https://youtu.be/PdmkeM_yVGg
PHYS 1101: Lecture Eleven, Part Five
The next force we run into often day to day life is a rope or a wire or a string being attached to this object.
Let me say something first about the value of that tension. Often it’s going to be a value that’s either given in the problem or you’ll have to determine what the value is based on the f equals ma prediction or the a equals f divided by m prediction.
And what I mean by that is you literally are going to have to set up the equation, the basic starting equation that ax is equal to the horizontal forces divided by m or perhaps ay is equal to the vertical forces divided by m or perhaps both. You’re going to have to set up these equations and have t be the variable that you’re trying to solve for. You’re going to have to let t be the value that’s required to make these true, to make Newton’s Laws true.
For direction, there’s two things you can conclude always about a force due to a rope or a wire, and that is that it’s always a pull and the direction of the force of this tension of this pull is always in line with the rope.
Here’s a simple experiment that you can do to emphasize this to yourself. Grab a small little section of hair. This would be like a rope. When you gently pull on that section of hair, it’s like a rope. So you obviously feel the tension in this collection of hair, of this rope, on your scalp you can feel it. You obviously feel that it’s a pull.
Try pushing with your hair. You can’t push. A rope can’t push. It can only pull. So the force on your scalp would be directed in line with the hair and it would be a pull.
If you change the direction, if you pull at a different angle, you can appreciate that the pull that you feel on your scalp is in that different direction. It’s always along the rope.
So here’s my excellent cartoon of a person’s head and their hair. And taking a little collection of their hair and somebody is pulling on it in that direction. The tension force then on this object, this person, because of this rope and this pull has to be along the direction of the rope. And it’s always a pull. It’s always away from that object.
If the person were to tug the hair off in a different direction, the direction would change. If a person tugs harder, this length of this arrow would have to get bigger.