Hard Problem of the Week

[gorka09]

How many mph is a car taveling if the wheels have diameters of 3 ft and turn 400 times a minute? (to the nearest whole number)

I would greatly appreciate it if you can help me!

 

[stapel]

With a diameter of d = 3', what is the circumference?

With 400 revolutions per minute, how many feet are covered in one minute?

How many minutes are in one hour? So how many feet are covered in one hour?

How many feet are in one mile? So how many miles are covered in one hour?

Eliz.

 

[gorka09]

Thank you!

 

[gorka09]

EEK! I got 1193702400 mph! I am horrible at math!

Can you help me find where I went wrong? These are the answers I got for your questions.

1. 9.42
2. 113
3. 226080

 

[stapel]

Can you help me find where I went wrong?

Certainly. Please show your work, and we'll be glad to check for errors.

Thank you.

Eliz.

 

[gorka09]

First, I got the circumference of 9.42 by multiplying pie times 3. Next I took 9.42 and multiplied it by 12. I got 113.4. Then I multiplied it by 60, then I multiplied it by 5280.

Sorry, I have to leave right now, but I'll be back on in about 30 minutes possibly.

 

[stapel]

First, I got the circumference of 9.42 by multiplying pie times 3.

That gives you the number of feet in one revolution.

Next I took 9.42 and multiplied it by 12.

Why?

Then...I multiplied it by 5280.

Why did you multiply the number of feet by the number of feet in a mile?

Think about the process, and then work through the steps in the order given.

Eliz.

 

[gorka09]

Ohhh.. I didn't realize that it was the number of feet in one revolution. Now do I multiply 9.42 by 400? 3768? Then do I multiply 3768 by 60?

oops.. I have to go again. I'll be on same time tomorrow. Thanks for the help!

 

[daon]

Just think for a minute.

You have 9.42 feet / turn. You have 400 turns / minute. How do you cancel the 'unit' turns? What are you left with?

Remember that your units in the end must be miles/hour.