

Gun Recoil

Bullet moves forward while gun recoils backward.

Rocket

Gases are expelled backward; rocket accelerates forward.

Swimming

Swimmer pushes water backward; water pushes swimmer forward.

Force Practice — 30 Questions

Q1

SI unit of force:

A. Joule
B. Newton
C. Watt
D. Pascal

Answer: B

Q2

Force is commonly described as:

Answer: Push or pull.

Q3

Newton's First Law is also called:

Answer: Law of Inertia.

Q4

Inertia depends primarily on:

Answer: Mass.

Q5

Greater mass means:

A. Less inertia
B. Greater inertia
C. Zero inertia
D. No relation

Answer: B

Q6

Newton's Second Law gives:

Answer: F=ma for constant mass.

Q7

Momentum equals:

Answer: mv

Q8

SI unit of momentum:

Answer: kg·m/s

Q9

Newton's Third Law concerns:

Answer: Action and reaction.

Q10

Action and reaction are:

A. Equal and same direction
B. Equal and opposite
C. Unequal
D. Always zero

Answer: B

Q11

Action and reaction act on:

Answer: Different bodies.

Q12

A 5 kg body accelerates at 2 m/s². Force?

Answer: 10 N

Q13

Mass=10 kg, acceleration=3 m/s². Force?

Answer: 30 N

Q14

Force=50 N, mass=10 kg. Acceleration?

Answer: 5 m/s²

Q15

Force=24 N, acceleration=4 m/s². Mass?

Answer: 6 kg

Q16

Mass=4 kg and velocity=5 m/s. Momentum?

Answer: 20 kg·m/s

Q17

Mass=10 kg moving at 8 m/s. Momentum?

Answer: 80 kg·m/s

Q18

A stationary object has momentum:

Answer: Zero.

Q19

Passengers tend to fall forward when a moving bus suddenly stops because of:

Answer: Inertia of motion.

Q20

A passenger tends to fall backward when a stationary bus suddenly starts because of:

Answer: Inertia of rest.

Q21

Dust leaves a carpet when it is beaten due to:

Answer: Inertia of rest.

Q22

Recoil of a gun illustrates:

Answer: Newton's Third Law.

Q23

Rocket propulsion illustrates:

Answer: Newton's Third Law.

Q24

Swimming involves:

Answer: Action-reaction forces.

Q25

If net force on a body is zero, acceleration is:

Answer: Zero.

Q26

If a body moves with constant velocity, its net force is:

Answer: Zero.

Q27

Which is a vector?

A. Mass
B. Momentum
C. Time
D. Temperature

Answer: B

Q28

A force changes velocity from 2 m/s to 8 m/s for a 3 kg body in 2 seconds. Force?

Acceleration=(8−2)/2=3 m/s².

Force=3×3

Answer: 9 N.

Q29

A 2 kg object initially at rest reaches 10 m/s in 5 s. Force?

a=2 m/s²

F=2×2

Answer: 4 N.

Q30

1 newton is equal to:

A. 1 kg·m/s
B. 1 kg·m/s²
C. 1 kg/m²
D. 1 J/s

Answer: B