Laws of Motion – JEE Mains Physics

• by Zenith LMS
• Updated April 2025

1. Force and Inertia

• Force: An interaction that changes or tends to change the state of motion of a body.
• Inertia: The natural tendency of a body to resist any change in its state of rest or motion.

2. Newton’s First Law of Motion

• Also known as the Law of Inertia.
• An object remains at rest or in uniform motion unless acted upon by an external force.

3. Momentum

• Momentum (p) = mass (m) × velocity (v).
• It is a vector quantity and is conserved in isolated systems.

4. Newton’s Second Law of Motion

• Force is the rate of change of momentum.
• In formula form:

  F = ma

  where F is force, m is mass, and a is acceleration.

5. Impulse

• Impulse is the product of force and time interval during which the force acts.
• Impulse = Change in momentum.
• Units: Newton-second (Ns).

6. Newton’s Third Law of Motion

• For every action, there is an equal and opposite reaction.
• Action and reaction forces act on different bodies.

7. Law of Conservation of Linear Momentum

• The total linear momentum of an isolated system remains constant if no external force acts on it.
• Applications:
  • Recoil of a gun.
  • Rocket propulsion.

8. Equilibrium of Concurrent Forces

• Concurrent forces act at a single point.
• For equilibrium:
  • Sum of all forces in horizontal and vertical directions must be zero.

9. Static and Kinetic Friction

• Static Friction: Friction that prevents relative motion between two surfaces.
• Kinetic Friction: Friction acting when two surfaces are in relative motion.

10. Laws of Friction

• Friction is independent of the area of contact.
• Friction is proportional to the normal force.
• Coefficient of friction (μ) relates frictional force (F) and normal force (N):

  F = μN

11. Rolling Friction

• Occurs when a body rolls over a surface.
• Rolling friction is generally much smaller than static or kinetic friction.

12. Dynamics of Uniform Circular Motion

• An object moving in a circle at constant speed experiences a centripetal force directed towards the center of the circle.
• Centripetal force (F_c) is given by:

  Fc = (mv²)/r

  where m = mass, v = speed, r = radius of circle.

13. Applications of Centripetal Force

• Vehicle on a Level Circular Road:
  • Friction provides the necessary centripetal force to turn the vehicle.
• Vehicle on a Banked Road:
  • Banking of roads reduces the dependence on friction and provides safer turns at higher speeds.