# Homework Help: Ball thrown at skater

1. Mar 4, 2015

### PierceJ

1. The problem statement, all variables and given/known data
A 63 kg ice skater, at rest on frictionless ice, tosses a 4.06 kg snowball with velocity given by v = ( -14.3 i + 17.2 j ) m/s, where the x and y axes are both in the horizontal plane. Find the subsequent velocity of the skater.

2. Relevant equations
p = mv
p = m1v1+...

3. The attempt at a solution
I started by finding the total momentum of the system by multiplying 4.06 by each part of the vector and I got:
( -58.058i + 69.832j )
Then I set that equal to the two masses added together times v.
( -58.058i + 69.832j ) = 67.06v
v = ( -0.8658i + 1.041j)

2. Mar 4, 2015

### Staff: Mentor

What is your question? Where are the units? What about significant digits?

3. Mar 4, 2015

### PierceJ

The question is there "Find the subsequent velocity of the skater."
The units are also there, m/s
4 significant digits

4. Mar 4, 2015

### PeroK

Is the skater moving in the same direction as the ball he has just thrown?

Or was the ball thrown at him, as the title of your post suggests?

Last edited: Mar 4, 2015
5. Mar 4, 2015

### haruspex

If the skater moves off with velocity w,and the masses are m (snowball) and M, what is the conservation equation (in symbols, not numbers)?

6. Mar 4, 2015

p = Mw + mv?

7. Mar 4, 2015

### Staff: Mentor

I meant what is your question, why did you post? I now realize that I misread the question, as I had in mind what you wrote for the title of the thread. But is the problem really "Ball thrown at skater," as PeroK also asked?

QUOTE="PierceJ, post: 5030654, member: 546115"]
4 significant digits[/QUOTE]
Why 4? Is this compatible with the data from the problem?

But you should start by answering haruspex's question correctly.

8. Mar 4, 2015

### PierceJ

I realize now that I have read the problem wrong, but even so I still don't know how to answer haruspex's question. I know this forum is designed for me to figure out with only a little help but I'm completely lost here.

9. Mar 4, 2015

### Staff: Mentor

The question is about conservation. What can you write that relates before the throw to after the throw?

10. Mar 4, 2015

### PierceJ

(m1+m2)( -14.3 i + 17.2 j ) = m1v1+m2v2?

11. Mar 4, 2015

### Staff: Mentor

Start from the momentum.

12. Mar 4, 2015

### haruspex

It says the skater throws, not catches, the snowball. What is the momentum of the system before the skater does so? What is the momentum of the snowball after the skater has thrown it?

13. Mar 4, 2015

### PierceJ

The momentum before is 0? Cause the guy isn't moving?
Momentum of the snowball after is its mass times the velocity given in the problem.

14. Mar 4, 2015

### haruspex

Right. So have another go at the equation.

15. Mar 4, 2015

### PierceJ

Ah I see, I got it now. Thank you for putting up with me haha.