Weight and Apparent Weight

  • Consider a man standing on a weighing scale.

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  • The Earth exerts a gravitational pull of magnitude mg on the man.
  • The man in turn exerts a downward normal contact force N‘ on the weighing scale.
  • What the weighing scale measures, is the normal contact force N‘ (or the action-reaction pair N), not the gravitational pull mg.
  • Even though these two force often have the same magnitude, they are fundamentally two different forces.
  • The term weight can be confusing because it can refer to either/both of these two forces.
  • Some people think of the gravitational pull as the true weight, and the contact force as the apparent weight.

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  • In fact, apparent weight is equal to true weight only when the man is at rest or travelling at constant speed.
  • If the man is accelerating upward, it must be because N > mg, so the apparent weight will be higher than the true weight.
  • If the man is having a downward acceleration, it must be because N < mg, so the apparent weight will be lower than the true weight.

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