There’s a couple of misconceptions that I’d like to make sure get cleared up here a bit. I don’t want to go into too much detail but I want to make sure to mention these as they may be important as you go deeper into your physics education.
First, friction is not a fundamental force. Friction is actually caused by the elemental force of electro-magnetism between two objects.
Secondly, friction isn’t “caused” by the roughness or smoothness of an object. Friction is caused by two objects, believe it or not, chemically bonding to one another. Scientists call it “stick and slip”.
Think about it this way. When you pull the wood in this experiment, notice that the force needed to get the board moving was more then the force was to keep it moving. The surface you were pulling the board on never got any rougher or smoother, it stayed pretty much the same.
So why was it harder to get the board moving?
When the board is just sitting there, the chemical bonds between the board and the surface can be quite strong. When the board is moving however, the bonds are much weaker. Here's what you need:
[am4show have='p8;p9;p11;p38;p92;' guest_error='Guest error message' user_error='User error message' ]
Download your worksheet here!
1. Write the different surfaces that you chose on a piece of paper.
2. Make a hypothesis. On a scale from 1 to 5 (or however many surfaces you chose) rate the surfaces you picked. 1 is low friction and 5 would be high friction. Write your ranking next to the surfaces on the paper.
3. Take your block or your book and attach a string to it.
4. Place your block on the surface to be tested.
5. If you have a spring scale, attach it to the string and carefully pull on your block until it just moves. What you will probably see, is that you will keep pulling and pulling until suddenly your block moves. Try to record the number that the scale said just before the block moved. It takes a little bit of practice to read that number so keep trying.
6. If you don’t have a spring scale, tie a rubber band to the string. Now put a ruler with the first inch at the end of the rubber band farthest from the block. Now pull on the rubber band holding it next to the ruler. When the block moves, record the number on the ruler where the end of the rubber band was. In other words, you are measuring how far the rubber band stretches before the board moves.
7. Remember, with the scale or the rubber band, this takes some getting used to so try not to get frustrated.
8. Write down your results next to your hypothesis.
9. The higher the number, the more friction there is between your board and the surface the board is on. In other words, the harder you had to pull to get the board moving, the more friction there is between the board and the surface.
10. Now analyze your data and see how the data matches your hypothesis. Which surface really had the most friction and which had the least. Write numbers 1 to 5 (or however many surfaces you chose) next to the results.
11. How did the data correlate with your hypothesis? Any surprises?
You’ve probably noticed with this experiment that the kind of surfaces rubbing together make a huge difference.
Flat, hard, smooth surfaces will have much less friction than a rubber, soft, or rough surface. Muddy, wet or icy surfaces will often have even less friction. So, if you remember what we talked about with shoes and tires, the job of the tread on a shoe and a tire is to cut through the lower friction water or mud and get down to the higher friction road or dry ground.
Something else I’d like you to notice is that friction acts differently depending on what something is doing. If you have ever had to push something heavy like a refrigerator you may have noticed that it was harder to get it to move than it was to keep it moving. This is because there are two types of friction; static friction and kinetic friction.
Static friction happens when something is resting on something else and not moving. Kinetic friction is when one thing is moving on something else. Static friction is usually greater than kinetic friction. This means that it is harder to get your fridge moving than to keep it moving. You may have noticed this during “What a Drag” (if not, go ahead and play with it some more). When you first got the board to move, your scale had measurements much higher than when it was actually moving. It was harder to get it moving than to keep it moving.
For the advanced students, here's a way to calculate the amount of force you're pulling with by figuring out how 'spring-y' your rubber band is...
[/am4show][am4show have='p9;p38;' guest_error='Guest error message' user_error='User error message' ]
Advanced Students: Download your Friction Lab here.
[/am4show]
First, friction is not a fundamental force. Friction is actually caused by the elemental force of electro-magnetism between two objects.
Secondly, friction isn’t “caused” by the roughness or smoothness of an object. Friction is caused by two objects, believe it or not, chemically bonding to one another. Scientists call it “stick and slip”.
Think about it this way. When you pull the wood in this experiment, notice that the force needed to get the board moving was more then the force was to keep it moving. The surface you were pulling the board on never got any rougher or smoother, it stayed pretty much the same.
So why was it harder to get the board moving?
When the board is just sitting there, the chemical bonds between the board and the surface can be quite strong. When the board is moving however, the bonds are much weaker. Here's what you need:
[am4show have='p8;p9;p11;p38;p92;' guest_error='Guest error message' user_error='User error message' ]
- A 6 inch long piece of 2 x 4 wood, or a heavy book
- A string
- A spring scale or a rubber band and a ruler.
- Paper
- Pen
- 5 or so different surfaces, table tops, carpet, chairs, etc.
Download your worksheet here!
1. Write the different surfaces that you chose on a piece of paper.
2. Make a hypothesis. On a scale from 1 to 5 (or however many surfaces you chose) rate the surfaces you picked. 1 is low friction and 5 would be high friction. Write your ranking next to the surfaces on the paper.
3. Take your block or your book and attach a string to it.
4. Place your block on the surface to be tested.
5. If you have a spring scale, attach it to the string and carefully pull on your block until it just moves. What you will probably see, is that you will keep pulling and pulling until suddenly your block moves. Try to record the number that the scale said just before the block moved. It takes a little bit of practice to read that number so keep trying.
6. If you don’t have a spring scale, tie a rubber band to the string. Now put a ruler with the first inch at the end of the rubber band farthest from the block. Now pull on the rubber band holding it next to the ruler. When the block moves, record the number on the ruler where the end of the rubber band was. In other words, you are measuring how far the rubber band stretches before the board moves.
7. Remember, with the scale or the rubber band, this takes some getting used to so try not to get frustrated.
8. Write down your results next to your hypothesis.
9. The higher the number, the more friction there is between your board and the surface the board is on. In other words, the harder you had to pull to get the board moving, the more friction there is between the board and the surface.
10. Now analyze your data and see how the data matches your hypothesis. Which surface really had the most friction and which had the least. Write numbers 1 to 5 (or however many surfaces you chose) next to the results.
11. How did the data correlate with your hypothesis? Any surprises?
You’ve probably noticed with this experiment that the kind of surfaces rubbing together make a huge difference.
Flat, hard, smooth surfaces will have much less friction than a rubber, soft, or rough surface. Muddy, wet or icy surfaces will often have even less friction. So, if you remember what we talked about with shoes and tires, the job of the tread on a shoe and a tire is to cut through the lower friction water or mud and get down to the higher friction road or dry ground.
Something else I’d like you to notice is that friction acts differently depending on what something is doing. If you have ever had to push something heavy like a refrigerator you may have noticed that it was harder to get it to move than it was to keep it moving. This is because there are two types of friction; static friction and kinetic friction.
Static friction happens when something is resting on something else and not moving. Kinetic friction is when one thing is moving on something else. Static friction is usually greater than kinetic friction. This means that it is harder to get your fridge moving than to keep it moving. You may have noticed this during “What a Drag” (if not, go ahead and play with it some more). When you first got the board to move, your scale had measurements much higher than when it was actually moving. It was harder to get it moving than to keep it moving.
For the advanced students, here's a way to calculate the amount of force you're pulling with by figuring out how 'spring-y' your rubber band is...
[/am4show][am4show have='p9;p38;' guest_error='Guest error message' user_error='User error message' ]
It can be difficult to maintain a constant speed by hand. It may be that slight variations in the drag speed are allowing friction to slightly take hold. Also, dragging certain materials across glass causes a static attraction. That attraction would cause the need for an increase in force to keep the book in motion. If possible try the experiment on a surface other than glass.
Hi Aurora! I had the same problem as someone else back in the comments. I put my big book on my glass desk and did the experiment and I had a greater stretch when it was at a constant speed than when I had to get it moving. Both of our problems happened on a glass surface. Could that be affecting it? If so why?
Yes, you’ve got it!
F = (spring constant) x (displacement)
Using algebra, if I want to find the spring constant, I divide both sides by displacement to get:
k = F/x
This is the same as saying 3 x 5 = 15 and 15/3 = 5 (we divided both sides by 3 in this case)
Hi:
Question, please.
In the video you state that Force is + to K(x), but the equation shown states that K=F/x. I assume this has to do with the idea that 6×8=48 and 48/6=8, but I am not clear on explaining the relationship to this problem. Can you clarify the explanation?
Thanks
Hi there,
When you look at the top edge of a surface of something, like the edge of a knife, while normally it looks straight and flat, when you look up close like under a microscope, you’ll ind find it’s not flat but jagged. Not everything that looks flat are really flat. Make sense so far?
Now some pieces of metal are really actually flat under a microscope because they have been so carefully machined. The trick is to know that atoms are constantly in motion (even in solids), and so the atoms on the very top surface edge are constantly wriggling and moving, and sometimes they break free and float freely. When this happens between two super-smooth flat pieces of material, the atoms can intermingle and eventually will form a bridge between the two and stick together.
in the experiment stick and slip I was wondering about what you said about 1:55 minuets into the video about two smooth pieces of metal. I don’t understand why they would “weld” together.
Hi Phoebe,
Good question! The friction force opposes motion, and it varies in strength depending on the type of materials that are moving against each other. Rough surfaces have more friction than smooth, for example.
When a boat moves through water, the layer of water near the boat gets dragged among with it and moves against the water that’s not moving, so the water pulls with a net force (drag) on the boat. The air also pulls on the boat as it moves above the water, but with a lot less drag than the water, so it’s usually neglected in a physics problem.
In high lever physics and engineering courses, you’ll learn how to calculate (with math) the drag force of objects using the “drag equation”, which depends on the mass density (mass per unit volume) of the fluid, how fast the fluid moves, and the surface area exposed to the flow.
For air the density at standard pressure and temperature is 1.2 kg/m3, and water is 998.2 kg/m3., so you can see how the drag force is affected by density alone.
This is Phoebe and i’m 10 years old.
I had a question on friction. Is there more friction in the air or the water?
Thanks!!
Phoebe
You should see more of a stretch when you get to to start moving from rest, and a smaller stretch during constant motion… try again?
Hi Aurora!
I pulled my big encyclopedia on a glass table. The rubber band stretches more when it moves at a constant speed! Why is that so?
Thanks!
– Dayini, 12 😀
If you look super-close at a rough surface, it will look like a jagged mountain range with lots of peaks and valleys. The rougher the surface, the bigger the mountains. When two of these surfaces scrape across each other, they “stick and slip” across the mountaintops.
How come rough surfaces have more friction?
Krishnaya Coleman Salgado
Hmm… that sounds odd. Log OUT at the bottom of the page, and then log IN on the main page here right under the video on the right: https://www.sciencelearningspace2.com/ Does that help?
I cannot access the video for the 9-12th why not?