# Acceleration and weight question

alexcc17

## Homework Statement

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

## The Attempt at a Solution

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

## Answers and Replies

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

alexcc17
I know that it is 9.8 m/s^2, but I'm not sure what gs is?

NoPoke
"g"s experienced by fighter pilots, astronauts, riders on a roller coaster. All the result of acceleration.

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

alexcc17
Gravity causes us to have weight. Are you saying I should multiply .9 by 9.8?

alexcc17
Gravity gives us mass. Are you saying I should multiply 9.8 by .9?

NoPoke
I'm not going to say anything yet about how you should calculate or not.

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

alexcc17
*weight. Gravity gives us weight.

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

alexcc17
Weight is an example of a force. The units of gravity are the same as an acceleration?

NoPoke
yes. so now use your f = ma formula and see where each piece you have fits. Beware units.

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

alexcc17
I'm still not sure. If I plug them in wouldn't I just get 180*.9? Or is the acceleration given just an arbitrary value and I need 9.81 instead?

NoPoke
forget about the additional acceleration for now. You are standing on a set of scales in a stationary lift how much do you weigh, how do you know how much you weigh.

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

alexcc17
You would weigh 180 * 9.81. You cancel units to determine what you end up with?

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

alexcc17
Ahh! 180/9.81= 18.3 as the mass.

NoPoke
lbs are an old unit and I suspect the question is using them to see if you are clear in your mind as to the difference between mass and weight. lbs are awkward because they are used for both mass and weight: the 1 lb weight = 1 lb mass conversion works on Earth because the acceleration on Earth is 1 gravity [or 1 "gee"]

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

alexcc17
The question actually says pounds, I just shortened it for the problem. If on the moon they weigh 1/6 less, then would it be (1/6)*180

NoPoke
so same mass but less weight on the Moon. How about on a moon a little smaller than ours where the acceleration due to that moon's gravity is 0.9m/sec/sec [please start giving answers with appropriate units as the correct selection of units is at the heart of the problem]

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

alexcc17
This is the part I don't know. Would it be 9.8/.9 * mass=180Ibs? So 16.5 lbs?

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

alexcc17
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.

NoPoke
On Earth my weight in lbs is the same as my mass in lbs. The equation f=ma in this scheme has to have a=1 in magnitude.

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.

alexcc17
Ok, converting pounds to kg I get that the person is 81.65. Now would I multiply that by .9?

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.

NoPoke
it is straightforward. but only once you can see the difference between mass and weight. You should then be able to see if the units in the problem are consistent or if some conversion is necessary.

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.

alexcc17
Can you show me how to convert the acceleration into gravities?

Homework Helper
Gold Member
Original question:

## 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/s2 [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/s2?

Last edited:
NoPoke
9.8m/sec/sec = 1 earth gravity

alexcc17
He is accelerating upwards in an elevator at .9 m/s^2. Ok, so 9.8/.9=1/x, so x=.092 gravities?

NoPoke
So the 180 lb man is in a lift standing on his bathroom scales. The lift is going up at constant acceleration equivalent to 0.092 "gee". He looks down at the scales and they now show ?

[@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]

alexcc17
This isn't an equation. Can you please just help with the equation part. It has been 2 hrs and this is incredibly frustrating. The scales aren't moving because he is in the elevator moving at the same speed. How do I solve the question, please please help with that.

Homework Helper
Gold Member
This isn't an equation. Can you please just help with the equation part. It has been 2 hrs and this is incredibly frustrating. The scales aren't moving because he is in the elevator moving at the same speed. How do I solve the question, please please help with that.

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?

alexcc17
Yes, we are on earth, moving upwards at an acceleration of .9 m/s^2.

NoPoke
The equation is f=ma

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.

alexcc17
This I understand! So... 180*(1.092)=196.56 lbs