# Homework Help: Collisions problem (How should I approach it)

1. Dec 13, 2003

### Hindi

Hi all,

I came across a problem on collisions on one of my professors old exams. The problem is:

http://home.comcast.net/~msharma15/problem_2.jpg

The way I am trying to approach it is by applying the conservation of linear momentum and energy, but the problem is that I still get left with 3 unknowns.

Here is what I know:
Before the collision, only block A has kinetic energy.

After the collision, the K.E. of system is (1/2 K.E. initial). block A has -1/2MV^2 and block B has 1/2MV^2.

The final collision is what confuses me. Should I just work with K.E.i (only block A moving) with conservation of linear momentum?

Any help would be greatly appreciated. Thanks!

Last edited by a moderator: Apr 20, 2017
2. Dec 13, 2003

### StephenPrivitera

by cons of p,
$$m_1v_{1i}=m_1v_{1f}-m_2v_{2f}$$
where v_2f is reckoned as negative
By the energy conditions,
$$m_1v_{1i}^2=\frac {1}{2}(m_1v_{1f}^2+m_2v_{2f}^2)$$
which gives two equations with two unknowns.

3. Dec 13, 2003

### Hindi

StephenPrivitera. I am just wondering why you made m_2v_2f negative in the first equation?? Should it be the other way around?

4. Dec 14, 2003

### Staff: Mentor

The equations should be:
$$m_1v_{1i}=-m_1v_{1f}+m_2v_{2f}$$
$$\frac {1}{4}m_1v_{1i}^2=\frac {1}{2}(m_1v_{1f}^2+m_2v_{2f}^2)$$
(where the speeds are all positive)