posted by casey .
a proton (mass 1.67 x 10^-27 kg) with a velocity of 1.10 x 10 7 m/s collides with a motionless helium nucleus, and the proton bounces back with a speed of 6.00 x 10 6 m/s. The helium nucleus moves forward with a velocity of 4.00 x 10 6 m/s after the bombardment. What is the mass of the helium atom?
momentum is conserved.
mHe*vhe + mp*Vp=mpVp' + mhe*vhe'
solve for mhe.
forward velocity is +, back velocity is -