Understanding Force in Physics

What is "Force"?

• A push or pull on an object
• Has magnitude (size) and direction
• Many different forces can act on an object at the same time
• Measured in Newtons (N)

Some Common Forces

• Gravitational Force (FG) - acts on all objects that have mass
  • directed towards the center of the Earth
• Friction (FF)
  • Resistive force that opposes motion or intended motion
  • Acts between two surfaces in contact
  • Depends on the type of surfaces
• Normal Force (FN)
  • Perpendicular force that acts in response to a contact force
  • Typically, a surface pushing up on an object
• Tension (FT)
  • Force from a rope, cable, or similar - can only pull, not push
• Water/Air Resistance (FDrag)
  • Opposes the motion of an object
  • Depends on the viscosity of the fluid and the area of the moving object
• Bouyancy (FB)
  • Upward force acting on an object submerged in a fluid
  • Equal to the weight of the fluid displaced
• Spring Force (FSp)
  • Acts when a spring is compressed or stretched, opposing the change in length
• Applied Force (FApplied)
  • Any external force that isn't specifically described
• It is ok to ignore air resistance unless it is mentioned or is a significant force

Free Body Diagram

• Tool used to illustrate the forces acting on a object
• Each force is drawn as an arrow coming FROM the object
• The length of the arrow corresponds to the strength of the force

### Newton's Three Laws

• Written by Isaac Newton in his book "Philosophiæ Naturalis Principia Mathematica" (1687)

Newton's First Law of Motion

• "An object either remains at rest or continues to move at a constant velocity, unless acted upon by a force."
• This is also known as the Law of Inertia:
• Balanced Forces - object remains at rest.
• Unbalanced Forces - object accelerates