lecture9

# lecture9 - Apparent Weight We have all had the experience...

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Apparent Weight We have all had the experience of feeling heavier or lighter when traveling in an levator elevator. If the elevator is at rest and then accelerates upward, we feel heavier. If the elevator is at rest and then accelerates downward, we feel lighter. The sensation of weight that we feel comes from the force that the floor of the The motion of the elevator can give rise to an apparent weight which is different from our actual weight. Why? e se sat o o e g t t at e ee co es o t e o ce t at t e oo o t e elevator applies to our feet. That is, it’s the normal force from the elevator floor.

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How do we calculate apparent weight? We use Newton’s 2 nd Law. Let’s look at the FBD for the guy in the elevator: We have his weight ( W ) downward, and the normal force ( F N ) from the floor upward. Newton’s 2 nd Law tells us that: = = y N y ma W F F Here, a y is the acceleration of the man and elevator. F N is the apparent weight that we feel. It’s the ormal force against o r feet normal force against our feet: apparent N W F = pparent ma W W + = ma mg + = ) ( pparent a g m W + = y apparent y y apparent Notice, if a y = 0, i.e. the elevator is not accelerating, then W apparent = mg , your true eight! weight! Remember, g is always a positive number, but a y could be positive or negative. If I cut the rope holding the elevator, then it accelerates downward at a y = - g , and your apparent weight goes to zero! You’re in free-fall!
Einstein’s Theory of General Relativity: The Principle of Equivalence If I drop an apple on the surface of the earth, I measure its acceleration

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## This note was uploaded on 08/04/2009 for the course PHYS 2001 taught by Professor Sprunger during the Fall '08 term at LSU.

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lecture9 - Apparent Weight We have all had the experience...

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