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Weight Acceleration of Gravity Objects that fall to the Earth all experience an acceleration. The acceleration due to gravity is g = 9.8 m/s 2 . This acceleration must be due to a force. The acceleration of a falling mass m is - g .

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Presentation Transcript
acceleration of gravity
Acceleration of Gravity
  • Objects that fall to the Earth all experience an acceleration.
  • The acceleration due to gravity is g = 9.8 m/s2.
  • This acceleration must be due to a force.
force of gravity
The acceleration of a falling mass m is -g.

The force on the mass is found from F = ma (action).

This gravitational force is F = -mg.

Force of Gravity

Kinematic view

Dynamic view

gravity at rest
Gravity at Rest
  • The force of gravity acts on all objects all the time.
  • If an object is at rest, the law of inertia says that the net force is zero.
  • There must be a force opposite to gravity that cancels it out.

F = -Fgrav =mg

Fgrav = -mg

normal force
Normal Force
  • The force that opposes gravity for objects on the ground is called the normal force.
  • It is perpendicular (normal) to the plane of the ground.
  • The force is a result of the law of reaction.

FN = mg

Fsb = -Fbs =mg

normal force and weight
Normal Force and Weight
  • The normal force pushing up against gravity can be measured.
  • We measure weight with a scale that measures normal force.
  • Weight is a force, not a mass.
  • Pounds measure weight, so force (not mass) can be measured in pounds.
accelerated weight
Accelerated Weight
  • An elevator is accelerating downward at 2.0 m/s2.
  • The person has a mass of 70 kg.
  • What weight is on the scale?
  • Add all the forces, but the net force is –ma = FN – mg.
  • Solve for FN = m (g – a)
  • The scale shows 56 kg.
weightlessness
Weightlessness
  • If the elevator accelerated downward at g, the normal force would become 0.
  • The person would feel weightless.
  • An object in free fall is weightless, but not massless.

Microgravity research at NASA

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