and so i need help:cry:

newton's second law states that force on an object is given by the rate of change in its momentum correct?

so f=dp/dt

if i expand dp, i get dmv.

f=dmv/dt

then now, i got to break up dmv. i'll let dt go over to F part.

fdt=mdv+vdm

now here is the part that i screw up. wads m and wads v?

i'll throw in some numbers so u all can punch. i got weird values.

assuming a car is travelling initially at 10ms-1, with the mass of the car+fuel being 100kg.

at time 1s later, the car is travelling at 100ms-1, with the mass of car + fuel being 10kg.

what is the force acting on the car during this 1s?

if i take it as Fdt= change of momentum, then i punch in numbers...

i shld get 0 resultant force as the momentum m1v1=m2v2 and hence no force.

yet if u use mdv+vdm.....

100kg (100-10)+ 10ms-1 (10-100)

u get a weird value.

i suspect i punched in wrong numbers for V and M. or my DV and DM is wrong. anyone can advise?