# Thread: Determine the perimeter of the stones?

1. ## Determine the perimeter of the stones?

In the Strongman competition, one of the competitions is to lift a rock. At a competition, the stones weigh 120, 140, 160 and 180 kg. Suppose the density is 2800 kg/m^3. Determine the perimeter of the stones.

I really do not know how to solve this, it would be nice if someone could explain!

2. ## Re: Determine the perimeter of the stones?

what shape are the stones? spheres?

note $V = \dfrac{m}{\rho}$

circumference of a sphere is $2\pi r$

3. ## Re: Determine the perimeter of the stones?

the stones are spheres. However, we do not know r. So i cant get out the circumference with 2*pi*r ...

4. ## Re: Determine the perimeter of the stones?

The volume of a sphere is given by $\displaystyle V= \frac{4\pi}{3}r^3$ so if you know the volume, V, you can find the radius, r. You are told that one of the spheres weighs 120 kg and that the density is 2800 kg/m^3. So the volume of that sphere is $\displaystyle \frac{120}{2800}= \frac{3}{70}$ cubic meters.

5. ## Re: Determine the perimeter of the stones?

so

p = m / v

v = 120/2800
V = 3/7 m^3

v = 4 * pi * r^3/3

R^3 = 0.10 m
R = 0.46 m

Circumference = 2 * pi * r
2 * pi * 0.46 = 2.93 m

My answer is wrong, because the correct answer should be one of the answers 136m, 144m, 147m, 150m, and 156m.

Do not know what I've done wrong.

6. ## Re: Determine the perimeter of the stones?

Originally Posted by selin
so

p = m / v

v = 120/2800
V = 3/7 m^3
Sigh. I told you previously that the volume is 3/70 cubic meters, not 3/7!

v = 4 * pi * r^3/3

R^3 = 0.10 m
R = 0.46 m

Circumference = 2 * pi * r
2 * pi * 0.46 = 2.93 m

My answer is wrong, because the correct answer should be one of the answers 136m, 144m, 147m, 150m, and 156m.

Do not know what I've done wrong.