Gravity vs imp acc

Mathieu_Courchesne

I did a small model that represents an object in a free fall.

When using /Grav card with a constant gravity of 1g, the gravity is decreasing during time and when using impacc, the gravity remains constant at 1g. See attached image.

 

For a free fall object I was expecting a constant acc of 1g. 

 

Why when using the grav card, the gravity does not remains constant?

 

What is the difference between the grav and impAcc card?

 

 

Thanks

Polyvios Romanidis

Response:

Hello Mathieu,

There is a difference between /IMPACC and /GRAV.

The /IMPACC is the total acceleration of a body.  No matter what happens this body is ‘feeling’ a specific acceleration.  So, this is a kinematic condition.  It is like replacing the acceleration parameter in Newton’s Second Law right side with a specific value, and with this as an input some forces apply to the object to create this acceleration.  For example:

The /GRAV works like an external force to the body and so represents an additional load.  The body’s acceleration is affected not only by /GRAV load, but also by other loads applied to the body.  Hence, in Newton’s Second Load /GRAV will create another Force unit in the left side of the equation and the acceleration could have any value the sum of the forces create.   For example:

Possible effects that can cause the acceleration to differ that much is another external force, some deformation existing in the body and may a rebound effect.

Polyvios Romanidis

If you want you can share more information about your model and check out why this happening!

 

Polyvios Romanidis

You can also have a look at the following KB article just released!

https://community.altair.com/csm/en/gravity-grav-vs-imposed-acceleration-impacc-load?id=kb_article&sys_id=5542dafe97380a54e3b0361e6253af92