Energy Loss in Head on Collision of Two Cars w/Same Speed and Mass

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Homework Help Overview

The problem involves a collision between two railroad cars of equal mass and speed, specifically examining the mechanical energy lost during the event. The subject area pertains to concepts of kinetic energy and energy conservation in physics.

Discussion Character

  • Exploratory, Mathematical reasoning, Assumption checking

Approaches and Questions Raised

  • The original poster attempts to calculate the mechanical energy lost using the kinetic energy formula but questions their result compared to a book answer. Some participants suggest checking the units of velocity and converting them to standard units.

Discussion Status

The discussion has progressed with participants providing guidance on unit conversion, which led to a revised calculation by the original poster that aligns with the expected book answer. However, there is no explicit consensus on the initial misunderstanding.

Contextual Notes

Participants are addressing the importance of using consistent units in calculations, particularly converting from km/h to m/s, which is a common requirement in physics problems.

mrsteve
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Homework Statement



Two railroad cars, each with a mass of 7,000kg and traveling at 90 km/h collide head on and come to rest. How much mechanical energy is lost in this collision?

Homework Equations



KE=\frac{1}{2}mv^2

The Attempt at a Solution



KE=\frac{1}{2}(7000)(90)^2=2.835*10^7

Mechanical energy = KE+PE. PE = 0.

Energy lost = 2KE or 5.67*10^7

Book answer is 4*10^6

Where am I going wrong? Thanks..
 
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If your result is in joules, the velocity units of km/h is not helping.
 
Convert the speed to standard units of m/s.
 
Ahh... So 90 km/h = 25 m/s.

\frac{1}{2}(7000)(25)^2=2.1875*10^6 => 2.1875*10^6(2) = 4.375*10^6 => 4*10^6

Thanks!
 

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