# How do you find the radius of a sphere if you don't know the volume?

For my geometry project, you have to find a sphere in your house and measure its volume and surface area. I have one of those stress balls as my sphere but I do not know how to find the radius so I can't find the volume nor the surface area. HELP ME!!

Relevance
• Anonymous
10 years ago

r = (3V/(4pi))^(1/3)

• Login to reply the answers
• My suggested answer assumes that the stress ball is hard and not easily squeezed out of shape - although a little distortion won't matter too much.

Do you have a measuring jug in your kitchen? You know, the sort of glass or plastic jug that has a scale on one side (in cubic centimeters, say), that you can measure the volume of a liquid that is in the jug?

If so, you can use it to answer your Q. Like this . . .

Fill the jug up to about half way with water and note the reading on the scale. Now put the stress ball in. If it floats, gently push it just beneath the surface of the water with a pin. Make a note of the new reading on the scale of the jug. The difference between the two readings = the volume of the stress ball.

You can calculate the surface area from:

1. Volume, V = (4/3).pi.r^3,

and

2. Surface Area, A = 4.pi.r²

So: from (1) r^3 = 3V/4.pi, so r can be calculated.

Then put this value of r into (2) to calculate Surface Area, A.

--------------------------------------------

Another idea: have you got a spare tennis ball, a knitting needle and a little paint?

If you carefully paint a thin line around the tennis ball, push the needle right through the centre of the ball and holding the ends of the needle, carefully roll the tennis ball (with the paint wet) along a piece of paper, (being careful not to 'over-roll' the ball), you can thus find its circumference.

From that, you can find the radius.

Not such an accurate method as the first - but it just might give you some other - and better - ideas!

.

Source(s): Me: I'm an engineer. I'm used to solving 'everyday' practical problems!
• Login to reply the answers
• You secretly know more than you think you know.

Think about if you looked at the sphere from the side, what would you see?

You'd see a circle right? That circle would be like a cross-sectional cut of the sphere.

The circle's perimeter is special. The circumference of a circle is 2PiR.

So you can just take a piece of string, hold one end with your finger, and wrap it around until the other end overlaps with the starting point, and mark off on the string how long the circumference is.

Then lay that string out next to a ruler, and you can find out the circumference.

:)

• Login to reply the answers
• The volume of a sphere is = 4/3 pi r^3........... Four thirds (3.14159) radius cubed.

The surface area = 4 pi r^2.............Four pi r sqrd.

The radius is equal to half the diameter. Once you know how wide ball is, divide

by 2 and you have the radius.

• Login to reply the answers
• Supposed V(sphere) =4/3 pi r^3 = 100 4/3 pi r^3 = 100 with pi = 3 (4/3) x 3 r^3 = 100 4r^3 = 100 r^3 = 100/4(used cube root both sides) r = cube root(100/4) (answer)

• Login to reply the answers
• Measure the circumference of the sphere at it's widest point. The circumference is equal to 2(pi)r.

• Login to reply the answers
• Submerse it in water to get the volume. The volume is equal to the water displaced. From there, plug it into this V = (4pr^3)/3

Source(s): mathematics
• Login to reply the answers
• but you can find it.

Hint: measure the distance around the sphere.

Source(s): my brain
• Login to reply the answers
• measure the circumference

• Login to reply the answers