Lesson 4.15: Vector Analysis and Forces Acting on an Object

Aug 13, 2024

Lecture Notes: Physics of Friction and Motion

Introduction

  • Discussion on a block of wood with a mass of 5 kilograms on dirt near the Earth's surface.
  • Coefficients of friction:
    • Static friction: 0.60
    • Kinetic friction: 0.55
  • Force applied: 100 Newtons

Concepts of Motion Without Friction

  • If no friction existed:
    • Only the 100 Newton force would act.
    • Net force = 100 Newtons.
    • Formula: Force = Mass x Acceleration (F = ma).
    • Acceleration = Force/Mass = 100 N / 5 kg = 20 m/s² (rightward).

Understanding Friction

  • Static Friction: The force needed to start moving an object.
  • Kinetic Friction: The force acting against motion once the object is moving.
  • Friction is always opposing the direction of applied force.
  • Normal Force: Equivalent to gravitational force when the block is stationary (49 Newtons).

Calculations for Static Friction

  • Budging Force: Force needed to overcome static friction.
  • Gravitational Force: 9.8 m/s² x 5 kg = 49 Newtons down.
  • Normal Force: Balances gravitational force = 49 Newtons up.
  • Budging Force Calculation:
    • 49 Newtons x 0.60 = 29.4 Newtons.
    • 100 Newtons is sufficient to overcome static friction.

Transition from Static to Kinetic Friction

  • Static Friction: Initial net force = 100 N - 29.4 N = 70.6 N (right).
  • Acceleration: 70.6 N / 5 kg = 14.12 m/s² (right).
  • Kinetic Friction: Starts once the block is in motion.

Calculations for Kinetic Friction

  • Kinetic Friction Force: 49 N x 0.55 = 26.95 Newtons.
  • Net Force when moving: 100 N - 26.95 N = 73.05 Newtons (right).
  • Acceleration when moving: 73.05 N / 5 kg = 14.61 m/s² (right).

Key Takeaways

  • Overcoming static friction requires more force than maintaining motion due to kinetic friction.
  • Initial acceleration is slightly lower due to static friction, but once moving, acceleration increases due to lower kinetic friction.
  • Understanding these principles is crucial for analyzing motion and force interactions in physics.

Conclusion

  • The lecture provides an in-depth understanding of how friction affects motion and the calculations involved in predicting the behavior of objects subject to forces on Earth.