Calculating Velocity of Wreckage After Collision

[Chica1975]

Homework Statement

A car of mass 950kg traveling at 60km/h in an easterly direction collided perfectly inelastically with another care of mass 1250kg traveling at 55.0km/h in a northerly direction. The design of the smaller care incorporated safety features which ensured that any occupants came to rest relative to the care in a tie of 250m/s. Both vehicles had one occupant of 75.0kg

calculate the velocity of the wreckage immediately after the collision.

Use conservation of momentum to determine the change in momentum of both vehicles

Homework Equations

m1v1 + m2v2 = m'1v'1 + m'2 v'2
I am not sure if the kinetic energy equation fits here too?
.5 m1 v1 E2 + .5 m2 v2 E2 = .5 m1 v'1E2 + .5 m2v'2E2
r= square root (xE2 +yE2)
p=mv

The Attempt at a Solution

I converted all the km/h to m/s
I calculated the momentum of each vehicle p=mv (I also included the weight of the driver for each). Using the answers I found the resultant.
I then rearranged the equation to find v and got the right answer in it.

I am now really struggling with the conservation of momentum - these always get me out. I can't seem to use the right equation and am just blank.

Can someone let me know = thank you. :)

 

[tiny-tim]

Hi Chica1975!

I am now really struggling with the conservation of momentum - these always get me out. I can't seem to use the right equation and am just blank.

As I said in the other thread …

You always need two equations to solve a collision question … one is always conservation of momentum, the other is either conservation of energy or some other constraint like "perfeclty inelastic" or " coefficient of restitution = 0.5".

In this case, your second equation (which you did use) is v_f1 = v_f2 …

you don't need, and mustn't use, conservation of energy!

(But you always need conservation of momentum)