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Force, Free Body Diagram

Force in physics​

Force in physics is an influence that can change the velocity of an object with mass. For intuition, it is like a push or pull on an object. Again, the velocity has both magnitude and direction, so what a force can potentially do:

  • Change the speed of motion
    • make an object accelerate
      • make an object at rest move
    • make an object decelerate
      • make a moving object stop
  • Change the direction of motion
    • make an object turn
    • make an object rotate
    • make an object change shape/size

Force has both magnitude and direction, so it can be modeled as a vector. The SI unit of force is newton (N=kg⋅m/s2N=kg\cdot m/s^2). Here are some typical ways to classify forces.

By practical closeness​

  • Contact force: frictional force, mechanical force, normal force, spring force, ...
  • Non-contact force: gravitational force, electrostatic force, magnetic force, ...

By pairwise stress​

  • Tensile force: a pair of forces stretching the object
  • Compressive force: a pair of forces squeezing the object
  • Shear force: a pair of unaligned forces twisting the object

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By source of origin​

  • External force: force coming from outside the system being considered
  • Internal force: force coming from inside the system being considered

Proposed by the Standard Model​

  • Gravitational force
  • Electromagnetic force
  • Weak nuclear force
  • Strong nuclear force

Free-body diagram​

A free-body diagram is a visual representation of forces acting on an object. It is used to determine the net force and analyze its effect on the motion of objects. Here are two examples:

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