1. The problem statement, all variables and given/known data http://www.jyu.fi/tdk/kastdk/olympiads/2009/Theo1_Question_Final.pdf This is a problem on some gravity phenomenon. I can't even get past the first part (though to be honest I'm probably not qualified even to look at these) The moon - earth system is changing. Because the tidal bulge axis is not aligned with the moon-earth axis, a torque shifts angular momentum from the earth's rotational into the moon's translation. Determine the conservation of momentum equation for the initial and final states of the system. In the final state, the moon's rotational velocity and the earth's rotational velocity will be equal; the moon will no longer move away from the earth afterwards. QUOTE: "Neglecting the contribution of the Earth´s rotation to the final total angular momentum, write down the equation that expresses the angular momentum conservation for this problem." 2. Relevant equations L = Iw 3. The attempt at a solution Initial angular momentum: I(earth)w(earth,initial) + I(moon,initial)w(moon,initial) = L Final angular momentum: I(earth)w(final) + I(moon,final)w(final) = L I(earth)w(earth,initial) + I(moon,initial)w(moon,initial) = I(earth)w(final) + I(moon,final)w(final) BUT their solution on http://www.jyu.fi/tdk/kastdk/olympiads/2009/Theo1_Answer.pdf I(earth)w(earth,initial) + I(moon,initial)w(moon,initial) = L = I(moon,final)w(final) I don't understand why they just blew off the angular momentum of the earth's rotation from the equation. It's obviously not w=0 that's for sure. Thanks for your help.