Question

When you weigh yourself on good old terra firma (solid ground), your weight is 130 lb. In an elevator your apparent weight is 100 lb. What are the direction and magnitude of the elevator's acceleration?
I know that the direction is downward because your weight it more than the apparent weight.
I know that Wa(apparent weight)=m(g-a) but I am confused on how to incorporate your weight as well?

Answer 1

I'm confused why they would do this question in lbs and not in kgs.

But either way, the 'm' in the formula is mass.

Weight = mass * gravity.

Answer 2

yeah i think that is why i am getting confused. But do I just use the apparent weight or do I have to include calculations of the actual weight?

Answer 3

130 = mg
100 = ma

130/g = 100/a

a = g 100/130

lb/lb cancels and we are left to play with the acceleration in whatever units g is in .

Answer 4

or am i underthinking this?

Answer 5

so the masses equal each other?

Answer 6

of course, the person hasnt changed ... their mass remains relatively constant inside and outside an elevator

Answer 7

I got 7.546 but it isnt correct.

Answer 8

i get 24.6 ish, ft/ sec^2

Answer 9

the acceleration is in m/s^2

Answer 10

you sure it is? if so then convert

130lbs = 578.26881 N

100 lbs = 444.82216 N

Answer 11

assuming g = 9.8; a = 7.538

some use g=10 .. what do you spose we should use?

Answer 12

if we are given lbs i think the answer is in ft/sec^2

Answer 13

Nope definitely wants it in m/s^2. This is the last question that I have to do and I cannot get it. I honestly have no idea...

Answer 14

can you attach a picture of it? or a link?

Answer 15

No picture :/ sorry. Its just a word problem and thats all it gives

Answer 16

well, converting to Newtons should have done it then

A = mg ; m = A/g
B = ma ; m = B/a

a = g B/A

it might seem simplistic, but a diagram might help? dunno

Answer 17

did you include the negative? or is there a different downward indicator?

Answer 18

and for the record, i get 7.538 using lbs and newtons. either way

9.8 (100/130) = 9.8 (444.82216 / 578.26881) = 7.538...

Answer 19

but it is going down and isnt acceleraton positive when going downward.

Answer 20

Idk the program says that it answer differs by more than 100% so maybe it is negative?

Answer 21

acceleration is negative (by convention/tradition)

when making an equation for motion, we have to start with a negative acceleration to suggest that its pulling us downward.

Answer 22

spose we toss a ball upwards

a = -g
v = -gt + k
d = -1/2 g t^2 +kt +c

for some given initial upward velocity, k, and initial height, c.

Answer 23

but the elevator is moving downwards so a=g right?

Answer 24

its accelerating downwards, so the acceleration is negative: its a vector pointing down.

Answer 25

if a=g, then we free fall

Answer 26

oh okay. So a=-7.538

Answer 27

if we let g=9.8, then yes.

Answer 28

see if there is some "decimal" rnding place they want. rnd to 2 decimals or whatever

Answer 29

yeah it was wrong and it was my last submission but thank you for your help

Answer 30

lets consider some stuff

the elevator is eating up some acceleration, so it may be that the elevator is g-a

9.8 - 7.538 = 2.262

does this make sense?

if the elevator is doing 9.8, then the acceleration we feel is 0, free fall

Answer 31

9.8 - (0) = 9.8 (elevator)

if the elevator is not moving

9.8 - (9.8) = 0 (elevator)

soo, 9.8, minus our apparent acceleration, is equal to the elevators acceleration.