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2.5.7Force and Translational Dynamics

Apparent Weight — AP PHYSICS 1 Revision Notes & Topical Questions

5 min read 9 parts Lesson 20 of 36 1 quiz

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Lesson summary

2.5.7 Apparent Weight — AP PHYSICS 1 notes: Define apparent weight as the normal force, and find it for accelerating systems and fluids.Apparent weight — the magnitude of the normal force exerted on a systemMeaning — the weight the object appears to have, shown by a scale reading…

  • At rest or constant velocity
  • Buoyant force
  • Elevator in space
  • Non-inertial frames
  • Equivalence principle
  • Resistance

Section 1 of 9Definition

Apparent weight
  • Apparent weight the magnitude of the normal force exerted on a system
Meaning
  • Meaning the weight the object appears to have, shown by a scale reading

Section 2 of 9Non-accelerating systems

  • At rest or constant velocity
    • FN = mg, so apparent weight equals actual weight

Section 3 of 9Accelerating systems (elevator)

Acceleration directionApparent weight
Same as weight (downward)FN = mg − ma, less than actual weight
Opposite to weight (upward)FN = mg + ma, more than actual weight

Section 4 of 9Method

1
Choose the positive direction.
2
Write Fnet = ma for the vertical forces.
3
Solve for the normal force FN.
4
Compare FN with mg.

Section 5 of 9Objects in fluids

Buoyant force
  • Buoyant force equals the weight of the fluid displaced
Apparent weight
  • Apparent weight equals the normal (or supporting) force
Effect
  • Effect immersion reduces the apparent weight

Section 6 of 9Equivalence principle

Statement
  • Statement an observer in a non-inertial frame cannot distinguish apparent weight from gravitational force
Elevator in space
  • Elevator in space accelerating at 10 m s-2 is equivalent to being at rest on Earth where g = 10 m s-2
Non-inertial frames
  • Non-inertial frames frames accelerating relative to an inertial frame, where objects resist the change in motion

Section 7 of 9More from the specification

1Reference frame
  • Coordinate system used by an observer.
2Equivalence principle
  • Acceleration and gravity are indistinguishable locally.
3Newton
  • Unit of force, kg m s-2.
4Resistance
  • Opposition to current, R = V / I, in ohms (Ω).
5Distance
  • Total length of path travelled, a scalar.
6Uniform
  • Same density throughout, so the weight acts at the centre.
7Satellite
  • A body that orbits a larger one; the Moon is the Earth's natural satellite.
8Gravitational field
  • Region where mass experiences gravitational force.
9Field strength
  • G = Fg/m.
10Field lines
  • Lines showing the direction and strength of a magnetic field.

Section 8 of 9Key terms

TermMeaning
Apparent weightMagnitude of the normal force on a system.
Buoyant forceUpward force equal to the weight of displaced fluid.
Equivalence principleAcceleration and gravity are indistinguishable locally.
Non-inertial frameAccelerating reference frame.

Section 9 of 9Exam technique

Avoid these mistakes
Saying a scale shows true weight in an accelerating elevator.
Using the wrong sign for acceleration.
Thinking weightlessness means zero gravity.

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