NotTim
  • 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.
Physics
jamiebookeater
  • jamiebookeater
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bahrom7893
  • bahrom7893
W = m*a When it's going up, acceleration is: g+a, when it's going down acceleration is g-a
bahrom7893
  • bahrom7893
a) total a = -9.8-3.2 = -13 W = 56*(-13)N
NotTim
  • NotTim
You see, here's what I did for a and b.

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bahrom7893
  • bahrom7893
b) total a = -9.8 + 3.2 = -6.6 W = 56*(-6.6)N
NotTim
  • NotTim
a) f=ma =(56)(3.2+9.81) =728.56N
NotTim
  • NotTim
Why is the force negative in both cases?
bahrom7893
  • bahrom7893
because weight is pushing down.
NotTim
  • NotTim
Ok. Give me a second to redo this...
bahrom7893
  • bahrom7893
same thing, just change the signs
bahrom7893
  • bahrom7893
you found the normal force, which has the same magnitude, but opposite direction
NotTim
  • NotTim
And its alright if my signs are negative? Because the answers are A) 3.7*10^2N and b) 7.3*10^2N
bahrom7893
  • bahrom7893
ohh yea they asked for magnitude. If they just asked for weight, ur answers wld be negative
bahrom7893
  • bahrom7893
since weight has magnitude and direction. In this case it's going down, hence the negative.
NotTim
  • NotTim
k. ill work with this. if i need help, ill call out
NotTim
  • NotTim
Hey @bahrom7893 ?
bahrom7893
  • bahrom7893
hey
NotTim
  • NotTim
the answer for A) was wrong. you probably explained this, but i forgot
NotTim
  • NotTim
We got -728.56N. The book says 3.7*10^2N
bahrom7893
  • bahrom7893
Wait in the answer, is the answer for a greater than the answer for b?
NotTim
  • NotTim
No.
bahrom7893
  • bahrom7893
A) 3.7*10^2N and b) 7.3*10^2N Aren't those the answers?
NotTim
  • NotTim
yup
bahrom7893
  • bahrom7893
oh yea.. obv. Hang on I know what we did wrong
NotTim
  • NotTim
for A), i did this: "56)(-3.2-9.81)
bahrom7893
  • bahrom7893
well it makes sense.. i just always confuse the signs of g. g has to be positive in this case.
NotTim
  • NotTim
would that make my value of 3.2N positive too?
bahrom7893
  • bahrom7893
http://www.intuitor.com/student/elevatorrProblem.php
bahrom7893
  • bahrom7893
it's the sign of g, i always get confused with it..
NotTim
  • NotTim
am I to consider 3.2N a positive or negative force?
bahrom7893
  • bahrom7893
well is it pushing down or up?
NotTim
  • NotTim
It's pushing down
NotTim
  • NotTim
If it was -ve, it would make sense mathematically, but not in terms of real world physics(for me)
bahrom7893
  • bahrom7893
then it's negative, but they're just asking for magnitude, or abs value of that force.
bahrom7893
  • bahrom7893
https://docs.google.com/viewer?a=v&q=cache:xp0UsLqIQAEJ:pleasanton.k12.ca.us/avhsweb/barnettdreyfuss/Physics/Documents/ElevatorHelp.pdf+&hl=en&gl=us&pid=bl&srcid=ADGEESgOMe38l7ow_fcC2jk4lULurodnv-4-vaORTq16eM87K7SLxLuGYv2O-HfEbFZyYeZwSx-qhMcpD5rNKvzT9Mr5TpwR1yAX_zgPO91ur3XVwrnSJ547HJqi-Z4aRe3xpDrjv1vn&sig=AHIEtbQpU7-PDOl_gkxu6JaT3QyU2AZ31w
NotTim
  • NotTim
ok. give me a moment to readi guess

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