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

Newton's Laws of Motion

Introduction

  • Newton's laws explain the relationship between the motion of an object and the forces acting on it.

  • There are three main laws: the first law (law of inertia), the second law (F=ma), and the third law (action-reaction).

Newton's First Law of Motion

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

  • Example: A box on a flat surface will not move if no force is applied.

    • If the box has a mass of 10 kg, the weight (force) calculated is:

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

    • The normal force, which acts upward from the ground, is also 98 Newtons (balancing the weight force).

  • This means the net force acting on the box is zero, and it remains at rest.

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

    • Example of Motion Against Friction:

      • A ball rolling on a rough surface (like carpet) stops quickly due to friction opposing the motion (net force).

      • Conversely, on an icy surface, less friction allows a puck to slide a longer distance with minimal force applied.

  • Concept of Zero Net Force:

    • If an object moves with constant velocity, it experiences zero net force (therefore zero acceleration).

  • Implication in Space:

    • In the vacuum of space, minimal friction allows objects like planets to maintain their motion indefinitely, as seen with the Earth orbiting the Sun.

Conceptual Applications of Newton's Laws

  • Understanding system force:

    • For objects at rest or moving with constant velocity, the net force is zero.

    • It's essential to relate net force and acceleration: if the net force isn't zero, acceleration must occur.

  • Momentum:

    • Momentum (p) is defined as mass (m) multiplied by velocity (v): p = mv.

Newton's Third Law of Motion

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

  • Example with Basketball:

    • As a person throws a basketball forwards, they recoil backward due to the equal and opposite force exerted by the ball.

    • This demonstrates that the action (throwing) results in a reaction (moving backward).

  • Relationship of Forces:

    • If a force of 200 Newtons is applied to the ball, an equal and opposite 200 Newtons acts on the person.

Applications of Newton's Laws in Problem-Solving

  • Questions on exams often require identifying net forces, applying formulas (F=ma), or utilizing concepts of acceleration.

  • **Example Problems: **

    • Constant Velocity Car: The net force acting on a car with constant velocity is zero, thus its acceleration is also zero.

    • Frictional Example: A car experiences a frictional force of 1500 Newtons; the engine applies the same amount to maintain constant speed.

    • Acceleration Calculation: A 10 kg box experiencing a 200 N force results in:

      • a = F/m = 200/10 = 20 m/s².

  • Explore varying mass and acceleration scenarios to understand how they affect applied force:

    • Increasing mass with constant acceleration reduces acceleration.

    • Express relationship: doubling mass while keeping acceleration constant doubles the force required.

Summary of Key Concepts

  • Newton’s First Law: Inertia - objects resist changes in motion.

  • Newton's Second Law: F = ma - the relationship between force, mass, and acceleration.

  • Newton's Third Law: Action-reactive forces - forces always exist in pairs.

Closing Remarks

  • Engage with practice problems to solidify understanding of these laws.

  • Apply these concepts to real-world and hypothetical scenarios for better comprehension.