Finding the speed v of the proton

[Janiceleong26]

Homework Statement

Homework Equations

Principle of conservation of momentum,
Σp_i=Σp_f

The Attempt at a Solution

Since the nucleus is stationary, initial momentum is zero.
Unified atomic mass constant, a.m.u = 1.66 x 10^-27
Mass of the new nucleus: (A-4) x a.m.u
Mass of proton: 4 x a.m.u
0= ((A-4) x a.m.u )(u) + (4 x a.m.u )(-v)
4v = (A-4) u
v= (A/4 -1) u
So I chose A,
But the answer is B, where have I gone wrong?

 

[gneill]

Why did you subtract 4 mass units from the atom when it only emitted a single proton? Were you thinking alpha particle?

 

[Janiceleong26]

Why did you subtract 4 mass units from the atom when it only emitted a single proton? Were you thinking alpha particle?

Oh yeah, I was.
thanks for the help