Collisions and the Speed of Two Pucks

[bowbe]

Homework Statement

A hockey puck moving along the +x axis at 0.5 m/s collides into another puck that is at rest. The pucks have equal mass. The first puck is deflected 37degrees below the +x axis and moves off at 0.36 m/s. Find the speed and direction of the second puck after the collision.

Homework Equations

1/2*m_a*v_ai^2 + 1/2*m_b*v_bi^2 = 1/2*m_a*v_af^2 + 1/2*m_b*v_bf^2
m_a*v_ai + m_b*v_bi = m_a*v_af + m_b*v_bf

The Attempt at a Solution

For part1, I am using the condensed formula of:
v_ai^2 = v_af^2 + v_bf^2
since the masses are equal and the second puck starts at rest, which gives me
v_bf = .3496 (says it is incorrect)

For part2, I know to separate the x-component and y-component to get (condensed again):
x:
0.5 = (.36)(cos37) + (v_bf)(cos[unknown])
y:
0 = (.36)(sin37) + (v_bf)(sin[unknown])

I could solve for the unknown angle if I knew v_bf.

Can anyone help me find what I am doing wrong?

 

[alphysicist]

Hi bowbe,

Homework Statement

A hockey puck moving along the +x axis at 0.5 m/s collides into another puck that is at rest. The pucks have equal mass. The first puck is deflected 37degrees below the +x axis and moves off at 0.36 m/s. Find the speed and direction of the second puck after the collision.

Homework Equations

1/2*m_a*v_ai^2 + 1/2*m_b*v_bi^2 = 1/2*m_a*v_af^2 + 1/2*m_b*v_bf^2

Did you give the whole problem above? This equation is for elastic collisions (kinetic energy conserved). Is this an elastic collision?

m_a*v_ai + m_b*v_bi = m_a*v_af + m_b*v_bf

The Attempt at a Solution

For part1, I am using the condensed formula of:
v_ai^2 = v_af^2 + v_bf^2
since the masses are equal and the second puck starts at rest, which gives me
v_bf = .3496 (says it is incorrect)

For part2, I know to separate the x-component and y-component to get (condensed again):
x:
0.5 = (.36)(cos37) + (v_bf)(cos[unknown])
y:
0 = (.36)(sin37) + (v_bf)(sin[unknown])

Here you have two equations with two unknowns, and so you can solve for both unknowns from just these two. (However, I would write this with one of these terms being negative.)