# Homework Help: What is the angle between the directions of motion of the helium atom

1. Jul 12, 2010

### ecogreen

1. The problem statement, all variables and given/known data
Question 1
An alpha particle travelling at velocity u makes an oblique elastic collision with a helium atom moving with a velocity which is negligible compared to u. After the collision the alpha particle moves in the direction inclined at 30 degrees to its initial direction of motion.

What is the angle between the directions of motion of the helium atom and the initial direction of motion of alpha particle?

Question 2
A ball is released from a point a few metres above the ground. By considering the ball earth system, discuss whether momentum is conserved before the ball reaches the ground.

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I don't really understand the situation in Question 1. For question 2, I do understand that momentum is conserved before the ball reaches the ground. Do I use m1v1=m2v2 or using total KE = total PE....

2. Jul 12, 2010

### bacon

Re: Momentum

1. It will help a lot to draw a picture of what's going on. I assume from the question that you have not been able to do this. The alpha particle hits "obliquely", that means not directly in the middle of the helium atom. Consequently the two particles will scatter in two dimensions and not just one. An "elastic" collision is one where kinetic energy is conserved.(click on the link on the word "elastic"). This will allow you to equate kinetic energy before and after the collision.
The problem gives you the direction of the alpha particle, draw that on a piece of paper. Also draw the general direction of the helium atom. You will need to resolve the velocity vectors of each particle into x and y components. Remember, momentum is conserved along the x axis and also along the y axis.