Inelastic collision physics help

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Homework Help Overview

The discussion revolves around an inelastic collision problem involving two cars, where Car A travels North at 35 km/hr and Car B travels East at 75 km/hr. Both cars have equal mass, and the participants are trying to determine the magnitude of the velocity of the combined wreckage after the collision.

Discussion Character

  • Exploratory, Assumption checking, Problem interpretation

Approaches and Questions Raised

  • Participants discuss the conservation of momentum and the relationship between mass and velocity in the context of the collision. There are attempts to calculate the final velocity of the wreckage, but confusion arises regarding the unknown masses and how they affect the calculations. Some participants express uncertainty about how to proceed without specific mass values.

Discussion Status

The discussion is ongoing, with participants exploring different interpretations of the problem. Some have pointed out that the masses cancel out due to their equality, while others are still grappling with how to express the final velocity without knowing the actual mass values. There is a mix of frustration and attempts to clarify the reasoning behind the calculations.

Contextual Notes

Participants note that the masses of the cars are equal, which simplifies some calculations, but they still struggle with the implications of this equality in determining the final velocity. There is a mention of a potential conservation of energy approach, but the lack of specific mass values complicates this method.

jtw2e
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Homework Statement



Car A travels North at 35km/hr.
Car B travels East at 75 km/hr.
Mass of Car A = Mass of Car B
Cars have a perfectly inelastic collision.

Homework Equations


What is the magnitude of the velocity of the tangled mess after the collision? (km/hr)


The Attempt at a Solution


Px = PAx + PBx = MB(0) + MA(75) = 75km/hr*MA

Py = PAy + PBy = MB(35) + MA(0) = 35km/hr*MB

SQRT[(MA*35km/hr)+(MB*75km/hr)]

PTotal= 82.76km/hr*MA

P=mv
v=P/m = (88.76km/hr*[STRIKE]M[/STRIKE]A)/([STRIKE]MA[/STRIKE]+MB)


tanθ = Py/Px = (35km/hr*[STRIKE]M[/STRIKE])/(75km/hr*[STRIKE]M[/STRIKE]) = 0.4666667

tan-1(0.46666667) = 25.01689 degrees.


I'm lost here... I have no idea how to get the mass OR velocity of the wreckage pile after the wreck... Please help. I have verified that my angle is at least correct.
 
Last edited:
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hi jtw2e! :smile:

(why do you keep using "STRIKE" ? :confused:)
jtw2e said:
P=mv
v=P/m = (88.76km/hr*[STRIKE]M[/STRIKE]A)/([STRIKE]MA[/STRIKE]+MB)
…
I'm lost here... I have no idea how to get the mass OR velocity of the wreckage pile after the wreck... Please help. I have verified that my angle is at least correct.


I don't understand :redface: …

you know the mass is MA + MB, and you've found v :confused:
 


tiny-tim said:
hi jtw2e! :smile:

(why do you keep using "STRIKE" ? :confused:)

I don't understand :redface: …

you know the mass is MA + MB, and you've found v :confused:


Hi tiny-tim,
I used strike to show that M/M = 1... i.e. those particular masses canceled. I have not found the V of the system after the collision, which is Ptotal/Masstotal. I haven't been able to find that value I don't know the mass of either car.

So I'm stuck with Vfinal = (88.76km/hr)/MB

I have two unknowns. Somehow I have to find the mass of the cars, OR perhaps there is some other way to find the V? I tried to start a Conservation of Energy approach but since I didn't know mass for the Kinetic Energies, that didn't help.
 
Last edited:
jtw2e said:
… I don't know the mass of either car …

yes you do! … the masses are the same! :smile:
 


tiny-tim said:
yes you do! … the masses are the same! :smile:

Two unknowns: velocity and mass. Need a value to calculate either one. The fact that they are the same simplifies things, but I don't see how it gives me a number... such as 1500kg or 2000kg, etc...
 
but they'll all cancel! :smile:
 


tiny-tim said:
but they'll all cancel! :smile:

v=P/m = (88.76km/hr*MA)/(MA+MB)

They don't all cancel. Only one of the masses cancels. You are left dividing by the other. Are my formulas in error?
 
Last edited:
jtw2e said:
v=P/m = (88.76km/hr*MA)/(MA+MB)

They don't all cancel. Only one of the masses cancels. You are left dividing by the other. If they all canceled we would be dividing by 0. Are my formulas in error?

of course they cancel …
MA/(MA + MB) = MA/(2MB) = 1/2 :rolleyes:
 


Sorry, the number should be 82.76km/hr (can't read my own writing).

tiny-tim said:
of course they cancel …
MA/(MA + MB) = MA/(2MB) = 1/2 :rolleyes:

Even so, V = 82.76 km/hr / .5M

It's still 1/2 times some number M. That number M is what I need to find V. Otherwise all I know is V = 165.5 / M
 
  • #10
you must have missed an M somewhere :wink:
 
  • #11


tiny-tim said:
you must have missed an M somewhere :wink:



It's been...a very... long two weeks. ;)

Thanks for putting up with my... frustration and confusion.
 
  • #12
jtw2e said:
It's been...a very... long two weeks. ;)

actually, it's only been 2 hours! :biggrin:

get some sleep! :zzz:​
 

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