# Homework Help: Sphere and radius

1. Sep 2, 2010

### afg_91320

1. The problem statement, all variables and given/known data
Two spheres are cut from a certain uniform rock. One has a radius 4.50cm. The mass of the
other is 5 times greater. Find its radius

2. Relevant equations
r1 = 4.50cm
r2 = ? when mass is 5x greater

3. The attempt at a solution
Looking at this problem I first thought of somehow integrating the volume of a sphere
which is 4/3$$\pi$$r3. But then i though i was just thinking too hard. it made more sense for the radius to be 5x the radius as i figure the mass was proportional to the radius.

so: r2 = 5(4.50cm) = 22.5cm

however i got this marked wrong as the answer was 7.69 cm. what wasn't I looking at clearly?

2. Sep 2, 2010

### Staff: Mentor

You started on the right track. Mass is proportional to volume, so the ratio 5 applies to the two radii how?

3. Sep 2, 2010

### afg_91320

well that would be proportional to the mass right? so if r2 is 5x in mass then r1 is 1/5(mass)? set up my equation to isolate r to get the radius...

4. Sep 2, 2010

### Staff: Mentor

Mass is proportional to volume. Write the fraction V1/V2 out fully, and that will show you a ratio involving some form of the radii. That's where the 5x comes into play...