The Student Room Group

Simple momentum question!!!

Ok, this Q is really easy, but for some reason, i got the wrong answers. :confused:

A truck of mass 1.2kg is travelling due west with a speed of 0.90m/s. A second truck B of mass 4.0kg is travelling due east towards A with a speed of 0.35m/s.
Calculate the magnitude of the total momentum of the trucks.

The trucks collide and stick together. Determine their velocity after the collision.

I think i'm losing my mind now, cause i can't do this Q. :confused:
Reply 1
is the answer 0.47ms-1 for final velocity
and 2.48 for total momentum?
Reply 2
Daban
is the answer 0.47ms-1 for final velocity
and 2.48 for total momentum?


Nope, that's what I got as well...but in the markscheme it says 0.32 kg m s^-1 for momentum and 0.062 m s^-1 for velocity!!
Reply 3
yes i got it
Reply 4
Daban
yes i got it


So how did you do it?
Reply 5
its pretty simple, the best way to do it is to draw a diagram.

truck A is moving west with momentum= 1.08kgms-1
truck B is moving east with momentum=1.4kgms-1

as Truck B has a higher momentum when they stick together they will have a total momentum of (1.4-1.08 = 0.32) due east
put this back into p=mv so 0.32kgms-1/5.2kg= 0.062ms-1
Reply 6
i think we were getting the wrong answer because we were assuming that the final momentum would be 0, the intial calculation would of been correct if one of the trucks were initally stationary
Reply 7
ohhh, thanks!
Reply 8
no problem
Reply 9
Oh wait...isn't the momentum of truck B -1.4? And then to get the total moment, you add them together...1.08+(-1.4) = -0.32? Or is that completely wrong? :redface:
Reply 10
If you specify direction, it all depends which way you take as positive.