# Homework Help: Calculate the mass of unknown nucleus

1. Aug 25, 2004

### james11

OK having a slight problem finding the mass of the unknown nucleus.

Any help rearranging to find this using Newton's Gravitational law.

A hydrogen nucleus and a nucleus of an ‘unknown’ atom are stationary, 5.0 x 10 ^-10 m apart. The mass of the hydrogen nucleus is mh = 1.7 x 10 - 27kg and the electric charge of the hydrogen nucleus is e = 1.6 x 10 ^-19 C. You may assume that the electric constant ke, = 9.0 x 10^9N m2 C-2 and that the gravitational constant G = 6.7 x 10^-11N m2 kg -2.

The magnitude of the gravitational force between the hydrogen nucleus and the ‘unknown’ nucleus is 9.3 x 10 ^-45 N. State whether this force is attractive or repulsive. Calculate the mass of the ‘unknown’ nucleus that is necessary to give a force of this magnitude.

any help would be great!

J

2. Aug 25, 2004

### james11

There are many other parts to the question that i have worked . I just need help on rearranging the equation for this part.

3. Aug 25, 2004

### Staff: Mentor

I assume you know Newton's law of gravity:
$$F = G\frac{m_1 m_2}{r^2}$$

This can be rearranged to solve for one of the masses:
$$m_2 = \frac{F r^2}{G m_1}$$

4. Aug 25, 2004

### james11

Yes I know Newtons Law. I already rearranged to this equation but my figures don't seem to add up.

I have tried a few times now with my calculator and i was missing a minus sign on one the powers.

I have solved the problem