As a professional tutor, I get to play all day with different academic subjects and ideas. For example, I had a student call the other day. He was dreading writing a school report about Galileo.
13 year old: “I have a report to write on Galileo. Do you have any ideas?”
Shawna: “Yes, bring your skateboard. See you at 4 p.m.”
What follows is a fun and easy physics activity that turned dreaded homework into a fun and interactive lesson.
I’ve been having so much fun exploring the sciences. Lately I’ve been reading up on Galileo. He was an out of the box thinker of his time (to say the least). When those around him believed that the earth was the center of the universe, he argued that the earth traveled around the sun.
Philosophers of the time argued with Galileo, they said, "The earth can’t be spinning. I don’t feel it spinning. If it was spinning, we’d all feel it. Clearly, we are not moving."
As well all now know, the earth is indeed moving. In fact, a quick look on the internet and you will find that the earth rotates on its axis at about 1,000 miles an hour at the equator, and it travels in its orbit around the sun at about 66,600 mph.
Wow, we are moving fast! Why don’t we feel the spin? Is the answer gravity? Well, yes, gravity keeps us all (including the atmosphere) from flying off as the earth spins, but why don’t we feel the spinning? This is where the skateboard comes in.
1. You’ll need a ball about the size of a tennis ball. A cloth ball that doesn't bounce is ideal (we used a dog toy).
2. A parent or a friend to mark the ball's landings.
3. A skateboard and helmet.
4. A safe flat surface to ride the skateboard.
Standing Still: Hold the ball and stand on your skateboard. Make sure your helmet and pads are in place. Do not roll the skateboard. Stay completely still.
Drop the ball straight down at your side.
Observe where the ball falls. Ask your parent or friend to note how the ball falls in relation to your foot. Does it fall in line with your ankle? You heel? Your instep? Make a note. Repeat this two or three times so you get an idea of where the ball lands.
Rolling: With the ball in your hand, get rolling on your skateboard.
While you are in motion, drop the ball straight down at your side just as you did when you were standing still. Ask your parent/friend to observe where the ball lands.
You will note that the ball falls exactly in line with your foot - just as it did when you were standing still.
Note that each time you drop the ball, moving or stationary, the ball falls in the same place relative to your foot.
Now try this:
While standing still, toss the ball up and catch it. Repeat.
Get moving along on the skateboard. Toss the ball up. Can you catch it?
Does it hit you in the face or can you toss it and catch it just as if you were standing still?
Wow. What is going on? Why does the ball fall in line with my body whether I am standing still or moving? Why can I toss a ball up and catch it even if I’m going down the street on my skateboard?
These are all very good questions.
According to Galileo, the movement of your body on the skateboard is transferred to the ball by your hand. You, on the skateboard, and the ball share the same motion. Because you share the motion, it is as if the motion does not exist. The motion is hidden because you and the ball are sharing the same motion. This is why the ball falls in alignment with your foot whether you are moving or standing still.
This is exactly why we do not feel the earth rotating on its axis. If the child on the skateboard represents the motion of the earth, then the ball represents us. We do not feel the motion of the earth spinning because we move right along with it.
In conclusion, Galileo’s explanation for why we don’t feel the rotation of the earth can be demonstrated using a child, a skateboard, and a ball. Now go try it and have fun!
By the way, my student experienced this activity, wrote it up, drew some illustrations, presented it to the class, and was awarded an A for his efforts. Go Kiddo!!!