- #1
Karol
- 1,380
- 22
Homework Statement
Two identical masses collide. one is at rest. what are their final velocities.
Homework Equations
The relative velocities before and after the collision are identical:
##v_1-v_2=-(u_1-u_2)##
The Attempt at a Solution
I draw the final velocities in the same direction:
[tex]\left\{ \begin{array}{l} mv_1=mu_1+mu_2 \\ -v_1=-(u_1-u_2) \end{array} \right. [/tex]
It gives $$u_2=0$$ which isn't true, u1 should be 0.
And more, i can switch between u1 and u2 in the drawing and it will also be good since they are drawn in the same direction, so, how do i know who stops?
I get this answer also if i use the conservation of energy formula instead of the relative velocities formula.
If i draw u1 backwards and change the formula in accordance:
##\left\{ \begin{array}{l} mv_1=mu_1-mu_2 \\ -v_1=-(u_1-u_2) \end{array} \right.##
I again get the same result: $$u_2=0$$
Apart from the question i posed, do i have to assume a direction and then write the formuls accordingly or i have always to write:
##\left\{ \begin{array}{l} mv_1=mu_1+mu_2 \\ -v_1=-(u_1-u_2) \end{array} \right.##