1. Force:
Definition : Force is a push, pull or hit that can change the state of motion or shape of an object.
What can a force do?
- Make a stationary object move.
- Stop a moving object.
- Increase or decrease speed.
- Change the direction of motion.
- Change the shape or size of an object.
Examples
- Pushing a trolley → trolley moves.
- Pulling a drawer → drawer moves.
- Hitting a hockey ball → its motion changes.
- Stretching a spring → its shape changes.
- Pressing a rubber ball → its shape changes.
| Hinglish: Force matlab kisi object ko push, pull ya hit karna. Force lagane se object ki speed, direction, motion ya shape change ho sakti hai. |
2. Balanced and Unbalanced Forces
Balanced Forces : When two or more forces acting on an object are equal and opposite, they are called balanced forces.
Result: Net force = 0. Balanced forces do not change the state of rest or uniform motion.
Net Force = 0
| Hinglish: Agar opposite directions mein equal forces lag rahe hain, to forces balance ho jaate hain. Net force zero hota hai, isliye motion state change nahi hoti. |
Example: 10 N ← [ BLOCK ] → 10 N
Unbalanced Forces :When the forces acting on an object are not equal, they are called unbalanced forces.
- Can change speed.
- Can change direction.
- Can change the state of motion.
Example: 5 N ← [ BLOCK ] → 10 N; Net Force = 5 N →
| Hinglish: Agar opposite directions mein lagne wale forces equal nahi hain, to unbalanced force hota hai. Object greater force ki direction mein move karega. |
| Important: An object does not require a continuous unbalanced force to maintain uniform motion. If net external force is zero, it can continue moving with uniform velocity. In everyday life, friction often opposes motion. |
3. Newton’s First Law of Motion
An object remains in a state of rest or of uniform motion in a straight line unless compelled to change that state by an applied force.
Simple Meaning
- An object at rest tends to remain at rest.
- An object moving uniformly tends to continue moving uniformly in a straight line.
- An unbalanced external force is required to change the state.
| Hinglish: Agar object rest mein hai, to woh rest mein rehna chahta hai. Agar object uniform velocity se straight line mein move kar raha hai, to woh waise hi move karta rahega jab tak koi unbalanced force uski state change na kare. |
4. Inertia
Inertia is the natural tendency of an object to resist a change in its state of rest or motion.
Inertia = resistance to change
Mass is a measure of inertia.
Hinglish: Inertia kisi object ki woh tendency hai jismein object apni current state ko change karne ka oppose karta hai. Rest mein hai → rest mein rehna chahta hai; motion mein hai → motion mein rehna chahta hai. |
Everyday Examples of Inertia
A. Bus suddenly stops
Passengers fall forward because the upper body tends to continue moving forward.
| Hinglish: Bus suddenly rukti hai, lekin body ka upper part inertia ki wajah se aage move karna chahta hai. |
B. Bus suddenly starts
Passengers fall backward because the upper body tends to remain at rest.
| Hinglish: Bus suddenly start hoti hai. Pair bus ke saath aage move karte hain, lekin upper body rest mein rehna chahti hai. |
C. Sharp turn
Passengers tend to move sideways because the body tends to continue in its original straight-line direction.
| Hinglish: Car turn karti hai, lekin body straight direction mein move karna chahti hai. |
D. Safety belt
Safety belts prevent the body from moving forward suddenly when a car stops.
| Hinglish: Sudden braking ke time body inertia ki wajah se aage ja sakti hai. Seat belt body ko rokne ke liye force provide karta hai. |
5. Inertia and Mass
Different objects have different amounts of inertia. Greater mass means greater inertia.
| Objects | Greater Inertia |
| Rubber ball vs stone of same size | Stone |
| Bicycle vs train | Train |
| ₹1 coin vs ₹5 coin | ₹5 coin |
Greater mass → Greater inertia
| Hinglish: Jitna zyada mass, utni zyada inertia. Isliye train ko move ya stop karna bicycle ke comparison mein difficult hota hai. |
6. Momentum
Momentum is the product of mass and velocity.
p = mv
- p = momentum
- m = mass
- v = velocity
SI unit: kg m s⁻¹
Momentum has magnitude and direction. Its direction is the same as the direction of velocity.
| Hinglish: Momentum object ke mass aur velocity dono par depend karta hai. Agar mass ya velocity badhegi, momentum bhi badhega. |
7. Newton’s Second Law of Motion
The rate of change of momentum of an object is proportional to the applied unbalanced force in the direction of the force.
| Hinglish: Newton ka second law batata hai ki force lagane se momentum change hota hai. Simple form: Force = Mass × Acceleration. |
F = ma
Derivation of F = ma
- Mass = m
- Initial velocity = u
- Final velocity = v
- Time = t
Initial momentum = mu
Final momentum = mv
Change in momentum = mv − mu = m(v − u)
Rate of change of momentum = m(v − u) / t
a = (v − u) / t
Therefore, F = ma
| Hinglish: Momentum ka change time se divide karne par rate of change of momentum milta hai. Kyunki a = (v − u)/t, isliye F = ma. |
8. SI Unit of Force
1 N = 1 kg m s⁻²
One newton is the force that produces an acceleration of 1 m s⁻² in an object of mass 1 kg.
| Hinglish: 1 Newton woh force hai jo 1 kg mass ke object mein 1 m/s² acceleration produce kare. |
9. Useful Forms of the Second Law
F = ma
F = m(v − u) / t
Ft = mv − mu
These forms are important for numerical problems.
10. Applications of the Second Law
Catching a Cricket Ball
A fielder moves the hands backward while catching a fast-moving ball. This increases the stopping time, decreases the rate of change of momentum, and reduces the force.
High Jump
Athletes land on sand or cushioned beds. These increase stopping time and reduce the force of impact.
| Hinglish: Soft sand/cushion body ko gradually stop karta hai. Stopping time badhta hai → force kam hota hai. |
11. Newton’s Third Law of Motion
To every action there is an equal and opposite reaction.
- Equal in magnitude.
- Opposite in direction.
- Act simultaneously.
- Act on different objects.
| Hinglish: Jab ek object doosre object par force lagata hai, to doosra object bhi pehle object par equal magnitude ka opposite direction mein force lagata hai. |
Examples
Walking : We push the ground backward. The ground pushes us forward.
Gun and Bullet : The gun exerts a force on the bullet and the bullet exerts an equal opposite force on the gun, causing recoil.
Sailor and Boat : When the sailor jumps forward, the boat moves backward.
Important: Action and Reaction Do Not Cancel
They do not cancel because they act on different objects.
| Hinglish: Action aur reaction equal aur opposite hote hain, lekin dono same object par nahi lagte. Isliye woh ek doosre ko cancel nahi karte. |
12. Conservation of Momentum
If there is no external unbalanced force, the total momentum of a system remains conserved.
m₁u₁ + m₂u₂ = m₁v₁ + m₂v₂
| Hinglish: Agar system par koi external unbalanced force nahi hai, to collision se pehle aur collision ke baad total momentum same rehta hai. |
13. Recoil of a Pistol
Textbook example:
Bullet mass = 0.02 kg, bullet velocity = 150 m/s, pistol mass = 2 kg.
0.02(150) + 2v = 0
3 + 2v = 0
v = −1.5 m/s
The negative sign means the pistol recoils in the direction opposite to the bullet.
| Hinglish: Bullet aage momentum lekar jaati hai. Momentum conserve karne ke liye pistol opposite direction mein recoil karti hai. |
14. Girl and Cart Example
Initial momentum = 40(5) + 3(0) = 200 kg m/s
(40 + 3)v = 200
v = 4.65 m/s
| Hinglish: Girl cart par jump karti hai aur dono ek saath move karte hain. Momentum conserve hone ki wajah se final velocity 4.65 m/s hai. |
15. Important Numerical Formulas
- Momentum: p = mv
- Acceleration: a = (v − u) / t
- Force: F = ma
- Force from momentum change: F = (mv − mu) / t
- Momentum-change form: Ft = mv − mu
- Conservation: m₁u₁ + m₂u₂ = m₁v₁ + m₂v₂
16 .Common Mistakes
Mistake: A continuous force is required to keep an object moving. Correct: Uniform motion can continue when net external force is zero.
Mistake: Balanced force means the object is always at rest. Correct: It may also be moving with uniform velocity.
Mistake: Action and reaction cancel. Correct: They act on different objects.
Mistake: Mass and inertia are unrelated. Correct: Mass is a measure of inertia.
Mistake: Momentum has only magnitude. Correct: Momentum has magnitude and direction.
Final Exam Revision
- First Law → Inertia
- Second Law → F = ma
- Third Law → Equal and opposite reaction
- Collision → Momentum is conserved
- Mass is a measure of inertia.
- Momentum = mass × velocity.
- 1 N = 1 kg m s⁻².
- Action and reaction act on different objects.
- Momentum is conserved when no external unbalanced force acts on the system.
Notes : Download here