Anyone who watches Olympic sports is bound to be puzzled: the athletes often seem to defy the laws of science that limit the rest of us. What you see is, of course, a result of thousands of hours of training, but there is also a bit of tricky science in it too. Earlier this week I showed you how by harnessing the understanding of physics, in particular, Newton’s Third law of motion, gymnasts can perform their incredible twists (The physics of Twisting). Today I want to talk about water resistance in relation to Olympic Swimmers.
If you live near a body of water, go with your kids to the beach and ask them to run in the water. Is it easier to run in the water or on the shore? How does running in the water up to their waists compare with running in the water that is up to their ankles? Water is more resistant to movement than air.
The Science of Swimming
The more of the body is submerged in the water, the harder it is to move. That’s why swimmers move as close to the surface as possible. Moving through less resistant air allows for better speed of movement than moving through water.
Water resistance goes up as the surface area of a submerged body increases. That is the reason swimmers try to make their bodies as compact as possible as they move through the water.
I decided to set up an experiment to investigate with my kids how water resistance slows larger objects moving through the water. We varied the shape of playdough (experiment #1) and the shape of aluminum foil wrapped around stones (experiment #2) to make it fall faster or slower through the water.
It’s not necessary to go into gravity today (unless you want to), but do mention that the reason the playdough is falling to the bottom of the glass is the gravitational pull between playdough and the Earth. You also can mention that besides working on water resistance and buoyancy, swimmers increase their speed by making their bodies as smooth as possible. They wear specially designed swimsuits, cover their hair with swim caps, and sometimes they even shave the hair on their bodies to be as streamlined as possible.
Water Resistance Experiment #1: Playdough
Glass cups/vases or jars
Fill your clear containers with equal amounts of water.
Take two equal pieces of playdough. (The best way to grab the same amount is to use an ice-cream scoop). Shape one into a ball and shape the other piece into a flat pancake. We decided to add a triangle piece too.
Ask questions: What do you think will happen? Do you think all the pieces are going to sink? Which one do you think will sink faster?
Use this Observation Sheet to record your predictions.
Get the stopwatch ready (we used the one on the phone) and drop each piece one by one recording the falling time.
Use the Observation sheet to record your findings.
In our experiment, both, play dough shaped like a ball and play dough shaped like a triangle, dropped down like a rock, while the pancake took some time to float down in a fluid circular fashion.
Water Resistance Experiment #2: Aluminum Foil
This experiment surprised and fascinated my kids. They predicted that aluminum foil crumpled into a tight ball would sink and a flat piece of aluminum foil will float, but all pieces floated.
We added stones we collected at the beach and experimented with stones in a foil in many different ways. First, we added two small stones and crumpled aluminum foil tight around them. When we put it in the water, it floated. While we stood fascinated, slowly it sunk.
Then we hid two stones in a flat, folded piece of aluminum and it shot down like a rock. What? hen we experimenting with folding the foil around stones in different ways to see, which fold made it sink faster or slower. Finally, we made an aluminum boat and put a couple of stones in it and it floated.
Glass cups/vases or jars
Fill the clear containers with an equal amount of water. Shape one piece of aluminum foil into a ball. Keep another piece flat. We added a third piece of the same size crumpled a bit, but not squeezed tightly into a ball. Use the Observation Sheet to make your predictions.
Put them all in the water and observe what happens. Next fold stones inside the aluminum foil pieces in many different ways to see, which combination will allow for fastest sinking time and what shape will float.
Two forces are at play in this experiment. Gravity is pulling things down in the water, while buoyancy pushes upward against gravity. Different objects behave differently in the water depending on their density. A crumpled ball of foil has more density because all of its weight concentrates in a small space. Our foil boat is spread out, less dense and therefore can float the objects that would normally sink on their own (i.e. stones). If you got interested in buoyancy try our Shark Experiment (we added oil to make “shark” heavier, but it actually caused it to float better).
Find more objects around the house that you can practice with.
What offers more water resistance?
This post is part of a series of Olympic Activities for Kids. Check it out for lots of fun ideas, including a Summer Olympic Challenge.