https://youtu.be/LvWGQYboknE
PHYS 1101: Lecture Eleven, Part Four
Now I want to go through the common forces that an object in our day to day experience can be subjected to. Every force, of course is a vector, so for that force I want to emphasize for you where the value typically comes from, for every force, and then the direction, the nature of the direction.
The first force on our list is the weight. All objects that are close to a very large, massive object, like a planet, experience a force toward the center of that planet, or that big mass. That translates to any object on the surface of the planet Earth experiences a force, and the magnitude of that force is the mass of that object in kilograms times the surface acceleration due to gravity, where g is 9.8 meters per second squared.
Okay, warning, because you use the value of g to determine the value of one force on this object, this does not mean that the acceleration, the a on the left side of your equation, does not mean that this is minus 9.8 meters per second squared. This a comes from whatever the real motion of the object predicts or says, what you really see with your eye, the change in velocity.
One of the forces here, in fact if you go with the y axis up, it’s going to be one of the y components for one of the forces, is going to be minus m times g.
For direction for the force due to gravity, I am going to write that it always points to the center of Earth. So from our perspective we’re just going to be focused on looking at this object sitting on the surface of the Earth, sitting on the ground, it always going to point down.
Notice for each of these forces the vector symbol for it, and of course the vector has to contain the information of magnitude and direction. The symbol I will use for the magnitude is always the same symbol just without the arrow. This says the magnitude of this vector W is equal to m times g.