Newton's Law of Motion - First, Second & Third - Physics

Newton's Laws of Motion

Introduction

  • Newton's laws of motion describe the relationship between the motion of objects and the forces acting on them.

  • There are three primary laws: First Law, Second Law, and Third Law.

Newton's First Law of Motion

  • Definition: An object at rest will remain at rest, and an object in motion will continue in motion at a constant velocity, unless acted on by a net unbalanced force.

    • Object at Rest: If a box (10 kg) is on a surface and no force is applied, it won't move.

      • Weight Force Calculation:

        • Weight force (FW) = M * g = 10 kg * 9.8 m/s² = 98 Newtons.

      • Normal Force: Equal and opposite to the weight force, also 98 Newtons for the box at rest.

    • Balanced Forces: If the net force is zero, the box remains at rest.

    • Unbalanced Forces: If force applied (e.g., overcoming static friction), the box can move.

Newton's Second Law of Motion

  • Definition: The acceleration of an object is directly proportional to the net force acting on it and inversely proportional to its mass.

    • Formula: F = ma

      • Example on Friction: A ball rolls on a carpet (high friction), quickly stops due to opposing friction.

      • On ice (low friction), a puck or ball continues to move for a longer distance.

    • Constant Velocity: If an object moves at a constant velocity, net force is zero; if net force is not zero, there’s no acceleration.

    • Acceleration: Defined as a change in velocity due to net force, if velocity changes then there’s acceleration.

      • Example Calculation of Force:

        • If mass doubles and acceleration remains constant, force also doubles.

        • If mass increases, acceleration decreases under constant force.

Relationship Between Mass, Acceleration, and Force

  • Increasing mass leads to increased force for the same acceleration.

  • If the net force is constant, increasing mass results in decreased acceleration.

Newton's Third Law of Motion

  • Definition: For every action, there is an equal and opposite reaction.

    • Examples:

      • When a person jumps and throws a basketball, the basketball moves forward while the person feels a reaction force. Acceleration differs due to mass.

      • In a boat scenario, throwing a ball forward causes the boat to move backward due to reaction forces.

      • In space, throwing a ball towards the Moon results in movement towards Earth due to the equal and opposite reaction.

Practical Problems and Applications

Example: A car travels on the road with a constant velocity.

  • What is the horizontal net force acting on the car?

    • Anytime an object is moving with constant velocity, the net force is always equal to zero.

  • What is the acceleration on the car?

    • If net force is zero, acceleration is also equal to zero.

Summary of Newton's Laws

  • First Law: An object at rest or in constant motion remains in that state unless acted upon by an unbalanced force.

  • Second Law: F = ma; the net force equals mass times acceleration, dictating how factors interact.

  • Third Law: Action and reaction forces are equal in magnitude and opposite in direction, influencing movements.

Final Notes

  • Understanding forces helps predict outcomes in real-life scenarios as demonstrated through practical examples.

  • Review applications of each law for problem-solving in physics.