# Homework Help: Rotational Motion and Law of Gravity

1. Nov 7, 2011

a) Two objects attract each other with a gravitational force of magnitude 9.30 x10-9 N when separated by 19.4 cm. If the total mass of the objects is 5.06 kg, what is the mass of each?

Heavier mass_______kg
Lighter mass______ kg

my attempt:
F=Gm1m2/r^2
so I rearranged to get m1 and m2 by itself and got (r^2)(F)/G=m1m2
5.247598=m1m2
but how do I get the mass for m1 and m2 separately is m1 + m2 = 5.06 kg?

b) A dental bracket exerts a horizontal force of 86.2 N on a tooth at point B in the figure. What is the torque on the root of the tooth about point A? (PLEASE SEE THE ATTACHMENT FOR THE PICTURE)

I know I need to break the forces up into components but I am lost on this question. Please help me! Thank you

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2. Nov 7, 2011

### Staff: Mentor

Yes, the total mass is given as 5.06 kg, so m1 + m2 = 5.06 kg. Replace one of the masses, say m2, with m2 = (5.06 kg - m1).

3. Nov 7, 2011

I used this and found my answer to be .433 and 4.627 but its wrong. Do i have my formula correct?

4. Nov 7, 2011

### Staff: Mentor

Your formula (Newton's law of gravitation) looks okay. Perhaps something went wrong in your math manipulations. If you post your calculations they can be checked.

5. Nov 7, 2011

actually I just solved it! It was just a silly error! But thank you for your help

Do you have nay idea how to do part b ?

6. Nov 7, 2011

### Staff: Mentor

What are your thoughts? What do you think you need to know in order to proceed?