ACT Math : How to find the diameter of a sphere

Study concepts, example questions & explanations for ACT Math

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Example Questions

Example Question #1 : Spheres

If a sphere has a volume of , what is its diameter?

Possible Answers:

Correct answer:

Explanation:

1. Use the volume to find the radius:

 

2. Use the radius to find the diameter:

 

 

Example Question #2 : Spheres

A sphere has a volume of . What is its diameter?

Possible Answers:

Cannot be determined from the information given

Correct answer:

Explanation:

This question relies on knowledge of the formula for volume of a sphere, which is as follows:  

In this equation, we have two variables,  and . Additionally, we know that  and  is unknown. You can begin by rearranging the volume equation so it is solved for , then plug in  and solve for :

Rearranged form:

 

Plug in  for V

Simplify the part under the cubed root

1) Cancel the 's since they are in the numerator and denominator.

2) Simplify the fraction and the :

 

Thus we are left with 

Then, either use your calculator and enter  Or recall that  in order to find that .

We're almost there, but we need to go a step further. Dodge the trap answer "" and carry on. Read the question carefully to see that we need the diameter, not the radius.

So

 is our final answer.

Example Question #3 : Spheres

A spherical plastic ball has a diameter of . What is the volume of the ball to the nearest cubic inch?

Possible Answers:

Correct answer:

Explanation:

To answer this question, we must calculate the volume of the ball using the equation for the volume of a sphere. The equation for the volume of a sphere is four-thirds multiplied by pi, which is then multiplied by the radius cubed. The equation can be written like this:

We are given the diameter of the sphere in the problem, which is . To get the radius from the diameter, we divide the diameter by . So, for this data:

We can then plug our newly found radius of two into the equation to find the volume. For this data:

We then multiply  by .

We finally substitute 3.14 for pi and multiply again to get our answer.

The question asked us to round to the nearest whole cubic inch. To do this, we round a number up one place if the last digit is a 5, 6, 7, 8, or 9, and we round it down if the last digit is a 1, 2, 3, or 4. Therefore:

Therefore our answer is .

Example Question #96 : Solid Geometry

A boulder breaks free on a slope and rolls downhill. It rolls for  complete revolutions before grinding to a halt. If the boulder has a volume of  cubic feet, how far in feet did the boulder roll? (Assume the boulder doesn't lose mass to friction). Round  to 3 significant digits. Round your final answer to the nearest integer.

Possible Answers:

Correct answer:

Explanation:

The formula for the volume of a sphere is:

To figure out how far the sphere rolled, we need to know the circumference, so we must first figure out radius. Solve the formula for volume in terms of radius:

 

Since the answer asks us to round to the nearest integer, we are safe to round  to  at this point.

To find circumference, we now apply our circumference formula:

If our boulder rolled  times, it covered that many times its own circumference.

Thus, our boulder rolled for 

Example Question #11 : Spheres

Find the diameter of a sphere whose radius is .

Possible Answers:

Correct answer:

Explanation:

To solve, simply remember that diameter is twice the radius. Don't be fooled when the radius is an algebraic expression and incorporates the arbitrary constant . Thus,

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