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Understanding Common Forces in Physics

Feb 22, 2025

5.6 Common Forces - University Physics Volume 1 | OpenStax

Learning Objectives

  • Define normal and tension forces
  • Distinguish between real and fictitious forces
  • Apply Newton's laws of motion to solve problems involving a variety of forces

Introduction to Forces

  • Forces are often described as push, pull, thrust, and weight.
  • Forces are grouped into categories relating to their source, transmission, or effects.

Catalog of Forces

Normal Force

  • Normal Force: Perpendicular force exerted by a surface to support the weight of an object resting on it.
  • Acts upwards to counteract gravity.
  • Illustrated by objects on horizontal surfaces or inclines.
  • Formula (horizontal surface): ( N = mg )
  • On an incline, normal force is less than the object's weight.

Example: Weight on an Incline

  • Skier's mass: 60.0 kg
  • Acceleration calculations without and with friction (45.0 N)
  • On a frictionless incline: ( a_x = g \sin(\theta) )
  • With friction: reduced acceleration due to opposing force

Tension

  • Tension: Force exerted along the length of a medium like a rope or cable.
  • Acts parallel to the connector and is a pulling force.
  • Example: A person holding a mass on a rope, tension equals the mass's weight.

Example: Tightrope Tension

  • Tightrope walker's weight causes wire tension.
  • Wire tension calculated using trigonometry.

Other Forces

Friction

  • Opposes motion or its tendency.
  • Acts parallel to surfaces in contact.
  • To be discussed in detail in the next chapter.

Spring Force

  • Spring Force: Restoring force proportional to displacement.
  • Described by Hooke's Law: ( F = -kx )
  • Acts in the opposite direction to displacement.

Real Forces vs. Fictitious Forces

  • Real forces have physical origins (e.g., gravity).
  • Fictitious forces arise in non-inertial frames (e.g., rotational effects).
  • Example: Coriolis effect observed in rotating frames like Earth.
  • Inertial frames: All forces are real and align with Newton's laws.

Conclusion

  • Discussed forces are real but not exhaustive.
  • Some forces are more fundamental or manifestations of the same force.
  • More on forces and physics will be covered in later sections.