Momentum in Collisions: Solving for Velocity and Kinetic Energy Conversion

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The discussion focuses on solving a physics problem involving a collision between two cars, emphasizing the importance of treating momentum as a vector quantity. The user initially calculated the total momentum before the collision but struggled with the correct method for determining the combined velocity after the collision. Guidance was provided to visualize momentum vectors and to calculate the total kinetic energy before and after the collision to find the energy converted. The user expressed confusion about finding the angle related to the collision's direction. Understanding the correct approach to part (a) is crucial for solving part (b).
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Momentum...Please urgent help!

My Question:
A 1500 kg car moving east at 38 m/s collides with a 1780 kg car moving south at 15 m/s and the two cars stick together.
(a) What is the velocity of the cars right after the collision? _______m/s ______o

(b) How much kinetic energy was converted to another form during the collision? ______kJ

My attempt:

So i used the momentum formula and got the total momentum before collision and made it equal to the momentum after the collsion but didn't get the right answer.

Before collision: car1:
p=m1v1=(1500kg)(38m/s)=57000

car2:
p=m2v2=(1780kg)(-15m/s)=-26700

p1+p2=P total
57000+(-26700)=30300

After collision: car1:
p=1500v1

car2:
p=1780v2

v1=v2 because of collision therefore

30300=(1500+1780)v
v=9.24m/s

when i took out the negative i got a different answer (25.5m/s) which was also wrong.

Part B I've no idea how to do:rolleyes:

Please tell me what I'm doing wrong!
 
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Remember that velocity, and hence momentum, are vector quantities and need to be treated as such when adding them together. I think you attempted to do this with the minus sign, but these vectors are not opposite in direction. They are at 90 degrees to one another. Draw a diagram of the momenta vectors. It may help you see how to combine them for the total.

Also, for part b, remember Kinetic energy is not a vector quantity. determine the total kinetic energy before the collision and compare it to thee total kinetic energy after the collision and determine the difference.
 


First of all...thanks Galileo's Ghost for all your help

Ok...part a: Yes i understand what your saying...my first try was without the negative sign and i still got that wrong...so it must be my method that's messed up

And how am i supposed to find the angle?

Lastly as long as i know how to get part a then i can get part b.
 


do you go to wayne state university?
 
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