Posted by Casey on .
At car (1140kg)traveling at 24.8 m/s and truck (12600kg) have a headon collision and then stick together. What is their final common velocity (m/s)? (assume the car is going in the positive direction)
What is the cars' change in momentum in the above question (kg m/s)?
When I calculate the velocity of the truck, I get 22.9 m/s which is correct.
The formula that I am using to find the common velocity is
pi= (velocity car)(mass car)  (velocity truck)(mass truck)
I then take the pi calculated and use it to find the final velocity
vf = pi/(mc+mt)
I get close to the right answer (4.36) but should have been 4.42.
For the last question. I believe that I am using the wrong formula.
mc*(vfvc)
The correct answer was 31100 but I get nowhere near this number.
Am I missing some key steps here?

Physics 
bobpursley,
You did not post the truck's initial speed, so I cannot check you. Is that really the mass of the truck?

Physics 
Casey,
Ohh sorry, the truck is going (24.8 km/hr) and the car is going 76.042 m/s

Physics 
Casey,
And yes, that is the right mass.