Acceleration and weight question

  • Thread starter alexcc17
  • Start date
  • #1
alexcc17
48
0

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.
 

Answers and Replies

  • #2
NoPoke
125
1
you have heard of "g force" I hope, so how many "gs" is the acceleration equivalent too?
 
  • #3
alexcc17
48
0
I know that it is 9.8 m/s^2, but I'm not sure what gs is?
 
  • #4
NoPoke
125
1
"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?
 
  • #5
alexcc17
48
0
Gravity causes us to have weight. Are you saying I should multiply .9 by 9.8?
 
  • #6
alexcc17
48
0
Gravity gives us mass. Are you saying I should multiply 9.8 by .9?
 
  • #7
NoPoke
125
1
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?
 
  • #8
alexcc17
48
0
*weight. Gravity gives us weight.
 
  • #9
NoPoke
125
1
yes. weight is an example of a _______ . The units of gravity are the same as an ________
 
  • #10
alexcc17
48
0
Weight is an example of a force. The units of gravity are the same as an acceleration?
 
  • #11
NoPoke
125
1
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.
 
  • #12
alexcc17
48
0
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?
 
  • #13
NoPoke
125
1
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 ]
 
  • #14
alexcc17
48
0
You would weigh 180 * 9.81. You cancel units to determine what you end up with?
 
  • #15
NoPoke
125
1
So the 180lb person is standing on their bathroom scales and they look down and is says 180*9.81?
 
  • #16
alexcc17
48
0
Ahh! 180/9.81= 18.3 as the mass.
 
  • #17
NoPoke
125
1
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?
 
  • #18
alexcc17
48
0
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
 
  • #19
NoPoke
125
1
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)
 
  • #20
alexcc17
48
0
This is the part I don't know. Would it be 9.8/.9 * mass=180Ibs? So 16.5 lbs?
 
  • #21
NoPoke
125
1
you appear to be saying that the person's mass changes when they are on the Moon.
 
  • #22
alexcc17
48
0
What about...
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
125
1
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.
 
  • #24
alexcc17
48
0
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.
 
  • #25
NoPoke
125
1
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.
 
  • #26
alexcc17
48
0
Can you show me how to convert the acceleration into gravities?
 
  • #27
lewando
Homework Helper
Gold Member
1,367
144
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:
  • #28
NoPoke
125
1
9.8m/sec/sec = 1 earth gravity
 
  • #29
alexcc17
48
0
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
125
1
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]
 
  • #31
alexcc17
48
0
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.
 
  • #32
lewando
Homework Helper
Gold Member
1,367
144
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?
 
  • #33
alexcc17
48
0
Yes, we are on earth, moving upwards at an acceleration of .9 m/s^2.
 
  • #34
NoPoke
125
1
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.
 
  • #35
alexcc17
48
0
This I understand! So... 180*(1.092)=196.56 lbs
 

Suggested for: Acceleration and weight question

Replies
4
Views
394
  • Last Post
Replies
14
Views
286
Replies
16
Views
686
Replies
4
Views
480
Replies
22
Views
1K
  • Last Post
Replies
3
Views
335
Replies
20
Views
474
Replies
8
Views
452
  • Last Post
Replies
3
Views
404
Top