Laws of Motion notes

Laws of Motion explains why objects start moving, stop moving, change direction or remain at rest. Aristotle believed force is necessary to keep a body moving, but Newton showed that force is required to change the state of motion, not to maintain uniform motion. The chapter introduces inertia, mass, force, momentum, impulse, equilibrium, friction and circular motion applications. Newton’s three laws form the core of mechanics and are used in blocks, pulleys, elevators, inclined planes and vehicle motion. For NEET, this chapter is very important because it gives direct conceptual questions, free body diagram problems, friction numericals and multi-body Newton’s law applications.

Force is an interaction that can change the state of motion or shape of a body. Forces may be contact forces such as normal reaction, tension, friction and spring force, or non-contact forces such as gravitational, electrostatic and magnetic forces. Inertia is the natural tendency of a body to resist any change in rest or motion, and mass is its measure. Newton’s first law states that a body remains at rest or in uniform motion unless acted upon by a net external force. The second law relates net force to acceleration, F = ma. The third law states that every action has an equal and opposite reaction on another body.

Linear momentum measures the quantity of motion of a body and is defined as p = mv. It is a vector in the direction of velocity. For a system of particles, total momentum is the vector sum of individual momenta. If no external force acts on a system, its total momentum remains conserved. Impulse is the effect of a force acting for a short time and equals change in momentum. This idea explains why catching a ball softly reduces force by increasing stopping time. Collisions are interactions of short duration where momentum is conserved if external impulse is negligible. Elastic collisions conserve kinetic energy, while inelastic collisions do not.

A particle is in equilibrium when the vector sum of all forces acting on it is zero. This means the particle has no acceleration; it may be at rest or moving with constant velocity. For forces in a plane, equilibrium requires both horizontal and vertical force components to separately add to zero. Many NEET problems involve strings, weights, inclined forces and concurrent forces meeting at one point. Free body diagrams and force resolution are the safest methods. Lami’s theorem is a shortcut for three concurrent, coplanar and non-parallel forces in equilibrium. It relates each force to the sine of the angle between the other two forces.

Friction is a contact force that opposes relative motion or the tendency of relative motion between surfaces. Static friction acts when there is no slipping and adjusts itself according to need up to a maximum called limiting friction. Once sliding begins, kinetic friction acts and is usually slightly smaller than limiting friction. The coefficient of friction measures roughness and equals friction divided by normal reaction in limiting or kinetic cases. Angle of friction is related to the resultant contact force, while angle of repose is the minimum inclination at which a body just begins to slide. Friction is necessary for walking, writing, vehicle motion and braking, but also causes wear and energy loss.

Circular motion applications use Newton’s second law toward the centre of the circle. A body moving in a circle requires centripetal force mv²/r, which is not a new force but the inward resultant of real forces such as tension, friction, gravity or normal reaction. In a vehicle moving on a curved road, friction or banking supplies the required centripetal force. Banking reduces dependence on friction by using a component of normal reaction. A conical pendulum moves in a horizontal circle, where horizontal tension gives centripetal force and vertical tension balances weight. In vertical circular motion, tension and weight change at different points, making top and bottom conditions important.

Problem solving in Laws of Motion depends on selecting the body or system, drawing a correct free body diagram and applying Newton’s second law along suitable axes. For a block on a horizontal surface, horizontal forces decide acceleration and vertical forces decide normal reaction. For an inclined plane, axes along and perpendicular to the plane simplify equations. Connected blocks and pulleys require separate FBDs and constraint relations because connected bodies often have related accelerations. Elevator problems modify apparent weight due to vertical acceleration. Friction-based problems require deciding whether friction is static, limiting or kinetic. Mixed NEET problems usually combine FBD, friction, pulley tension and acceleration equations.