- #1
Karol
- 1,380
- 22
Homework Statement
A body of mass 3 kg slides on a friction less surface with velocity 4 m/s and collides elastically with a resting mass of 2 kg. calculate the final velocities.
Homework Equations
Conservation of momentum: [itex]m_1v_1+m_2v_2=m_1u_1+m_2u_2[/itex]
Conservation of energy: [itex]\frac{1}{2}m_1v_1^2+\frac{1}{2}m_2v_2^2=\frac{1}{2}m_1u_1^2+\frac{1}{2}m_2u_2^2[/itex]
The Attempt at a Solution
I solved those two equations and got a correct answer. but my question is as follows:
If we solve, as a general case, the two above equations we get a third equation:
[tex]v_1-v_2=-(u_1-u_2)[/tex]
Which states that the relative velocities before and after the collision remain the same.
When i solved this third equation together with the first equation, that of the conservation of momentum i got, of course, a wrong answer.
Can you explain? both ways seem correct.