Friction in COSMOSmotion

[polykracker]

I'm having trouble understanding the way friction works in COSMOSmotion.

I have a 1 kg block constrained to move along the x-axis by a translational joint. There is an action only constant force of 10N acting the +x direction, gravity is in the -y direction @ 9.81m/s^2, and the friction in the joint is 0.25. According to elementary dynamics the acceleration should be constant @ 7.55 m/s^2 but that is not the result that I am seeing, what am I missing here?

 

[cbrn]

Hi,
please describe the disuniformity you are seeing.
As you say, F=ma is not to argue about: the block should accelerate with a=7.5475 m/s^2...
BUT... are you sure you entered the joint parameters such as to have constant friction (static and dynamic) and not different values for each (or a ratio or an equation relating the two) ?
Also remember that the step between iterations can also play a role in giving you "not-smoothed" curves of acceleration-vs-time.

Regards

 

[polykracker]

With the joint friction the block will have an initial positive acceleration then gradually move to a negative acceleration (duration of ~.2secs over a 1 sec sim) then jump to 0 acceleration and remain that way for the rest of the acceleration. For the joint paramaters I chose 'use materials option' and dry steel for both. I am running CosmosMotion 2007 SP4.0

 

[cbrn]

Hi,
I'd check carefully the definition of the force.
How is the plot of the velocity vs time? And displacement? Do they match with the one of the accel?
In fact, your accel plot is typical of an impulse force, or of a force acting only transitorily.
Also consider that, when using "material definition", the dynamic coefficient of friction and the static one are different. Check that the dynamic friction doesn't overcome the active force (but first of all I'd check the force itself!!!).

Regards