force required to push an object

Hi,

I am writing a simulation in which  there is a robot in a crate-world. 
When the robot pushes a crate I want to know the force it requires to 
push this crate. (The crate is basically a cube and the robot is in the 
shape of a four-wheeled box. ) Is it possible to get this value?

Thanks,
Mithila
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