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Forces
Will the box in the picture move?
So, is this where you stand lazing about? I'm not. I'm supporting the wall. Looks more like the wall is supporting you - you know, normal force? Normal force? Yeah, I know about that. Normal force - it's the force that the wall is-- where should I even begin? You do know what a force is, physically speaking, I mean? Of course, a force is sort of, hmm... It's hard to explain.
What is a force? Forces are something that can push or pull on things. When Kim pushes this glass, the glass is influenced by a force from his side. We can illustrate that force with an arrow, like this. If Kim is pushing hard enough, the glass will accelerate, it starts moving. But if Kim doesn't push hard enough, nothing happens. There are more forces at work here.
The earth's gravity is a force of attraction, and it pulls the glass down toward the table and because of that, it takes more effort to push the glass across the table. This is friction, and that's a force, too. Frictional force points in the direction opposite to that of the movement. If the force that Kim applies is greater than the force of friction, the glass will accelerate. Check this out. Less than the friction, greater than the friction, less than the friction, greater than the friction.
Oops, I forgot gravity. Even though we can illustrate a force using an arrow, forces can be seen in real life. But we can measure them. To measure a force, you can use one of these: a dynamometer. It's a tube attached inside another tube with a spring. The greater force you use to extend the tubes, the more the spring will stretch out.
Then you can read off how strong the force is using this scale. Forces are measured in newtons. If we attach the dynamometer to the glass and let the glass hang down like this, the glass is pulled down by the gravitational force. And now we can measure how strong that force is. Earth's gravity pulls on the glass with a force of two newtons. To measure the friction instead, we attach the end of the dynamometer to the glass and pull gently on the other end.
Now we pull a bit harder, increase the force until the glass starts to move. At that moment exactly, we read off the force on the dynamometer’s scale. It takes a force of one newton for this glass to start moving across the surface. That's how strong the force of friction is. A force, that's something that pushes or pulls, and that can make things move. But that's not all.
Forces can make an object change speed, direction of motion, or shape. Oh, oh, I have the perfect thing for which to show that. [inaudible] That's it. I can press on the balloon with my muscle force and change its shape. And here comes the speed and direction. Very mature, but hey, I'm pushing the wall here with a force, don't I? Yeah, a small one. But the wall doesn't change its speed, direction, or shape. There - the normal force again. The greater is the force that you direct toward the wall, the greater is the force with which it pushes back at you. I understand that.
I am a great force and the world is against me. Come here and help me clean the glass and I'll explain later. I don't think I can do that. What if the wall falls down?