What Went Wrong in My Elastic Collision Calculation?

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SUMMARY

The discussion centers on the calculation of final velocities in an elastic collision using the equations V1f = (m1 - m2 / m1 + m2) * V1i and V2f = (2m1 / m1 + m2) * V1i. The user calculated V1f as -1.917 m/s and V2f as 0.6645 m/s, questioning the discrepancy in results. The confusion arises from the negative value of V1f, which indicates a direction change post-collision, while V2f remains positive. The user also notes the importance of precision in calculations, as advised by their teacher, to account for a 2% tolerance in the system.

PREREQUISITES
  • Understanding of elastic collision equations
  • Familiarity with basic physics concepts such as momentum and energy conservation
  • Proficiency in algebraic manipulation of equations
  • Knowledge of gravitational acceleration (g) values, specifically 9.81 m/s²
NEXT STEPS
  • Review the principles of elastic collisions in physics
  • Study the significance of direction in velocity calculations
  • Learn about the impact of precision in numerical calculations
  • Explore common pitfalls in using physics equations for collision problems
USEFUL FOR

Students studying physics, particularly those focusing on mechanics and collision theory, as well as educators seeking to clarify common misconceptions in elastic collision calculations.

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Homework Statement



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Homework Equations



V1f = (m1 - m2 / m1 + m2) * V1i
V2f = (2m1 / m1 + m2) * V1i
.5mVi^2 + mgyo = .5mVi^2 + mgy1

The Attempt at a Solution



V1 = (2gyo)^(1/2) = (2*9.8*.34)^(1/2) = 2.5815 m/s
a) V1f = (.650 - 4.40 / .650 + 4.40) * 2.5815 = -1.917 m/s
b) V2f = (2*.650 / .650 + 4.40) * 2.5815 = .6645 m/s

I don't understand why part a is wrong and part b is right. Doesn't make any sense to me since the question is asking for the speed after the collision in both part a and b. What did I do wrong?
 
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I worked out v1 independently and got -1.918.
Surely the 4th digit difference is not significant?
I used 9.81 for g and got 2.583 for the initial speed.
 
Delphi51 said:
I worked out v1 independently and got -1.918.
Surely the 4th digit difference is not significant?
I used 9.81 for g and got 2.583 for the initial speed.

My teacher told us to input a lot of digits to be sure our answer is recognized since there is a 2% tolerance built in. Either way our answers would match. I think there must be something wrong with the system to say it is wrong.
 

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