Inside: Dynamic and creative activities for hands-on exploration of Newton’s 3rd Law of Motion. Twisting soda can, cats, and a couple of twisting challenges. Physics has never been as much fun.
Like most Russian girls, I once dreamed of a career as a professional gymnast. I was born hyper-flexible (and even now in my 40s I can still do splits, backbends, and some jumps) and when I was young my parents were offered a place in a government-funded gymnastics school. That kind of school is probably not what you would normally envision. Trainees were not allowed to leave the school from Monday through Friday. The training schedule was grueling and the academic learning took time only in the time left after the training sessions were done. My parents declined the offer.
An important thing to know about sports is that a great athlete is not just a strong person with good coordination and body/mind control. A great athlete is usually someone who understands physics and can use that knowledge to his advantage. There are many different aspects of physics that can be covered using gymnastics – energy, force, and friction – but today I want to concentrate on twisting.
Newton’s Third Law of Motion
In order to understand twisting, one has to start with Newton’s 3rd law of motion: for every action, there is an equal and opposite reaction. The harder a gymnast pushes against the beam, the higher is her lift into the air. Any time you want an upward movement, you have to start with a downward push. The “down” and “up” are two equal forces working in opposite directions.
The concept of “equal” was hard for my kids to grasp at first. When I told them that by kicking a ball they are exerting a force on a ball that is called action. They agreed. But when I told them that the ball is exerting an equal force on their foot called reaction, they vehemently disagreed.
How can it be true?! The ball is doing no such thing! Mom is wrong!
If the ball didn’t exert the force on your foot, you wouldn’t even feel it when you kicked the ball. You can feel the force of the ball when you feel compression on your foot at the moment of kicking.
The physics of twisting is taught in college-level mechanic courses, but as Albert Einstein said, if you can’t explain it simply, you don’t understand it well enough.
In preparation for the Olympics, I thought it would be fun to do a couple of hands-on activities that demonstrate the concept of twisting to my little kids in a simple way. We started by watching the twisting of a falling cat. (No cat was hurt during this experiment). Second, we used the power of water to twist a can of soda. And finally, we watched women’s gymnastics from the last Olympics and used our bodies to practice twisting (tip: it’s a great gross motor activity for kids who like to move).
Do Falling Cats Defy The Laws of Physics?
Until the camera was invented scientists had a hard time explaining how cats manage to land on their feet without defying the laws of physics. (If you are curious, here is the first-ever cat video made in 1894). How can cats possibly manage to turn in the air, with the absence of force giving it an initial rotation? Newton’s 3rd law of motion specifically states that there must be two equal forces working in opposite directions and the only forces working on a falling cat are gravity and friction. How can a cat use these forces to turn?
Physicists analyzed falling cats and discovered that all falling cats perform the same nifty trick. As they fall, (1) they bend in the middle to create two different axes of rotation: that is, the front half of their body rotates about a different axis than the rear half. (2) They turn in the front legs while leaving the back legs extended. The result is that the front and back of their bodies start rotating in opposite directions creating their own rotational force.
In short, by rotating sections of its body in opposite directions, cats manage to move their body as a whole. (It also helps that cats do not have a collar bone and can turn their heads 180 degrees to look down as they fall).
To help my kids understand the concept of axis and how the rotation happens I made them a toilet paper roll cat. The body consists of two rolls so that I could bend “the cat” in the middle and rotate the front and the rear parts in opposite directions. I put some stripes on the back of our cat to make the rotation more evident. If you don’t like the idea of your cat being so unnaturally long just cut one paper roll in half and use that.
Twisting Soda Can Experiment
In this experiment we made a soda can twist by filling it with water and strategically placing holes along the bottom of it to harness the gravitational force. The created rotational force surprised and fascinated my kids to no end. It’s a great outside experiment for summer and deserves its own post (coming up this week).
Hands-on Twisting Fun for Kids
Did you ever wonder why gymnasts do cartwheel before going into a backflip? A cartwheel is a twisting force that tends to cause rotation! Ask kids what are some other things that help us twist our bodies? How about the position of our hands? Ask kids to spin in one place while keeping hands straight down by their bodies. Now ask them to spin while spreading hands to the side? Which position of their hands creates better spinning? Hands out or hands down?
Look at this video of a gymnast performing her vault routine in slow motion. Doesn’t she seem to defy all the rules of physics by staying in the air so incredibly long with no apparent force to hold her there? More time in the air means more time to complete somersaults. And how does she get more time in the air? By jumping down as hard as possible to spring high into the air.
The kids really enjoyed watching this long video. here are many nice slow-motion sequences.
After watching gymnasts perform, we decided to create a series of twisting challenges by changing different variables to see if we can produce better twisting without any special equipment.
Remember how the upward movement is created by downward push? We decided to vary how hard we jump on a pillow to see if it will change how much we can twist.
(Safety note: make sure the kids are jumping on a soft surface like grass or carpet and the carpet is securely attached to the floor. Make sure the pillow is not covered with slippery material like silk).
- Stand on one end of your working space with a pillow about a foot or two in front of you.
- Start running and jump on the pillow with some force.
- Immediately jump off the other end trying to twist the body 180 degrees in the air.
- Now see, if you can perform a 360 degrees rotation.
- Try this a couple of times.
My kids just couldn’t get enough of this experiment.
- Stand in front of a pillow.
- Jump on the pillow and immediately jump off trying to twist the body 180 degrees. Then 360 degrees.
- Try this a couple of times.
Which position of the pillow allowed for better twists?
Now try varying the position of arms: swinging arms, extended straight overhead, straight out to the sides, glued to the body, etc. Would jumping on one leg allow for better twists? Or maybe try shoes and no shoes?
Would you like a whole month of Olympic-related activities, crafts, books and more to try with your kids? Click here for inspiration from the best kid bloggers from around the world. And for extra fun, try an Olympics Challenge for Kids that you can print off kcedventures blog.
What are you favorite Olympic sports? What are your kids’ favorite Olympic-related activities?