When I taught college physics I told my students to approach a problem in steps.
Step the first: Draw a picture (gets both sides fo the brain working on the problem).
Step B: Determine if the body in the problem is accelerating or not (it determines what set of equations you’ll use - acclelerations are different from constant motion [or constant non-motion]) and chose your orientation reference (which direction is X and which is Y).
Step 3: Draw all forces (if forces are involved in the problem).
Step “the next”: Determine if you can use conservation laws to solve problem.
(the rest of the steps don’t apply here, and I don’t want to be verbose)
Step B is the one here. Since your tuchas is in the chair, you are correct about the 170 lbs of force downward. Now, since your keyster is not accelerating (in our frame of reference) there must be a force counteracting (equal and opposite) the gravitational force. In this instance, since the seat of the chair is perpendicular to the direction of the gravitational force, it is completely the normal force. Were the seat at an angle w/ respect to the gravity force, we need to find the component of the friction force that opposes gravity, and we’re suddenly talking sine and cosine and stuff that doesn’t really need to be brought up here.
Hope it helps
of at least confused on an entirely different plane.
Yes, and no. The problem is that you assume that the gravity force remains constant. It does remain fairly constant in magnitude. However, the direction is constantly changing. Thus the counteracting force is in fact itself.
Think of it this way: in spring, the Earth is on one side of the sun. It is getting pulled by gravity towards the sun in direction +x. I’m defining the x axis as a line through the sun and the Earth at the equinox. + is going out from the sun through the Earth at the fall equinox. Now is spring.
Okay, fast forward to fall equinox, and gravity is pulling -x; it is pulling the opposite direction.
But what keeps Earth from hitting the Sun? It has tangential motion. It has a velocity that is tangential to the sun. It is moving. So start at spring. As it gets pulled toward the sun in the +x direction, it moves in the +y direction. Pretending that the Sun applies a uniform +x field on Earth, you will see that as the Earth moves closer to the sun, it moves sideways past the edge of the sun, until summer hits. Now the Earth is at x=0, i.e. even with the sun, but it is at 1 a.u. in the y direction. Stop, change the field so that it pulls uniformly -y. But the velocity is now +x. Same situation occurs, the Earth falls down to y=0, but reaches it at x= - 1 au. That is the Fall equinox.
Realistically, the solar field is not a uniform parallel linear force, but a radial force. However the change in direction of pull is continuous, and the change in location of the Earth is continuous. More complex, same result. The Earth falls around the sun, only acted on by the force of gravity, but also retaining tangential motion.
And yes, the motion is from the left over angular momentum, which makes it an inertial effect.
AWB asked:
The second force is structural resistance. The chemical and atomic forces holding the chair together and the forces holding your butt together. We call this a “normal” force because it acts perpendicular to the surface of the chair, or we call it a “reaction” force because it is a response to the force of gravity.
Centrifugal force is not the opposing half of centripetal force. That would also be a reaction force.
Let’s look at what those words mean, and imply.
Centrifugal force is “center fleeing” - it is radial directly away from the center. It assumes that circular motion is something in itself, that occurs and then maintains itself without an applied force. Like a linear force acts and then inertia will carry it until acted upon by another force, it is implying that the body is inertially circling and will continue to circle until made to not circle. A centrifugal force would be a force trying to make it uncircle. As you can see, this is not a correct representation. Centrifugal forces can be thought of if you are using a self-reference frame for the feeling your body has trying to fight the circular motion, but that is a “fictitious” force.
Fictitious forces do have some use in physics. Currently gravity is considered to be a fictitious force, using relativity. I don’t know enough about geodesics and rheimann space to worry about it, so I take their word for it.
Now lets look at centripetal force - that is “center seeking” force. What you have is your standard classical Newtonian case. An object is put in linear motion. It’s inertia will keep it in linear motion. To make it curve, you apply a sideways (radial) force to turn the path. Keep applying the force, the path curves. Stop the force, the path straightens in the new direction. For the case of driving in a car, that radial force is the turned wheels, and the friction with the ground. The car turns, thus it pulls your body with it.
Er, guys. The question in my post was rhetorical. I was just showing that the force from my chair is just like centrifugal force. It isn’t generated by anything, it is the reactionary force of an original force being applied.
