Ch 3Laws of Motion — Class 11 Physics, concept-first.

Every object in the universe interacts, in some way, with every other object around it -- a cool breeze interacts with a tree, the tree in turn interacts with the Earth beneath it, and every living sp…

Building on Galileo's insight that motion does not require a continuously applied force, Newton formulated three precise laws that together form the complete foundation of classical mechanics:

Statement. Every object continues in its state of rest, or of uniform motion in a straight line (constant velocity), unless an external force acts on it.

Statement. The force acting on an object equals the rate of change of its momentum:

Statement. Whenever an object 1 exerts a force on an object 2, object 2 simultaneously exerts an equal and opposite force back on object 1: Both forces act along the line joining the two objects.

This subsection draws out several important consequences of the three laws.

Having stated the three laws, this section develops the standard toolkit for applying them to real mechanical systems: drawing a free body diagram, analysing motion on an inclined plane, analysing blo…

A free body diagram (FBD) is the essential first tool for analysing motion with Newton's laws. It is built with the following systematic steps:

For a block of mass sliding on a frictionless incline at angle to the horizontal, two forces act: the weight (vertically down) and the normal force (perpendicular to the incline surface).

Consider two blocks of masses and () placed in direct contact on a smooth (frictionless) horizontal surface, and pushed together by a single external horizontal force applied to the outer face of one…

When two masses are joined by a light, inextensible string over a frictionless, massless pulley, analysing each mass separately (its own free body diagram) and then combining the resulting equations g…

A set of forces is said to be concurrent if all their lines of action pass through one common point (they need not lie in the same plane); when concurrent forces do lie in one plane, they are called c…

Statement. If three coplanar, concurrent forces keep a point in equilibrium, then the magnitude of each force is proportional to the sine of the angle between the other two, with the same constant of…

There are three fundamental conservation laws in mechanics — of total energy, total linear momentum, and total angular momentum.

When a very large force acts for a very short time (a bat striking a ball, a foot kicking a ball), it is called an impulsive force, and the physically useful quantity is its impulse — the accumulated…

If even a very gentle force parallel to the surface is applied to an object resting on a table, it may not move at all — an opposing force from the surface resists this tendency to slide.

Before an object begins to slide, the applied force parallel to the surface is met by an equal and opposite resisting force from the surface — this is the frictional force.

Static friction () is the force that opposes the initiation of sliding motion of an object resting on a surface.

Once the applied force exceeds the maximum static friction and the object begins to slide, the surface continues to exert an opposing frictional force — now called kinetic friction (, also called slid…

When an object is pushed at an angle to the horizontal (force directed into the surface as well as along it), the applied force resolves into (parallel to the surface) and (perpendicular, pressed into…

The angle of friction is defined geometrically: it is the angle between the normal force and the resultant of the normal force and the maximum static friction force (i.e. ).

Consider a block placed on a plane inclined at angle to the horizontal. For small , the block stays put; as is increased, at some critical angle the block just begins to slide down — this critical ang…

Two everyday applications of the angle of repose:

The invention of the wheel was pivotal to human civilization precisely because rolling motion involves far less friction than sliding motion.

Friction is not purely a nuisance — it is essential for walking, for vehicles to move (tyre-road friction), and for braking systems (kinetic friction between brake pads and wheel).

Having covered linear motion under Newton's laws, this final section turns to circular motion. A crucial distinction: linear motion can occur with zero force (Newton's first law), but circular motion…

For a particle in uniform circular motion (constant speed, radius ), the centripetal acceleration is , directed toward the centre of the circle.

For a vehicle of mass moving at speed on a level (unbanked) circular road of radius , three forces act: weight (down), normal force (up), and — critically — the static friction between tyre and road,…

Since a leveled road's safe turning speed is capped entirely by the (often unreliable, weather-dependent) coefficient of friction, real roads and railway tracks instead raise the outer edge relative t…

Circular motion can be analysed from two very different vantage points: (i) an inertial frame (at rest, or moving at constant velocity, watching the rotation from outside), where Newton's laws hold in…

Although centrifugal force is fictitious (pseudo), its effects, as experienced by an observer in the rotating frame, are entirely real and consequential.

Although the Earth is usually treated as (approximately) an inertial frame, it is strictly non-inertial — it spins about its own axis with angular velocity ( hours s, so ).

Both centripetal and centrifugal force have identical magnitude, , but represent fundamentally different physical descriptions of the same circular motion, depending purely on which reference frame (i…

This unit's key results, gathered together:

The unit's concept map traces the logical structure of the chapter: Aristotle's ("force motion") view is contrasted with Galileo's ("force change of motion") view, which crystallises into Newton's fir…

This closing exercise set tests every idea developed in the unit, across five question formats:

Sample papers and previous-year board questions for this subject.