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.

- NotTim

- jamiebookeater

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- bahrom7893

W = m*a
When it's going up, acceleration is: g+a, when it's going down acceleration is g-a

- bahrom7893

a) total a = -9.8-3.2 = -13
W = 56*(-13)N

- NotTim

You see, here's what I did for a and b.

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## More answers

- bahrom7893

b) total a = -9.8 + 3.2 = -6.6
W = 56*(-6.6)N

- NotTim

a) f=ma
=(56)(3.2+9.81)
=728.56N

- NotTim

Why is the force negative in both cases?

- bahrom7893

because weight is pushing down.

- NotTim

Ok. Give me a second to redo this...

- bahrom7893

same thing, just change the signs

- bahrom7893

you found the normal force, which has the same magnitude, but opposite direction

- 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

ohh yea they asked for magnitude. If they just asked for weight, ur answers wld be negative

- bahrom7893

since weight has magnitude and direction. In this case it's going down, hence the negative.

- NotTim

k. ill work with this. if i need help, ill call out

- NotTim

Hey @bahrom7893 ?

- bahrom7893

hey

- NotTim

the answer for A) was wrong. you probably explained this, but i forgot

- NotTim

We got -728.56N. The book says 3.7*10^2N

- bahrom7893

Wait in the answer, is the answer for a greater than the answer for b?

- NotTim

No.

- bahrom7893

A) 3.7*10^2N and b) 7.3*10^2N
Aren't those the answers?

- NotTim

yup

- bahrom7893

oh yea.. obv. Hang on I know what we did wrong

- NotTim

for A), i did this: "56)(-3.2-9.81)

- bahrom7893

well it makes sense.. i just always confuse the signs of g. g has to be positive in this case.

- NotTim

would that make my value of 3.2N positive too?

- bahrom7893

http://www.intuitor.com/student/elevatorrProblem.php

- bahrom7893

it's the sign of g, i always get confused with it..

- NotTim

am I to consider 3.2N a positive or negative force?

- bahrom7893

well is it pushing down or up?

- NotTim

It's pushing down

- NotTim

If it was -ve, it would make sense mathematically, but not in terms of real world physics(for me)

- bahrom7893

then it's negative, but they're just asking for magnitude, or abs value of that force.

- 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

ok. give me a moment to readi guess

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