It is amazing how much of introductory Physics we all learn before we turn one year old. We learn it so well and internalize the information that we forget that we even know it. It is just a part of our core knowledge, like breathing and eating (other things we learn as babies).
My daughter is just learning these things for the first time. She is just beginning to explore motion. Here are some of the things that she has learned so far:
- The smooth floor at mommy’s yoga class is good for sliding on. You can scoot while sitting up to get closer to the good toys (like yoga blocks and straps).
- The rug is best for rolling over and standing up and sitting up.
- The counter is not good for standing up on.
- Sitting can be accomplished by either pulling on mommy’s hands, or both pushing on the floor and pulling on something.
- Rolling is easiest if you first push up on your arms, or push your butt up into the air, or kick your legs up into the air.
- Small things move when you pull on them. Parents and furniture do not.
Friction and Forces
So, my daughter has learned all about forces & friction: The smooth floor at the yoga studio has very small coefficients of both static and kinetic friction – that means it is easy to get things moving and keep them moving. In order to move, she needs to apply a force that is larger than the force of friction in order to accelerate from her current position.
One of the first things we learn in Physics is that the sum of all the forces on an object is equal to the mass of the object (mdaughter) times its acceleration (adaughter). My daughter needs to scoot with a force greater than that of friction, and then her movement will be governed by the following:
The harder she pushes off (larger Fdaughter), the faster she accelerates. Or if the force of friction is smaller, it’s easier to move. That’s why she can scoot on the smooth floor but not on the rug yet. Her small mass also makes it easier. I have to push much harder to scoot on the floor than she does, but then again, I’m a bit stronger as well.
Okay, so sliding is easiest on surfaces with less friction – smooth floors, countertops, icy surfaces, etc. But sometimes sliding does not help her in her attempts to move.
When she is trying to stand, she needs her feet to stay still (not slide) while she pulls up. When she’s sitting on the counter and she pulls on my hands, she just slides instead of standing up. So, for standing, she needs a big friction force.
Newton’s Third Law
She has just discovered that pulling up on my hands seems to be similar to pushing down on the floor. Why? Newton (he was a smart guy) said that every action has an equal and opposite reaction. When she pulls down on my hands, my hands (without me moving) pull up on her. That helps her sit up. When she pushes down on the floor, the floor pushes up on her, also helping her to sit up.
Of course, if she pulls on smaller objects or objects that are on a smooth surface, sometimes they move! That’s just friction again – the force of friction is directly related to the weight of the object. Heavier objects experience have a bigger friction force keeping them immobile. Lighter objects experience less friction and so it takes a smaller force to get them in motion.
It is a common misconception that she can pull lighter objects because she pulls on them harder than they pull back. That’s never true. The forces are always equal and opposite. When my daughter pulls an object to her, it is because the force of her pull is greater than the force of friction (or other force) holding the object in place.
My daughter has been rolling over for quite some time, but she’s always hated to be on her belly, so she refused to roll onto her belly. And she was so unhappy when on her belly that she just cried instead of thinking to roll onto her back.
In the past couple of weeks, though, she has remembered that she can roll over. She stays on her belly exactly as long as she wants and then rolls over. She’s also realized that she can continuously roll over and over again to move across the room. (Crawling on her belly has occurred to her but she has not yet discovered how to make that work).
We know that we need a big force to move in a line. My daughter needs to learn to push or pull (or kick) off with more force in order to scoot along or crawl. What about rolling? Is it just a matter of force? How do we rotate?
Think of opening a door. When you push a door open, it matters both how hard you push the door and where you apply that force. If you try to open a door by pushing on it very close to the hinge, you have to push very hard to get it to open. But if you push on the edge of the door furthest from the hinge, you do not have to push very hard at all. So rotating objects seems to depend on how hard you push (or pull) but also where you apply that force – how far from the point of rotation.
When my daughter lies flat on the ground and tries to roll over, she doesn’t go anywhere. She would have to have very strong abdominal muscles to make her body rotate – the force of her abs is applied very close to her center of rotation.
But she’s too smart for that. She’s already learned about torque (force times distance from axis of rotation) and gravity. So, when on her back, she kicks her legs up in the air (and sometimes her arms too), rotates just a little bit and uses gravity to make her legs and arms fall to the side and bring her around all the way. When on her belly, she can push up on her arms first, then just push off a little on one arm and her body falls to one side and rolls back over (kicking your butt up in the air and kicking off works just as well). She has already learned that she needs a lever arm to rotate. What a smart kid.
Physics in Motion
My daughter has not yet mastered the art of motion, but she learns more about it every day. She is quickly mastering the Physics involved and will soon be ready to study energy, power, fluid motion, electricity and magnetism. By the time she’s five, I imagine she will be doing her graduate work in Physics. Or perhaps, she will be done with that and have moved on to learning new languages and cultures by then.
I love to watch her learn. If only we could all learn with the wide-eyed, fearless innocence of a baby. Not to mention the amazing brain capacity to learn a hundred new things every day. I guess if my brains were working at that speed all the time, I would also need two naps and eight meals a day.