# Inelastic collisions with constant momentum

• haha0p1
In summary, the conversation discusses the calculation of kinetic energy before a collision and the discrepancy of the required answer of 1/2 mv². The expert agrees with the calculation and notes a potential error in the book. It is mentioned that the books specifies "identical" objects, but it is clarified that objects of the same mass are all that is required kinematically. The use of "identical" may be for economy of words and to convey the idea of symmetry. However, it is pointed out that the book actually states "the same two objects", implying that they are the same as in the preceding question.
haha0p1
Homework Statement
The total momentum before the collision in an inelastic collisions is 0, but the total kinetic energy before the collision is 1/2mv². Calculate how the total kinetic energy before collision is 1/2mv².
Relevant Equations
Ek=1/2mv²
Kinetic energy before collision =1/2 mv² + 1/2 mv² = mv² (since energy is a scalar quantity, the direction does not matter). Kindly tell why am I not getting the required answer i.e: 1/2 mv². Am I doing the calculation wrong?

Last edited by a moderator:
Hi,

I agree with your calculation. An unfortunate error in the book.

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haha0p1
It's interesting that the book specifies the "same object" or "identical" objects, where all that is required kinematically is objects of the same mass!

PeroK said:
It's interesting that the book specifies the "same object" or "identical" objects, where all that is required kinematically is objects of the same mass!
Truee

PeroK said:
It's interesting that the book specifies the "same object" or "identical" objects, where all that is required kinematically is objects of the same mass!
I think it's economy of words. "Identical objects" is shorter than "objects of the same mass" and conveys the idea of symmetry. "Identical" becomes relatively conciser when the masses also carry equal charges.

PeroK
PeroK said:
It's interesting that the book specifies the "same object"
No, it says "the same two objects". Presumably the same two as in the preceding question.

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