Monday, January 23, 2012

Centripetal Force: The Real Force

This is my disclaimer:

I am not a physicist. I am not a teacher. My expertise and credentials, if one may call them that, are in Geography, Biology, and to some extent, Photography and Astronomy (I consider being an active Amateur Astronomer for over 30 years somewhat of an expertise in general astronomy).

However, I am highly motivated to share science of all types, and specifically, scientific methods, and critical thinking. This is why I like to post about science topics. I get things wrong at times, but try very hard to do my homework and to not get things wrong. I enlist the help and expertise of others to make sure I get stuff right. 

The short of it is I will share information which I find interesting and useful and hope you will enjoy and share back. 


Last week I choked my way through a pitiful attempt of an explanation of Centrifugal Force. This week, I will put two brain cells together and try and explain Centripetal Force, or as some* have called it, Who Gives a Shit?

Easiest way I know to explain it is to go with this example:

Ball on a string.

Find a ball attached to a string. Go ahead; I'll wait. 

Got it?
Ok, now swing the ball around on the string. Faster! Faster! 
Oops. Sorry. First, call the glazer, then take the ball and string outside. 

Now swing the ball around you like a Crusader with a Ball and Chain** 
There, you've done it. You've demonstrated centripetal force. Go back inside, pop open a bottle of wine and congratulate yourself. So what did you do?

Back to Isaac Newton and his Laws of Motion. Remember the first one? An object in motion tends to stay in motion and an object at rest tends to stay at rest until acted upon by another force? Yep. That one.

The object in motion (ball) was really traveling in a straight line, or at least it would have if it weren't attached to the string. The ball was accelerating, but the string was causing it to change direction constantly, therefore, the net result was that the ball was continually being pulled toward the center, which is the end of the string in your hand. The net force was center seeking, or centripetal. 

The outward force that you felt was not centrifugal, but simply the velocity of the ball in its original path. The ball was trying to fly off in one direction. You felt the ball's mass and velocity. Test this (outside) by swinging it again and letting go of the string. 

Zing! Off it goes. This works even better if you have a kid to retrieve the ball and string each time. It's like a game of fetch, but with college credits possible.

The super-simplified explanation. Possibly. And I managed to work in the words "you felt the ball's" without being skeezy. 

Now go out there and play physics!

*Everyone I've tried to explain this to.

**An actual ball and chain. Don't go to the other place, gentlemen, or you will see a demonstration of the skillet in flight force. Which also strikes me (badoom-cha!) as sexist.

Being the nice guy I am, I'm including, at no extra charge, links to two websites that explain this correctly, with diagrams and everything.
Diagrams and equations and stuff.
Another nice site: Flash required


  1. I totally agree it works better if you have a kid to go get the ball.

    And you dog looks EXACTLY like Winnie, my childhood pet. What a sweet pooch you have.

  2. I have to go play physics now. See ya.

  3. You seriously did not wait long enough for me to retrieve a ball and string! Slow your roll man!! Okay... got it. Swinging, spinning it, dammit... I have vertigo. This is making me ill. Aside from the nausea and tripping everywhere, this was quite cool! -- Mrs. Snarky

  4. I exempt myself from participating in the actual "ball on a string" experiment *because* when I was in 7th grade I did my science experiment for the annual Science Fair on centripetal force. I was a gymnast at the time and had pictures of me doing giants(circular rotation) around the uneven bar (bar = end of string in hand; me = ball; (is that right???)). I can barely remember the details, but I did win either 2nd or 3rd place in the fair?!