# Discussion Forum

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## How to increase a load in a physical time

Hi everyone, I started to realize a preliminary breast computational model with COMSOL: first have to study the gravitational effect on it, by increasing the gravity progressively from zero at initial stage to g in a physical time of 60 s. How can I do this? How can I increase progressively this load in 60 s? Thank you a lot,

Maria

4 Replies Last Post 2019年1月17日 GMT-5 上午11:57

Posted: 2 years ago

COMSOL's graphical user interface is interpreted, so that in the various text fields where you specify your physics you can type in expressions that depend on variables, coordinates, and (most important for you, Maria), time. Time is "t". So you can type a mathematical expression of you choice involving "t" to have a load evolve with time. Note that many advanced mathematical functions are preimplemented for you, see the Component > Definitions > Functions nodes.

Best,

Jeff

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Jeff Hiller

Posted: 2 years ago

COMSOL's graphical user interface is interpreted, so that in the various text fields where you specify your physics you can type in expressions that depend on variables, coordinates, and (most important for you, Maria), time. Time is "t". So you can type a mathematical expression of you choice involving "t" to have a load evolve with time. Note that many advanced mathematical functions are preimplemented for you, see the Component > Definitions > Functions nodes.

Best,

Jeff

Thank you for your kind attention, Jeff. I tried to define a piecewise function, but the result was the following: "The following feature has encountered a problem: -feature: undefined value found in the stiffness matrix. Undefined value found. -detail: undefined value found in the stiffness matrix.

I defined the variable g with the expressione x/t^2, in order to obtain the correct unit, and maybe for this reason I obtain some result that are undefined. So I don't know how to solve this problem. Could you help me? I'm not able to realize which is the error.

Thank you so much, Maria

Posted: 2 years ago
Updated: 2 years ago

H Maria,

A couple of thoughts: * The problem mau come from the division by zero at t=0 for your variable g=x/t^2. * Also, when you specify a function (as opposed to a variable), you need to pass arguments to it when you use it. So, say you define a function called myfunction which is a function of x and t, you need to use the syntax myfunction(x,y) to use it somewhere else in your model.

If that does not resolve the problem, I'll invite you to send your model file (.mph file) to the COMSOL support team, support@comsol.com, with a mention of this Discussion Forum thread. That's the more appropriate environment to get support questions handled by COMSOL support staff and it brings in resources at our end not available in the Discussion Forum.

Thanks,

Jeff

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Jeff Hiller