What was the mistake in solving for the speeds of two marbles after a collision?

[anotherphysicsgeek]

Homework Statement

A 49.0 g marble moving at 1.90 m/s strikes a 28.0 g marble at rest. Note that the collision is elastic and that it is a "head-on" collision so all motion is along a line.

What is the speed of 49.0 g marble immediately after the collision?
What is the speed of 28.0 g marble immediately after the collision?

Relevant Equations

1. m1v1i+m2v2i=m1v1f+m2v2f
2. 1/2m1v1i^2+1/2m2v2i^2=1/2m1v1f^2+1/2m2v2f^2

Solved equation 1 for v1f and then substituted into equation 2 and solved for v2f. Got 2.22 as the answer, but it said the answer is incorrect.

 

[Hill]

Homework Statement: A 49.0 g marble moving at 1.90 m/s strikes a 28.0 g marble at rest. Note that the collision is elastic and that it is a "head-on" collision so all motion is along a line.

What is the speed of 49.0 g marble immediately after the collision?
What is the speed of 28.0 g marble immediately after the collision?
Relevant Equations: 1. m1v1i+m2v2i=m1v1f+m2v2f
2. 1/2m1v1i^2+1/2m2v2i^2=1/2m1v1f^2+1/2m2v2f^2

Got 4.4333 as the answer

This is the answer to what? There are two questions.

 

[anotherphysicsgeek]

This is the answer to what? There are two questions.

Sorry it was the answer to the second question. Also going back and re-plugging in the numbers I got 2.22, but it's still wrong.

 

[PeroK]

Sorry it was the answer to the second question. Also going back and re-plugging in the numbers I got 2.22, but it's still wrong.

You need to show your work. Also, take a look at the Latex guide for maths equations, under the help pages.

In a question like this, where one mass is initially at rest, I would automatically simplify the relevant Equations. If you reply to this post, you'll see the Latex I used. I prefer $u, v$ to $v_i, v_f$. E.g.
$$m_1u_1 = m_1v_1+m_2v_2$$$$\frac 1 2 m_1u_1^2 = \frac 1 2 m_1v_1^2 +\frac 1 2 m_2v_2^2$$

 

[haruspex]

Sorry it was the answer to the second question. Also going back and re-plugging in the numbers I got 2.22, but it's still wrong.

That's not far off. Maybe a rounding error. Keep at least three sig figs until the end.
But it is silly having to guess. As @PeroK says, post your working.

 

[anotherphysicsgeek]

Thank you guys, and I'll fix that in the future. I ended up finding the correct answer and it was an algebra mistake.