A neutron with a mass of 1.67 × 10-27 kg traveling east with a kinetic energy of 2.00 × 10-21 J collides perfectly elastically with a helium nucleus with a mass 6.68 × 10-27 kg that is initially at rest. After the collision, the neutron has a kinetic energy 1.80 × 10-21 J. What angle was the neutron deflected during the collision? I am able to calculate the values of the final velocities. I just don't know what to do with the information provided and the information I calculated to get the final angle of deflection.

You need to use conservation of momentum in order to find the angle at which the neutron was deflected. Although, I would need to see an image of the problem or know if the helium nucleus was being deflected at an angle as well.

You can split up momentum into x and y components, similar to force. I.e

Pxi=PxF

PyY=PyF

Py:mvsin(theta)=mvsintheta

Px: mvcos(theta)=mvcos(theta)

Hopefully these equations can help you solve the problem.

a) lower because the sound-wave fronts reach the platform at a frequency lower than the frequency at which they are produced

explanation :3

If the train is leaving the train station, then the people who are standing on the platform would hear a sound with a lower frequency since the train is moving further away. ^^