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Physics Revision Notes: Force, Momentum, and Newton's Laws

This lecture covers the fundamental concepts of force, including its definition, SI and CGS units, and conversion factors. It explains the classification of contact and non-contact forces, types of fundamental forces, levers, centripetal and centrifugal forces, torque, and moment of inertia. Finally, it explores Newton's three laws of motion and friction through conceptual questions and practical examples.

Teaching
Physics
English · 6 topics
Checked against the video on 25 Oct 2026

Made by AI from the YouTube video UP SUPERTET SCIENCE Class-4 | बल (FORCE) | TOPIC WISE VIDEO | उ.प्र. प्राथमिक शिक्षक भर्ती 2026 (opens YouTube in a new tab) by Balaji Study Adda, then checked against it. All credit for the teaching goes to the creator. Tap a topic’s time to confirm a fact in the video.

Physics Revision Notes: Force, Momentum, and Newton's Laws

English6 topics · 6 MCQs · 14 flashcardsWatch the video (opens YouTube in a new tab)

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In short

This lecture covers the fundamental concepts of force, including its definition, SI and CGS units, and conversion factors. It explains the classification of contact and non-contact forces, types of fundamental forces, levers, centripetal and centrifugal forces, torque, and moment of inertia. Finally, it explores Newton's three laws of motion and friction through conceptual questions and practical examples.

Definition and Units of Force

A force is an external push or pull that changes or tends to change the state of motion or shape of an object.
Must knowThe SI (MKS) unit of force is the newton (N), which equals 1 kg·m/s².
In the CGS system, the unit of force is the dyne.
Must know1 newton = 10⁵ dyne, and conversely, 1 dyne = 10⁻⁵ newton.

System	Unit of Force	Symbol	Extended Value / Relation
SI (MKS)	newton	N	1 N = 1 kg·m/s²
CGS	dyne	dyne	1 N = 10⁵ dyne

Classification and Fundamental Types of Forces

Contact forces act through physical touch and include muscular force, frictional force, tension force, normal reaction, air resistance, and fluid resistance.
Non-contact forces act at a distance and include gravitational force, magnetic force, and electrostatic force.
Gravitational force is an attractive force acting between any two bodies with mass in the universe.
Electromagnetic force comprises electrostatic force between two static charges and magnetic force between two magnetic poles.
Weak nuclear force acts during radioactive beta decay between electrons and antineutrinos, having an intensity roughly 10⁻¹³ times that of strong nuclear force.
Must knowStrong nuclear force is the powerful short-range attractive force that binds protons and neutrons inside an atomic nucleus, operating over a range of about 10⁻¹⁵ m.

Muscular Force, Levers, and Circular Motion Forces

Muscular force is the force exerted by the muscles of humans and animals to perform physical work.
A simple machine is a device operating on the principle of moment of force to do work conveniently.
A lever is a simple rigid or curved rod free to rotate about a fixed pivot point consisting of three points: fulcrum, effort, and load.
Must knowIn a first-class lever, the fulcrum is between the load and effort (e.g., scissors, hand pump handle, seesaw).
Must knowIn a second-class lever, the load is between the fulcrum and effort (e.g., nutcracker/sarota, lemon squeezer).
In a third-class lever, the effort is applied between the fulcrum and load (e.g., pair of tongs/chimta, farmer's plough).
Centripetal force acts on a body traversing a circular path and is directed inward toward the center of the circle (F = mv²/r).
Centrifugal force is a pseudo/reaction force directed outward away from the center, utilized in devices like a cream separator machine.

Class of Lever	Middle Component	Examples
First Class	Fulcrum	Scissors, hand pump, seesaw
Second Class	Load	Nutcracker (sarota), lemon squeezer
Third Class	Effort	Tongs (chimta), farmer's plough

Torque, Moment of Inertia, and Types of Friction

Torque (moment of force, τ) is the product of applied force and perpendicular distance from the axis of rotation (τ = F × L), measured in newton·meter.
Moment of inertia (I) is the property of a body by which it resists changes to its rotational motion about an axis (I = mr²).
Balanced forces occur when multiple forces act on a body such that their net resultant force is zero.
Unbalanced forces produce a non-zero resultant force, causing a change in the body's speed or direction of motion.
Static friction balances an applied force on a stationary body, and its maximum value before motion begins is called limiting friction.
Kinetic friction opposes relative motion between moving contacting surfaces and is less than limiting friction.
Rolling friction occurs when an object rolls over a surface; rolling friction is the weakest, while static friction is the strongest.
Friction is highest between solid-solid surfaces and lowest between air-solid surfaces.

Mass, Weight, and Newton's Laws of Motion

Mass is the quantitative measure of a body's inertia; it remains constant regardless of whether the object is on Earth or the Moon.
Weight (W) is the force with which Earth attracts an object toward its center (W = mg), which varies with gravitational acceleration.
Must knowNewton's first law of motion is also known as the law of inertia, stating that a body remains at rest or in uniform motion unless acted upon by an external unbalanced force.
Must knowNewton's second law of motion gives the measure and definition of force: the rate of change of momentum is directly proportional to applied unbalanced force (F = ma).
Momentum (p) is the product of mass and velocity (p = mv), having the SI unit kg·m/s.
If a body is stationary (v = 0), its momentum is zero; if velocity is halved, momentum is also halved.
Acceleration of a body is zero when no external unbalanced force acts on it.
When two bodies move at the same speed, the body with greater mass has higher momentum.

Applications of Newton's Third Law and Inertia

Must knowNewton's third law of motion states that to every action there is always an equal and opposite reaction acting on two different bodies.
Rocket propulsion functions on the principle of conservation of momentum, which is governed by Newton's third law.
The recoil of a rifle when a bullet is fired illustrates Newton's third law.
Swimming in water is enabled by pushing water backward, generating an equal forward reaction.
When pushing a stationary truck that does not move, the applied force is balanced by an equal opposing static friction force between the tyres and road.
Jumping out of a moving vehicle is dangerous because a person's feet abruptly stop upon hitting the ground while the upper body continues moving due to inertia of motion.
A boat moves backward when a sailor jumps forward onto the bank due to the reactionary force.
A ceiling fan continues to rotate for some time after being switched off due to the property of inertia.

Key terms

Force: An external influence that changes or tends to change the state of rest, motion, or shape of an object
Inertia: The inherent tendency of a body to resist any change in its state of rest or uniform motion
Momentum: The product of the mass and velocity of a moving object (p = mv)
Torque: The rotational equivalent of linear force, given by force multiplied by perpendicular distance from the pivot (τ = F × L)
Moment of Inertia: The measure of a body's resistance to angular acceleration about an axis of rotation (I = mr²)
Limiting Friction: The maximum static friction force exerted just before a body begins to slide
Centripetal Force: An inward-directed force acting on an object traveling along a circular path toward the center of curvature
Centrifugal Force: An apparent outward force experienced by an object moving along a curved path in a rotating frame

Formulas

F = ma: Newton's second law relating force, mass, and acceleration
p = mv: Linear momentum of a body
J = F × t: Impulse exerted on a body over a time interval
J = Δp: Impulse equals change in momentum
W = mg: Weight of an object under gravitational acceleration
F = mv² / r: Centripetal force acting on an object moving in a circle of radius r
τ = F × L: Torque (moment of force) about an axis
I = mr²: Moment of inertia of a point mass about an axis

Quick revision

Read these the night before the exam.

The SI unit of force is newton (N) which equals 1 kg·m/s².
1 newton equals 10⁵ dyne, and 1 dyne equals 10⁻⁵ newton.
Strong nuclear force operates across an extremely short range of approximately 10⁻¹⁵ m.
Scissors and hand pumps are first-class levers because the fulcrum lies in the middle.
A nutcracker (sarota) and lemon squeezer are second-class levers where the load is positioned in the middle.
A pair of tongs (chimta) is a third-class lever where the effort is applied in the middle.
Rolling friction has the minimum value among friction types, whereas static friction has the maximum.
Newton's first law of motion defines the property of inertia.
The SI unit of momentum is kg·m/s, and momentum is a vector quantity.
Rocket propulsion is based on the principle of conservation of momentum and Newton's third law.
Action and reaction forces always act simultaneously on two different bodies.

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