# Center of mass particle problem

1. Dec 9, 2003

### pringless

A 4.01g particle is moving at 1.36 m/s toward a stationary 9.43g particle. With what speed does the heavier particle approach the center of mass of the two partciles?

i dont get how cm relates to finding speed

The mass of a star like our sun is 347000 Earth masses, and the mean distance from the center of this star to the center of a planet like our Earth is 6.63*10^8 km. Treating this planet and start as particles, with each mass concentrated at its respective geometric center, how far from the center of the star is the center of mass of the planet-star system? Answer in units of km.

2. Dec 9, 2003

### Staff: Mentor

Well, how do you find the center of mass of a two particle system?

3. Dec 9, 2003

### pringless

is it like V_cm = m1v1+m2v2 / m1 + m2?

4. Dec 9, 2003

### NateTG

You probably meant
$$V_{cm}=\frac{m_1v_1+m_2v_2}{m_1+m_2}$$ which is correct. You should be carefull to put your parens in where necessary.

5. Dec 9, 2003

### pringless

thx nate..
i have one more question for that first problem.

how would you calculate the magnitude of momentum of the lighter particle relative to the center of mass?

6. Dec 9, 2003

### NateTG

Linear momentum is mv.
Since you know the mass already, so all you have to do is figure out the relative velocity.