how to set friction force to zero when velocity is zero

[jim cibulka]

Hi all!  

I am new to using VPython and I just cannot seem to be able to get my friction force to zero when the velocity is zero . . . I want my students to explore a program I made to investigate balanced forces . . . Thanks!

[Kevin Karplus]

On Tue, Oct 13, 2015 at 8:45 AM, jim cibulka <jaci...@gmail.com> wrote:

I am new to using VPython and I just cannot seem to be able to get my friction force to zero when the velocity is zero

​That looks like a physics problem, not a Vpython problem.  What formula are you using to compute friction force (which of the many friction models are you trying to implement)?​

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Kevin Karplus   kar...@soe.ucsc.edu    http://www.soe.ucsc.edu/~karplus

[Paul Camp]

Have you tried an if statement? It would basically serve the purpose of a Heaviside step function.

On Tue, Oct 13, 2015 at 11:45 AM, jim cibulka <jaci...@gmail.com> wrote:

Hi all!  

I am new to using VPython and I just cannot seem to be able to get my friction force to zero when the velocity is zero . . . I want my students to explore a program I made to investigate balanced forces . . . Thanks!

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--

Dr. Paul J. Camp

"The beauty of the cosmos derives not only from unity in variety
       but also from variety in unity"
                 -- Umberto Eco
                     The Name of the Rose

[Bruce Sherwood]

To put it in other words: If you are modeling sliding friction with iterative calculations of position and velocity, it is extremely important at each step to check whether the velocity will go to zero in the next time interval, adjust the time interval to the time when the velocity will go to zero, update the position, then set the frictional force to zero. 

[Bruce Sherwood]

To be more precise, I was thinking of the case of an object sliding to a stop under the sole influence of a friction force. If an additional force is applied, the situation is more complicated. Assume for simplicity that the static frictional force is equal to (or less than) the frictional force when the object is moving. Then if the net force is zero when the velocity is zero, the object will stop and remain there. If the net force is not zero, and the zero velocity is passing through zero rather than stopping, you have to reverse the direction of the friction force.