Collision of subatomic particles

[fffff]

A sub-atomic particle os mass 0.113 u collides head-on and elastically with a stationairy neutron. The neutron moves off with a
speed of 3.8 x 10^6 (power 6).

I am having a number of difficulties with this question. For instance, what's the mass of one neutron in terms of u
please forgive me for asking such a newbie question but what does a 'head-on collision mean'

and why is it that the equation for principle of momentum
(m1u1+m2u2=m1v1+m2v2)
would not necessay apply if the sub-atomic particle had a much greater initial speed.

what does this mean?
and why is it that we are applying the equation in the first place.

Thanks

 

[rsk]

A neutron has a mass of 1 atomic mass unit - a proton likewise (in reality they're slightly different but I assume this question wants you to use 1u)

Head on just means you don't have to consider angles.