Overcoming Friction

The sun is finally out again, and although we’re not really feeling it’s warmth just yet, we wanted to enjoy the rays nonetheless.  One of the things that the kids love to do in our backyard when they aren’t playing on the swing set is to climb “Boot’s Mountain”.  Part of our yard is a steep hill that goes back into the woods.  The kids run up to the top where the woods meet the yard and declare that they have climbed Boot’s mountain (named after an episode of Dora).  I decided that we could have a quick and fun lesson that not only let everybody go outside for a short while, but also incentivized the kids to come back in before we all froze. 

I tell the kids after naptime that we can all go outside for a little while.  You would have thought I told the kids that we were going back to Disney World for the joy that they showed.  First we’re going to pick out some balls to play with.  We go over to the ball bin and look inside.  I tell the kids that we should get one ball each and take it outside.  Briana picks out a Mickey Mouse beach ball.  Abigail grabs a yellow softball…um, we don’t have a softball…I guess Abigail grabs a softball that one of our friends is missing (if you’re looking for your softball and have been to our house recently, call me).  I grab a golf ball.  We take our treasures out to the yard. 

We go outside and play on the swing set for a few minutes and throw our balls around for a while when Abby screams out “I climbin’ a Boot’s mountain.”  Yeah, it’s time.  I tell everyone to take the balls to the top of Boot’s mountain.   Abby takes mine too.

  

Then, with no additional information given, I tell the kids to roll their ball down Boot’s mountain.  Whose ball went the farthest?  Briana’s smooth ball did (though in this case it was mostly because the force of Briana’s throw exceeded the force of Abby’s throw with the golf ball and her softball).  What happened?

“My ball rolled all the way down because it was the roundest.  Round things roll good.” Briana explains to Abby.

“Do you know what stopped it from continuing to roll down to the bottom?” 

“Gravity made it go on the ground so gravity stopped it on the ground.”

A very scientific answer presented by Briana.  I like how her mind works.  I explain that gravity pulls something out of the air and onto the ground, so when you threw the ball, gravity pulled it out of the sky and onto the ground and then it rolled. Something called friction stops things from rolling.  Friction is what happens when two things touch, like the ball and the ground, and it slows the moving thing down.  So friction stopped our balls from rolling all the way down Boot’s mountain.  Just like you told Abby, round things roll well because they have a little friction with the ground.  Bumpy things have more friction with the ground.

Quick aside:

Yes, we all know that gravity is still pulling on the ball as it rolls down the hill, but any scientists out there are going to have to give me some poetic license today for simplicity’s sake.  Toddlers remember.  ;)  Incidentally when I told my husband what we were going to do, he asked if I was going to show them F=ma and F(f)=mu*F(n).  Um, no.  We need to learn addition first. 

But for anyone with older kids, here are the full forces acting on the balls after the kids push them (assuming no other forces, like more pushing, the wind, etc):

With N being the Normal force perpendicular to the plane, m is the mass of the ball, g is gravity, theta is the angle of the hill, and f is the force of friction.  Note that since the ball doesn’t move perpendicular to the hill, that is bounce upwards, (ie N=m*g*cos(theta)), you can simplify the diagram by taking those off.

But back to our day.

We collect the balls and roll them again.  Briana’s goes the farthest and she amends her previous explanation to, “My ball is the roundest AND it has the less friction so it went the farthest.  Abby stopped her ball so Abby is friction.”  The kids always crack me up.  You’re right, Briana, in this case, Abby represented friction.

Abby chasing her ball down the hill

We roll some rocks and what we call “monkey brains” because that’s what our neighbor’s kid said they were called and we have no other word for the giant green balls that fall from our tree.  We talk about which things we think have lots of “bumpy friction” and what we think has less.

Then I tell the kids that the ground has friction too.  “Yep, the grass makes it bumpy, so you’re right Mommy!”  says Bree.  “Good job Mommy.” Abby adds, though she’s not really paying her full attention to our discussion because throwing stuff down the mountain has Abby tickled pink. 

Don't mind my shadow - the cell phone's camera is limited.

Eventually, Bree’s ball doesn’t go the farthest and she is frustrated.  My golf ball, powered by Abby, goes the farthest and this defies science to Briana.  Like Walter Bishop on Fringe, Briana does not like to see the laws of physics disrupted so she comes up with her own theories “Abby’s grass is less bumpy than my grass.”  Nope.  “Abby…hmmm…waaaaaa!”  Like I said, frustrated.  I give Briana a big hug and Abby does too while saying “It’s ok Breezy.  I hug a Breezy.”  Then I tell the kids that friction is only one thing acting on our balls.  The other thing is called the Kid’s Force.  Force is how hard you push the ball and force can be stronger than friction.  Go and kick your ball and see what happens.  It beats Abby’s ball now.  It just needed more force.

Bree loves the explanation of force and has a blast rolling balls and then kicking them when they stop and shouting, “I have to apply Force! Kick!”

I wonder what would happen if we had less friction.  What do you think about the hardwood floor in the kitchen?  Is it bumpy?  “No, it has little friction.”  So, what would happen if we roll our balls somewhere with less friction?  “They will go and go and GO!”  Oooh, let’s go find out…and get in out of the cold.

When we get coats off and settled in, Bree has collected a new set of balls.  Some really spiky ones and some plastic ones with a lot of holes in them that I got from Gymboree, a completely deflated volleyball, a mini basketball from their hoop, balls from the ball pit, and the balls that we had outside, minus the golf ball about which Abby says “I can’t find it.  Ooooh lost.”  I prep dinner while a million balls come flying past me.  I listen to the girls talking to each other about how much friction each ball has with respect to the other balls.  I hear a lot more of “I gave that ball Force.” from Bree or “A Force kick” from Abby.

We spend 2 afternoons checking out different types of balls on different flooring (the wooden deck, the hardwood, the Berber carpet, the regular carpet, tile).  While I wanted the takeaway to be an understanding that friction slows things down, I am happy with the kids takeaway, that force can make something go.

One final thought – you don’t need to be a scientist to teach your kids to understand and enjoy science.  There a millions of sites online that can help you with the basics.  That’s what I have to do when we talk about fashion or any other area where I have no real training or understanding.  I think that it is very important to expose your kids to lots of diverse ideas when they are young.  Not at all so that they master them.  You do it so that they are familiar enough to know that they can understand them.  That way they aren’t scared when these types of concepts are introduced much later in school.  It’s about feeding their curiosities and their confidence in their own abilities to learn about their environments.

<3 Pedigreed Housewife