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

Newton's Laws of Motion Overview

  • Key Principles: Newton's laws describe the relationship between the motion of an object and the forces acting on it.

Newton's First Law of Motion

  • Definition: An object at rest stays at rest, and an object in motion continues in motion unless acted upon by a net force.

    • Example: A box on a surface that remains stationary due to balanced forces (weight force and normal force).

      • Weight Force Calculation:

        • Weight = Mass (M) x Gravitational Acceleration (g)

        • For a 10 kg box:

          • Weight = 10 kg x 9.8 m/s² = 98 Newtons

      • The normal force also equals 98 Newtons to maintain equilibrium, resulting in a net force of zero.

    • Practical Example: To move the stationary box, an unbalanced force must overcome static friction.

  • Motion Continuation: An object in motion will not stop unless acted upon by an unbalanced force (like friction).

    • Example: A ball rolled on carpet stops quickly due to friction.

    • On ice, a puck slides for longer distances due to reduced friction and inertia.

Newton's Second Law of Motion

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

    • Formula: F = ma

    • If a mass is constant, increasing acceleration raises net force proportionally and vice versa.

  • Understanding Acceleration:

    • Acceleration is defined as the change in velocity over time: ( a = \frac{V_f - V_i}{t} )

    • With zero net force acting, the object maintains constant velocity; acceleration equals zero.

Momentum

  • Definition: Momentum (p) is the product of mass and velocity.

    • Momentum Formula: ( p = mv )

    • Change in momentum relates to force: ( F = \frac{\Delta p}{\Delta t} )

  • Impulse-Momentum Theorem: Impulse (force × time) is equal to the change in momentum.

Newton's Third Law of Motion

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

    • Example: When a basketball is thrown, the thrower recoils in the opposite direction.

      • Force exerted on the ball equals the force felt by the person throwing it (same magnitude, opposite direction).

Real-life Applications

  1. In Space: An astronaut throwing a ball in one direction experiences a force propelling them in the opposite direction.

  2. Skater Example: A heavier skater pushes a lighter skater, causing the lighter skater to accelerate more than the heavier skater due to differences in mass but equal forces exerted.

Problem-Solving Examples

  1. Car with Constant Velocity:

    • Net force = 0; if friction is counteracted by engine force, acceleration = 0.

  2. Box on Frictionless Surface: A force of 200 N on a 10 kg box results in an acceleration of 20 m/s². (F = ma)

    • Final speed after 8 seconds will be 160 m/s.

  3. Acceleration to Reach Speed: From rest to 500 m/s with constant 20 m/s² acceleration takes 25 seconds.

  4. Net Force Calculation: Applied force minus friction yields net force; then use that to calculate acceleration.

Summary of Key Equations

  • First Law: Net force = 0 suggests constant velocity.

  • Second Law: ( F = ma )

  • Third Law: Action = - Reaction forces.

Conclusion

  • Understanding these laws and principles provides insight into mechanics and helps in problem-solving scenarios in physics.