Question

I agree with this and see were it comes from

net force = (delta p)/t

so momentum is conserved when net force equals zero...

net force = (delta p)/t
0 = (delta p)/t
however the following is not true

0 = (delta p)/t = P - Po

were p is momentum and Po is used to indicate inital and P final

can you please help me prove that when no net forces act on an object momentum is conserved... because I don't see how the t in the equaion just gets to magically dissapear... I was think along the lines of you can't divide by zero therefore delta p must be equal to zero which lead me to get

P = Po

but then I rembered that I was actually doing math and that you can divide by zero

so I don't get it please show me how the t just magicall dissapears

I agree with this

(P - Po)/t = 0
P/t - Po/t = 0
P/t = Po/t

how come i can just take the t out

but don't see were this comes from

Answer 1

The more correct way to write the change of momentum equation is

F = (delta p)/(delta t)

If there is a time-varying force, the above equation applies for short time intervals, in the limit as delta t approaches zero. Using calculus notation, one writes

f = dp/dt

If there is NO force, p never changes, regardless of the time interval.