NotTim
Determine the magnitude of the apparent weight of a 56 kg student standing in an elevator when the elevator is experiencing an acceleration of (a) 3.2 m/s^2 downward and (b) 3.2 m/s^2 upward.
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bahrom7893
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W = m*a
When it's going up, acceleration is: g+a, when it's going down acceleration is g-a
bahrom7893
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a) total a = -9.8-3.2 = -13
W = 56*(-13)N
NotTim
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You see, here's what I did for a and b.
bahrom7893
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b) total a = -9.8 + 3.2 = -6.6
W = 56*(-6.6)N
NotTim
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a) f=ma
=(56)(3.2+9.81)
=728.56N
NotTim
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Why is the force negative in both cases?
bahrom7893
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because weight is pushing down.
NotTim
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Ok. Give me a second to redo this...
bahrom7893
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same thing, just change the signs
bahrom7893
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you found the normal force, which has the same magnitude, but opposite direction
NotTim
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And its alright if my signs are negative? Because the answers are A) 3.7*10^2N and b) 7.3*10^2N
bahrom7893
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ohh yea they asked for magnitude. If they just asked for weight, ur answers wld be negative
bahrom7893
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since weight has magnitude and direction. In this case it's going down, hence the negative.
NotTim
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k. ill work with this. if i need help, ill call out
NotTim
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Hey @bahrom7893 ?
bahrom7893
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hey
NotTim
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the answer for A) was wrong. you probably explained this, but i forgot
NotTim
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We got -728.56N. The book says 3.7*10^2N
bahrom7893
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Wait in the answer, is the answer for a greater than the answer for b?
NotTim
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No.
bahrom7893
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A) 3.7*10^2N and b) 7.3*10^2N
Aren't those the answers?
NotTim
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yup
bahrom7893
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oh yea.. obv. Hang on I know what we did wrong
NotTim
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for A), i did this: "56)(-3.2-9.81)
bahrom7893
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well it makes sense.. i just always confuse the signs of g. g has to be positive in this case.
NotTim
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would that make my value of 3.2N positive too?
bahrom7893
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it's the sign of g, i always get confused with it..
NotTim
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am I to consider 3.2N a positive or negative force?
bahrom7893
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well is it pushing down or up?
NotTim
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It's pushing down
NotTim
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If it was -ve, it would make sense mathematically, but not in terms of real world physics(for me)
bahrom7893
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then it's negative, but they're just asking for magnitude, or abs value of that force.
NotTim
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ok. give me a moment to readi guess