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:

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14 Responses to “What a Drag!”

  1. 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.

  2. 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?

  3. 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)

  4. 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

  5. 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.

  6. 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.

  7. 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.

  8. Michelle Eckstein says:

    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

  9. 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?

  10. Nazliatul Aniza Nordin says:

    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 😀

  11. 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.

  12. Carmenza Coleman Salgado says:

    How come rough surfaces have more friction?
    Krishnaya Coleman Salgado

  14. Stephanie Mayo says:

    I cannot access the video for the 9-12th why not?