# Princip. of linear impulse and momentum , diff. btw. impulsive force and non-impulsiv

1. Jan 29, 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

2. Jan 29, 2012

### tiny-tim

hi tj00343!
i don't understand

friction isn't for a very-short time …

obviously friction over a finite distance (and time) will change the momentum

can you give a more specific example?​

3. Jan 29, 2012

### pabloenigma

Re: princip. of linear impulse and momentum , diff. btw. impulsive force and non-impu

And Momentum is conserved only when there are no external forces acting on the system.
∫Fdt gives the change in momentum for both impulsive and non-impulsive forces,so what is the problem?

4. Jan 29, 2012

### tj00343

Re: princip. of linear impulse and momentum , diff. btw. impulsive force and non-impu

I thought that momentum is conserved when there are no external forces on the system or the forces acting are non-impulsive forces ....I'm confused because in problems my professor solved , in some problems there was external forces acting on the system ,but they were not considered ,if for example 2 balls collide , their weights and normal forces are external to the system yet we apply conservation of momentum to find their velocities ........

5. Jan 29, 2012

### tiny-tim

ah, but momentum is a vector,

so conservation of momentum is a vector equation

(and so is Newton's second law)

so it works in each direction separately …

in your professor's examples, the weights and normal forces are vertical,

so there is no horizontal external force or impulse,

so horizontal momentum is conserved

6. Jan 30, 2012

### tj00343

Re: princip. of linear impulse and momentum , diff. btw. impulsive force and non-impu

ahhhhh thank youuu tiny tim................and pabloenigma