https://youtu.be/sLaKQ8D2X9Q
PHYS 1101: Lecture Eleven, Part Six
Next category for us to consider are those caused by surfaces. There is what’s called a normal force. This is a contribution of force that helps, that supports the load. And then there’s what’s called friction force and that’s just due to roughness that exists between this object and this surface. You always have a normal force. If any object is touching a surface, there is always going to be a normal force. Any contact, no matter how light represents some level of force.
Friction force we’re only going to see if the object is directly sliding against a surface or if there’s a tendency for the object to want to move. Imagine you could turn this surface into a slick, lick sheet of ice. If you did that, would the object start sliding? If that’s the case, you will have what’s called static friction. There is a frictional contribution from the surface that’s keeping the object from sliding. I’m going to go through each of these a little bit more carefully to give you a feel for the size and the direction.
Let’s now do the first one and talk about the normal force. This is the force that any surface is always providing simply by being in contact with that object. I need to think about the magnitude of this force which I’ll denote “F” “N” because we will use “F” sub “N”, “N” for normal and an arrow over it to represent this vector, that force. We need to think about the magnitude and the direction.
The magnitude for “F” “N” is always determined by solving Newton’s Second Law. I mean by that the same thing I meant up here for tension. It’s the value based on the “F” equals “A” “M” prediction.
In terms of direction, the critical thing to remember is that a surface can only push, never a pull. And the direction it can push is always perpendicular to that surface.
Let me do a couple of quick sketches. Let’s say we have an object sitting on a slope at this angle. We have an object sitting on a flat horizontal surface. For either scenario, because it’s contact with the surface there is a normal force on this object. The direction of it is always perpendicular to the surface. And the magnitude of it, how long this vector would be, I can only conclude the value after I have set up and algebraically solved my “A” equals “F” over “M” equations with “F” “N” as a variable.
They call it the normal force because normal is a mathematical term that means the same as perpendicular. Mathematicians say that this vector is normal to this surface.
To emphasize how the value of “F” “N” is determined I’ve got a couple of little thought experiments I want you to consider. Start out and imagine that I have on two supports something like a meter stick, kind of a long flexible stick and I put this red block or object in the middle of it. You can probably imagine pushing down on this block that a thin meter stick, a thin stick like this will be springy.
There would be the force due to gravity and then as I went around the block I would realize it’s touching this surface so I would have to have a normal force, a supporting force. That’s the only force on the object. It’s up. Surfaces can only push, so I would draw force up.
How long would I draw it? I would draw it long enough so that my visual or mathematical balance of these forces is zero because the acceleration of that block is zero. “F” net has to be zero. There’s no net up down force.
Now let’s say someone comes in and pushes down on this block. I’m sure you can imagine if I came in, somebody came in pushed down on this block, I would increase the load on this thin meter stick. It would deflect more. What does my free body diagram look like now?
Here’s my dot for the object. The force due to gravity has to be the same. I’m actually going to draw that off to the side just so slightly. It would be the exact same length it was above, looks like I drew it a little bit too long. Whatever this length is has to stay the same. In fact, let me copy this down here because that’s just the nature of this block. It’s made up of so many molecules. It has so much mass. Force due to gravity stays the same.
Now let’s go around the block. What forces do I have touching it? I still have a surface so I have a normal force. As I keep going around, now I’m in real contact with some other object, this person pushing. The person is pushing down. That’s going to be another force down.
My first pass at my free body diagram, I’m going to put sub “A” “N” “G” here for the force due to Angela. My first pass is to just get the approximate direction correctly for all of these forces. Then I start thinking about size. How big is “F” Angela? That is determined by the pressure that I would feel in my fingers at this point of contact.
If I push down harder I’m going to feel more pressure, so right at this contact point the force is larger on this object. This would get bigger and bigger or smaller so this is really set by how hard Angela pushes. Then we know that the force due to gravity is set just by the number of molecules in the block. That doesn’t change.
But as I draw this normal force, I want you to think about Question 13 to answer. Did I draw this normal force with the appropriate length? This is where you need to go back and force between our fundamental equation of Newton’s law, Newton’s Second Law. Whatever the acceleration for that object is it has to be consistent. This vector has to be visually consistent with the vector sum of these forces. “A” for the block is definitely zero. I only have vertical forces. They all have to balance.
This is fixed, the weight force, the magnitude. The force due to Angela is fixed, it’s a certain amount. What force had to get bigger is my next quiz question for you.