- #1

alexcc17

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## Homework Statement

How heavy does a 180Ib person feel if while they are accelerating at a rate of .9 m/s^2?

## Homework Equations

## The Attempt at a Solution

I thought I would use F=W=ma, so .9 * 180, but it says that is wrong.

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- Thread starter alexcc17
- Start date

- #1

alexcc17

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How heavy does a 180Ib person feel if while they are accelerating at a rate of .9 m/s^2?

I thought I would use F=W=ma, so .9 * 180, but it says that is wrong.

- #2

NoPoke

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you have heard of "g force" I hope, so how many "gs" is the acceleration equivalent too?

- #3

alexcc17

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I know that it is 9.8 m/s^2, but I'm not sure what gs is?

- #4

NoPoke

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If that still doesn't help then what causes you to have a weight?

- #5

alexcc17

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Gravity causes us to have weight. Are you saying I should multiply .9 by 9.8?

- #6

alexcc17

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Gravity gives us mass. Are you saying I should multiply 9.8 by .9?

- #7

NoPoke

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Gravity does not give you mass so what does gravity do?

- #8

alexcc17

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*weight. Gravity gives us weight.

- #9

NoPoke

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yes. weight is an example of a _______ . The units of gravity are the same as an ________

- #10

alexcc17

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Weight is an example of a force. The units of gravity are the same as an acceleration?

- #11

NoPoke

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How much would you weigh on the moon where gravity is 1/6th that of the Earth.

- #12

alexcc17

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- #13

NoPoke

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[Physicists always have a set of scales with them in lifts :D ]

- #14

alexcc17

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You would weigh 180 * 9.81. You cancel units to determine what you end up with?

- #15

NoPoke

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So the 180lb person is standing on their bathroom scales and they look down and is says 180*9.81?

- #16

alexcc17

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Ahh! 180/9.81= 18.3 as the mass.

- #17

NoPoke

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How much does a person who weighs 180lb on earth weigh on the Moon?

- #18

alexcc17

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- #19

NoPoke

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(I'm ignoring that 1/6 less is not the same as 1/6th)

- #20

alexcc17

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This is the part I don't know. Would it be 9.8/.9 * mass=180Ibs? So 16.5 lbs?

- #21

NoPoke

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you appear to be saying that the person's mass changes when they are on the Moon.

- #22

alexcc17

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9.8*m=180 m=18.4

18.4 * .9 =16.5 lbs??

Can you please give some hints since I am really lost here.

- #23

NoPoke

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try here http://www.bbc.co.uk/schools/gcsebitesize/science/edexcel_pre_2011/space/gravityforceandweightrev1.shtml its four pages but all very quick [10 minutes or thereabouts]. It avoids the issue of lbs weight = lbs mass on earth, and by using the SI system forces you to think about units.

- #24

alexcc17

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I don't want to sound rude, but I've been at this one problem for over an hour and it really seems as though it should be pretty straight forward.

- #25

NoPoke

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For myself I kept the lbs weight and converted acceleration into earth gravities aka "gee"s The numbers in the question look a lot like what I might see if I took my bathroom scales into a lift and selected to go up or maybe down.

- #26

alexcc17

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Can you show me how to convert the acceleration into gravities?

- #27

lewando

Homework Helper

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Original question:

Is this person in free fall, accelerating at .9 m/s^{2} [edit: headed towards the surface of a small planet], or standing on the surface of a planet, acceleration = 0, where the acceleration due to gravity is .9 m/s^{2}?

## Homework Statement

How heavy does a 180Ib person feel if while they are accelerating at a rate of .9 m/s^2?

Is this person in free fall, accelerating at .9 m/s

Last edited:

- #28

NoPoke

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9.8m/sec/sec = 1 earth gravity

- #29

alexcc17

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He is accelerating upwards in an elevator at .9 m/s^2. Ok, so 9.8/.9=1/x, so x=.092 gravities?

- #30

NoPoke

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[@lewando, I agree the question is unclear, but I don't believe that lack of clarity is what is preventing alexcc17 from getting an answer that makes sense]

- #31

alexcc17

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- #32

lewando

Homework Helper

Gold Member

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We'll get through this. Stay calm. We are your friends. Put down the butter knife. Being on an elevator is new information. Are we on Earth in this elevator?

- #33

alexcc17

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Yes, we are on earth, moving upwards at an acceleration of .9 m/s^2.

- #34

NoPoke

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weight = mass x (acceleration due to gravity + additional acceleration from motion)

lbs are awkward

weight in lbs = mass in lbs x acceleration in earth gravities

weight in the lift going up = 180 x(1 + ???? ) lbs

===

and because the question is unclear there would also be the weight an astronaut experiences inside a rocket accelerating at 0.9m/sec/sec or on a moon a little smaller than Earth's moon with a surface gravity of 0.9m/sec/sec.

Along with a decending lift 180x(1-????) lbs and even a car accelerating from the lights where the passenger will feel himself being forced back into the seat but the set of scales on the floor will still show 180lbs.

- #35

alexcc17

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This I understand! So... 180*(1.092)=196.56 lbs

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