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## MikeAqua

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Topic # 203129 19-Sep-2016 09:50

If an object of certain mass in sitting on spring, does the act of it's compressing the spring absorb some of it's weight.

For example: -

A) I place an object on a scale and the scale reads 100kg.

B) If I place it on a spring (pretend the spring as no mass) and place the assembly on the same scale under the same conditions will it now read less?

I realise the mass of the object is unchanged.  I am interested in the downward force acting on the scale.

Mike

## Technofreak

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Reply # 1635712 19-Sep-2016 09:52

The scales will show the same weight, ignoring as you say the weight of the spring.

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## SaltyNZ

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Reply # 1635714 19-Sep-2016 09:53
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The weight is still 100kg plus the mass of the spring. All the forces have to balance. If they don't, then the object isn't sitting on the scale - it's accelerating in some direction.

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These comments are my own and do not represent the opinions of 2degrees.

## shk292

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Reply # 1635724 19-Sep-2016 10:15
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In the steady state, Salty's statement below is true.

When the weight is first put on the spring, the force transmitted is zero (Hooke's Law).  As the spring compresses due to the weight on it, the force transmitted increases from zero to the full weight of the supported weight.

SaltyNZ:

The weight is still 100kg plus the mass of the spring. All the forces have to balance. If they don't, then the object isn't sitting on the scale - it's accelerating in some direction.

## MikeAqua

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Reply # 1635737 19-Sep-2016 10:46

shk292:

In the steady state, Salty's statement below is true.

When the weight is first put on the spring, the force transmitted is zero (Hooke's Law).  As the spring compresses due to the weight on it, the force transmitted increases from zero to the full weight of the supported weight.

SaltyNZ:

The weight is still 100kg plus the mass of the spring. All the forces have to balance. If they don't, then the object isn't sitting on the scale - it's accelerating in some direction.

Thanks both.

Mike

## geoffwnz

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Reply # 1635740 19-Sep-2016 10:52

shk292:

In the steady state, Salty's statement below is true.

When the weight is first put on the spring, the force transmitted is zero (Hooke's Law).  As the spring compresses due to the weight on it, the force transmitted increases from zero to the full weight of the supported weight.

SaltyNZ:

The weight is still 100kg plus the mass of the spring. All the forces have to balance. If they don't, then the object isn't sitting on the scale - it's accelerating in some direction.

I wonder if a simple test of this would be to place both the spring and the weight on the scales side by side (or possibly zero the scales with just the spring on them).  Note the reading.  Then move the weight from the scales to the spring.  Note new reading.
Am I simplifying it too much or would this work?

## SaltyNZ

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Reply # 1635745 19-Sep-2016 11:02

geoffwnz:

I wonder if a simple test of this would be to place both the spring and the weight on the scales side by side (or possibly zero the scales with just the spring on them).  Note the reading.  Then move the weight from the scales to the spring.  Note new reading.
Am I simplifying it too much or would this work?

No, that's exactly right. But the answer 100kg only applies to the steady state, i.e. everything has stopped moving. If the mass is still moving - compressing the spring down - then the scale's reading will be lower. As the spring compresses it will push back against the gravitational force imposed by the mass. It pushes back more forcefully as it compresses, approximately linearly, until either they balance, or the spring is fully compressed.

If the spring is not fully compressed, the mass will overshoot, and then (assuming everything is moving directly up and down) you end up with a harmonic motion, which will gradually die out due to things like air resistance and imperfect elasticity of the spring.

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## eracode

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Reply # 1635747 19-Sep-2016 11:05

If the scale shows less than the total weight - hey presto, you've invented an anti-gravity device!

## linw

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Reply # 1635870 19-Sep-2016 13:48

eracode:

If the scale shows less than the total weight - hey presto, you've invented an anti-gravity device!

Could make a fortune with that

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