# I'm in dire need of momentum help.

1. Feb 18, 2008

### TheNormalForc

If P = mv, then P^2/(2m) is:

I know the answer is Kinetic Energy, I just don't know the proof. Help?

A 70kg man and his 40kg daughter stand together on skates in the middle of a frozen lake. They push apart. Immediately after the push the father has an initial velocity of .8 m/s in on direction and his daughter has an initial velocity of v in the opposite direction.

Find the magnitude of v, the daughter's speed:

The question continues to ask for other variables and the like, which I could all calculate if I knew the daughter's speed, which I can not figure out. Please do help.

2. Feb 18, 2008

### Kurdt

Staff Emeritus
I'm sure you know that $E_k = 1/2 mv^2$. You can work out v in terms of momentum and substitute.

Remember that momentum is conserved. You can use this to work out the daughters speed.

3. Feb 18, 2008

### lavalamp

For the first question you ask, I just answered an almost identical one here.

For the second part. You know that their momentum must be the same, but in opposite directions. Since you know the fathers momentum is mv = 70*0.8, you can set it equal to the daughters and solve for her velocity.

4. Feb 18, 2008

### TheNormalForc

The simplicity of my delima really disheartens me, you know?

Thank you both.

5. Feb 18, 2008

### lavalamp

Ah don't sweat it too much, any day you learn something new is a good day right?

6. Feb 18, 2008

### TheNormalForc

That's true, but it's sometimes frustrating to ask for help. Atleast with the advent of the internet I can ask behind anonimity.

7. Feb 18, 2008

### Kurdt

Staff Emeritus
I wouldn't be embarrassed or frustrated about asking for help. How else do we learn.

8. Feb 18, 2008

### TheNormalForc

Well, while we're on the subject of helping.

If the coefficient of kinetic friction between the daughter's skates and the ice is mew = .1, how far will she slide before she is brought to a stop by the friction force?

9. Feb 18, 2008

### Kurdt

Staff Emeritus
We help, we don't give answers . What have you tried so far or what are your thoughts about tackling this problem?

10. Feb 18, 2008

### TheNormalForc

I calculated that the average force exerted on the daughter is 160N, and that the magnitude of her velocity is 1.4 m/s. I know F=ma is a relevant formula to the question, but I'm stuck from that point on.

11. Feb 18, 2008

### Kurdt

Staff Emeritus
The average force from friction will not be 160N. The force of friction is $F = \mu_k N$, with N the normal force. Once you have the force you can work out the acceleration from F = ma. Then you will need to use the kinematic equations.