Now let’s talk about the other ever present force on this Earth, and that’s friction. Friction is the force between one object rubbing against another object. Friction is what makes things slow down.

Without friction things would just keep moving unless they hit something else. Without friction, you would not be able to walk. Your feet would have nothing to push against and they would just slide backward all the time like you’re doing the moon walk.

Friction is a very complicated interaction between pressure and the type of materials that are touching one another. Let’s do a couple of experiments to get the hang of what friction is.
Here’s what you need:

1. Aurora says:

Oh, no! Thanks for letting me know. I am updating all the shopping lists soon, so I’ll put this correction on the list also. So sorry about that!

2. Nhasan112 says:

Hello.
FYI: the Protractor is missing from the list of materials. We didnt realize until we gathered everything else and started to do the experiment.

3. Tamara Howard says:

Does it matter the type of surface??? I wanna know if smooth or jagged would work! Help!

4. Aurora says:

To measure the angle that you lift the books up to. That’s the angle you need in your calculations.

5. Tamara Howard says:

Why would you need a protractor? It doesn’t make any sense.

6. Aurora says:

This is part of the lesson in Unit 1 on Friction.

7. Aurora says:

No, the word is TREAD which means the bottom part of your shoe that contacts with the sidewalk.

8. Tamara Howard says:

You need a thread for this experiment right?

9. Tamara Howard says:

Why would part of this video refer to kinetic energy? I know what kinetic energy is but I just wonder why part of that video refer to kinetic energy.

10. Aurora says:

Sorry for the trouble! We’re updating the videos to new players, and the new players seem to be breaking the links for the worksheet downloads. My team is working on a solution right now, so please check back soon!

11. Linda Miller says:

Hi,

I was able to access the direct link provided to Dom, but I can’t seem to access any of the other student worksheets (and I am logged in). Help 🙂

12. Aurora says:

You need to be logged in first. THe link is working – here’s the direct link to it: https://sciencelearningspace.com/standardcontent/docs/Unit1-tracking-threads.pdf

13. Dom L. says:

Hi there,

The link to the student worksheet doesn’t seem to work. Is there another way that we can get it?

Thanks

14. Aurora says:

The force of friction depends on the mass of an object as it is affected by gravity, otherwise known as the “normal force”. So yes, the heavier the object, the more friction force it will experience. But friction is also dependent on something called the “coefficient of friction,” which changes according to the type of material that the object is sliding on. For example, a brick sliding on concrete would have a higher coefficient of friction than a skate on ice.

As far as the astronaut on the moon, remember that the moon does have a gravitational force, it’s just much smaller than that of earth. So if you trip on the moon, you would still experience friction.

15. sheri albee says:

HI,
Does the friction also count on how HEAVY something is?
Like with moving a heavy box compared to sliding regular-sized books across the floor?
I know that Gravity is also involved at least somewhat with friction, so does that also explain it?
Would you need gravity in order to actually heat up your astronaut boot on the moon?
With no gravity, wouldn’t the motion of that kick or that rub send you flying into space, causing you to flip, or causing you to bump your head in your helmet?
I know that’s a lot of questions, but just wondering 🙂
-S.

16. Daniel Ohanessian says:

I just realized that if you put a stuffed animal on LEGO, it will slip at 5º. But if you put a stuffed animal on a pillow, it falls at 85º. Try that experiment!

17. Aurora says:

No, not permanently. There are stick-on sprays that can help, but they rub off easily. I am sorry I can’t be of more help! The good news is you now know how to test the shoes before leaving the store…

18. Jodi Schreiber says:

Grrrrr… we discovered that Ryan’s BRAND NEW basketball shoes have less friction than many of the other shoes we tried. This was then proven on the basketball court two days later, when he was sliding all over the place! Help us Aurora! Is there anything we can do to make his shoes grip the court better? Applied Friction 101.

19. Aurora says:

Yes, using the same shoe with different rocks shoved inside the shoe for weight will narrow your variables down to one for that experiment – nice job figuring that out! If you keep the weight the same, then you’re only testing the two surfaces. If you test them all on the same surface, then you’ve narrowed down the surfaces to one… etc. 🙂 Keep me posted!

20. beth tuggy says:

Thank you for responding to our question.
That is what we were trying to do. We were trying to eliminate some of the variables by using the same shoe…same size, same materials, same surface. The only variable was the weight. I see what you mean about using three different shoes and stuffing them to make them all weigh the same, but then you would still have different materials and different tread surface. The original experiment peaked my sons curiosity so we were specifically trying to see how weight affects friction. Do heavier things have more friction or less friction? Don’t we need to use the exact same shoe and only vary the weight to determine that?

21. Aurora says:

There are two different things going on here. What you should really do is take the three shoes and stuff the lighter ones with something so they weigh the same amount as the heavier one and then try it in order to determine if it’s the weight or the material/surface roughness that the shoe is made from. Also is the surface area or the footprint area of the shoes the same size? Different? In science we try to test only one variable at a time, that way we know what’s causing our results. In this case you have at least three variables: shoe weight, surface area (footprint size), and shoe material/surface roughness.

To answer your race car question, the answer is yes, but mainly because it takes less energy (power) to accelerate a lighter weight quickly than a heavier weight. Which means that the lighter car can accelerate out of a turn or out the starting gate faster than a heaver car. Does that make sense?

22. beth tuggy says:

We tested athletic shoes that were around the same age but one was much larger and heavier than the other. We were surprised that the lighter one had more grab even though there was less surface area touching the book. This caused my son Jonathan to question if the weight and an affect on the results. So, when we finished we did another experiment with the lighter shoe and put different weights into the shoe to see if the weight affected the friction on an incline. Our hypothesis was that on an incline the heavier shoe would have less grab/friction… based on the findings from our first experiment. We thought that would be true because the weight of the heavy shoe would pull it down the incline where as the lighter shoe would kind of just stick there. We used three different weights in the same shoe. The heavier shoe began sliding at 40 degrees and the medium weight shoe began sliding at 48 degrees and the lightest shoe began sliding at 52 degrees. So we concluded that the lighter shoe had more grab/friction than the heavier shoe. Is this true? It seems like it would be the opposite on a flat surface because don’t they strip race cars down to make them very light weight so they have less friction on the course?

23. Aurora says:

Great question! A tire without treat does in fact have more traction because of it’s increased surface area contacting the road. This is why race cars use treadless tires called “racing slicks”. But… This is only true if the road is absolutely dry (and free of sand, gravel, oil, etc.) If there’s even a slight bit of water on the road, the traction of the racing slick goes down way below that of an ordinary tire (we call it hydroplaning). You see, the treads on a tire are specially designed to move water into the little grooves of the tread so that the rubber can touch the road. When you don’t have treads, the rubber suddenly ends up gliding on a thin sheet of water and has very little traction (think about slipping on a wet floor). While they might quickly postpone a NASCAR race due to rain, most bosses, schools, etc. are reluctant to say “Oh, it’s raining, no need to come in today!”

For more details on this, check out:
http://auto.howstuffworks.com/auto-racing/nascar/nascar-basics/nascar-rain1.htm

To answer your second question (the flip-flop vs. the rain boot). The reason the treadless flip-flop didn’t hang on as well is because of the type of rubber it’s made of (this makes a big difference). If you took two identical rain boots and wore out the tread on just one of them, the one without tread would have more friction on a DRY surface (as long as the boot and surface were clean).

If you try this, let me know how it goes!

Al (Director of Supercharged Science)

24. Carole Turner says:

I am confused. You said the race car tire has no tread so that it can have better friction, but in your answer above, you said bald tires tend to slip more than a tire with new treads. You also said that the shoe with the most surface touching the board would have the most friction. So wouldn’t that mean a “bald” shoe would hang on better than a shoe with tread? In our experiment, the rubber boot with good tread had the most friction and the “bald” flip-flop slid down first.

Ava Turner age 9

25. Aurora says:

Oops – didn’t realize we omitted a piece of equipment from the list. Thanks for the eagle eye – we’ll get that added right away.

26. Meghan MacKenzie says:

We were surprised to find our worn out croc had more friction that a brand new snow boot and tennis shoe. Does this have anything to do with the foam or the wieght?

Also could you add protrctor to your list of materials? We ended up using a ruler but I would have liked to have the expeirment ready with the materials you used in the video.

27. Aurora says:

There’s a difference between static and kinetic friction. The static friction is the friction when you are going from stop to motion (when the shoe initially starts to slide), and the kinetic is the friction felt during motion (a hockey puck sliding on ice has low kinetic friction compared to a show sliding down a ramp).

Not all friction has to do with weight – there’s also surface texture and electrostatic charges involved too! You can read more about it in the reading section of the friction experiments.

If you think tires, you know from experience that bald tires tend to slip more than a tire with new treads. (You can wrap a fat rubber band around a toy car wheel to demonstrate the difference with your child.) And in the mathematical side of things, the amount of area doesn’t even enter in the equations when calculating the friction force – it was to do with what the material is made up of and how much it weighs.

You can take your kids to a part with slides for a more practical approach with this: have them slide down slides on different materials (wool sweaters, cotton shirts, paper bags, towels, saran wrap, plastic bags, carpet squares… anytime you have on hand). Time their decent, measure the length of the slide, and you can estimate how fast they were traveling (Hint: find a looong slide and start timing after they really get going, and be sure to measure the distance that you timed). Have fun!

28. annette vellenga says: