# Forces required to accelerate a mass on a spring

1. Jun 15, 2009

### JohnDear

Hi,

I am currently working on a project and have become stuck on what should be a relatively simple problem (I thought).

Basically I am using a DC motor to provide a force on a crank that compresses a peice of rubber.

The movement includes some acceleration.

I am trying to use the motor current to find out the motor torque (use torque constant), and then force profile of the peice of rubber.

What i have done is perform the movement without the rubber, data log the motor torque.

Then perform the movement with the rubber, data logging the new motor torques.

Without thinking too much about it, I then subtracted the free motor torque from the motor torque with rubber, and said the result was 'torque due to compressing the rubber'. Calculated force from that, and results seem ok.

My question is:

Does acceleration effect the results when using my method? If so how? And how could i minimise this under circumstances where acceleration must be high?

Also if anyone has some information on this type of procedure I'm using, (papers etc) that would be great.

thanks,
John

2. Jun 15, 2009

### JohnDear

After thinking about it I think my approach is fair.

In the free motion case F = ma.

In the rubber compression case F = ma + k.d (k the spring constant of rubber assuming it behaves linearily).

I see no reason why I can't treat the two force components independently.

Only issue I can see is the effect of loading on friction (not accounted for in my model, but inherent in both my measurements), I think this may increase with loading, but can assume the increase to be insignificant.

Anyone with greater insight than me into this type of problem would love to hear from you,

thanks,
John