What Is the Velocity After Two Cars Collide?

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Homework Help Overview

The discussion revolves around a collision problem involving two cars, one weighing 2000 lbs traveling north at 60 mph and the other weighing 5000 lbs traveling east at 40 mph. The cars stick together after the collision, and participants are tasked with finding the resultant velocity and direction post-collision using the principle of conservation of linear momentum.

Discussion Character

  • Mixed

Approaches and Questions Raised

  • Participants explore the calculation of momentum for each car, questioning the necessity of converting weights from pounds to slugs. There is discussion about how to properly add momentum vectors, considering their directions, and the implications of using different units.

Discussion Status

Several participants have provided insights into the vector nature of momentum and the importance of considering directional components. There is acknowledgment of the confusion surrounding unit conversions and the relationship between weight and mass. Some participants have expressed their understanding of the process while others continue to seek clarity on specific points.

Contextual Notes

Participants are grappling with the implications of using pounds versus slugs in momentum calculations, particularly in the context of the problem's requirements and the potential for rounding errors in conversions. There is also a mention of how the mass of the cars affects the outcomes, with questions about the independence of results from unit choices.

  • #31
Boy, if I have ever seen a plea for metrication, this is the one !

On the other hand it's good to realize -- as entro did -- that momentum really has the dimension mass times length divided by time, in whatever units are in fashion !
 
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  • #32
NickAtNight said:
You are welcome.

Oh, the SI system does not get off quite so easy either.

If you weigh yourself on the bathroom scales and get 68 kg (150 lbs).

This is also Force units. So ##kg_f##

And the ##G_c## for the SI system is ##9.8 kg*m/kg_f/s^2 ##

So ##kg_f = kg_m##

If you step on the same scale on the moon, you would only weigh 11.6 kg_f. But when you do the conversion, you will still be 68 kg mass.From that other source.
No, the SI doesn't have that mess. There is no such thing as "kg_f".
The bathroom scale measures a force of 667 N and converts it for you to m=F/g = 68 kg.
On the Moon the bathroom scale measures ~110 N and converts it (wrongly) to about 11 kg because it is not calibrated for use on Moon. A properly calibrated scale there would convert 110 N to 68 kg as well. If scales would be used in different environments frequently, one would add a test mass to the scale to make it self-calibrating.
 
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