# Princip of linear impulse and momentum, Difference btw. impulsive/nonimpulsive forces

1. Jan 28, 2012

### tj00343

when applying the principle of linear impulse and momentum , how do I know if the force should be considered impulsive or non-impulsive , how should I know if I should consider it in the equation , I already know that an impulsive force is a force that is applied for a very short time ,but in some problems forces such as the normal force were considered impulsive ,for example , there is one containing a crate where the only forces applied are the weight ,normal force ,and friction and still momentum was not conserved , for example , the princip. of impulse and momentum is m(v1) + ∑ ∫ (F)dt =m(v2)
when do I consider the integral to be 0 and momentum conserved
Thank You

Last edited: Jan 28, 2012
2. Jan 29, 2012

### rude man

Re: princip of linear impulse and momentum, Difference btw. impulsive/nonimpulsive fo

Momentum is always conserved for any system not subject to an external force. Even with friction, momentum is conserved but you have to include the momentum change of everything connected with your system, including possibly the Earth itself!

So if you have a frictionless table with inelastic collisions, momentum is still conserved even though kinetic energy is not, because the lack of friction means no communication with anything beyond the cue balls themselves.

The momentum integral ∫Fdt = Δp is similarly applicable. Again, if there is friction, that means whatever the friction is with must be included in the momentum conservation equation.

I would need a more explicit problem description, for example your "crate" problem, to go any further.