# Homework Help: Transforming momentum between inertial reference frames

1. Jun 5, 2013

### bkraabel

1. The problem statement, all variables and given/known data
A bug of inertia $m_B$ collides with the windshield of a Mack truck of inertia $m_T \gg m_B$ at an instant when the relative velocity of the two is $\boldsymbol v_{BT}$.
(a) Express the system momentum in the truck’s reference frame, then transform that expression
to the bug’s reference frame, and in so doing remove $m_B\boldsymbol v_{BT}$ from the expression. (Remember, in the bug’s reference frame, the bug is initially at rest and the truck is moving.)
(b) Now express the system momentum in the bug’s reference frame, then transform that expression to the truck’s reference frame, and in so doing remove $m_T\boldsymbol v_{BT}$ from the expression.
(c) Is there something wrong here? How can we change the momentum by a small amount $m_Bv_{BT}$ doing the transformation one way and by a large amount $m_Tv_{BT}$ doing the transformation the other way?

2. Relevant equations
Take the bug's direction as the positive direction. System momentum in bug frame is
$\boldsymbol p_{sys,B}=-m_T\boldsymbol v_{BT}$
System momentum in truck frame is
$\boldsymbol p_{sys,T}=m_B\boldsymbol v_{BT}$

3. The attempt at a solution
I can see that the magnitude of the momentum is much larger in the bug frame, but I don't get the part about removing $m_B\boldsymbol v_{BT}$. It doesn't seem necessary or even possible. I understand that the absolute magnitude of the momentum in different inertial reference frames is not important. What is important is the difference between momenta in two inertial frames. This difference should be the same in the two frames.

2. Jun 5, 2013

3. Jun 5, 2013

### bkraabel

No special rules given for momentum transformation, just the regular Galilean transformation rules for transforming velocity between different frames. But applying a Galilean transformation just gives you the equations we've already written above.

4. Jun 5, 2013

### Simon Bridge

You can just write down the equation by inspection because, basically, you know the outcome in advance... or you can start with one and formally apply the transformation step-by-step and demonstrate you get the same thing.