×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Friction in COSMOSmotion

Friction in COSMOSmotion

Friction in COSMOSmotion

(OP)
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?

RE: Friction in COSMOSmotion

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

RE: Friction in COSMOSmotion

(OP)
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

RE: Friction in COSMOSmotion

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

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources