Gravity and Weight: Understanding the Relationship between Mass, Force, and Acceleration, Slides of Physics

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Apparent 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.

Normal Weight

 We measure weight with a scale that measures normal force.

• W = mg

 Weight is related to mass by the gravitational field g.

False Weight

 A vertical acceleration can change the weight.

 The normal force on the floor is our sense of weight.

• Downward acceleration reduces weight
• Upward acceleration increases weight

 Mass is unchanged.

 Newton’s law of acceleration  F = ma  net force, F = - mg + FN.

 Solve for the normal force  - mg + FN = ma  FN = ma + mg  FN = m ( a + g )

 Apparent mass based on g  m (^) app = FN / g

Weightlessness

 If the elevator accelerated downward at g , the normal

force would become 0.

• FN = m ( g – a ) = m ( g – g ) = 0

 The person would feel weightless.

 An object in free fall is weightless, but not massless.

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