Quick Physics Question

[Heavenguard]

Can a centripetal force be negative? I'm studying Uniform Circular Motion in my Physics AP Class, but I can't really remember much about it xD, but I just need to know if this can be negative.

Top
∑F = FC
Ft + Fw = FC
Ft + mg = m(v₂/r)
Ft = m(v₂/r) – mg

Bot
∑F = FC
Ft – Fw = FC
Ft – mg = m(v₂/r)
Ft = m(v₂/r) + mg

At the top, does FC become negative? I know its center-seeking and all so im not too sure. There both the same objects with the same mass going through the same acceleration, velocity and period.

[i luffs yeww]

Never taken a physics class, love, but I don't think it could be negative? How would force be negative?

http://answers.yahoo.com/question/in...8054435AAX7V0s

[Heavenguard]

A force can be negative if its compared to other forces. That's where im trying to ask about, kinda confused if that would apply to the FC, because its never shown on a free body diagram but is said to be center-seeking.

[TomTuff]

a negative centripetal force would go away from the center of gravity right?

[i luffs yeww]

Fair enough. Get on MSN. ^^

Also, my sig. :3 check it out if you have teh tiem. <3

[Heavenguard]

A negative centripetal force would yeah, it would go away I guess... but if you think about it, their both going in the same direction in the case. Since gravity goes downward from the object, and at the top of this objects motion; the FC would go downwards as well...


O (object)
|
\/FT & FW (& FC)

[TomTuff]

A negative centripetal force would yeah, it would go away I guess... but if you think about it, their both going in the same direction in the case. Since gravity goes downward from the object, and at the top of this objects motion; the FC would go downwards as well...

i have no idea what FC is, i'm just in chemistry (i'm a sophomore). But just applying logic, since the distance of an object orbiting around another larger object, perfect centripetal force in which the object moves neither closer nor further from the center object, the amount of force would be 0 since inertia and gravity cancel eachother out... a positive centripetal force would result in a move closer to the center object, and a negative centripetal force would mean the opposite; this goes against the definition of what centripetal force is, yes? So it's not centripetal force?

again, though, that was just some vomit from my uneducated-in-physics mind, so don't expect any degree of scientific accuracy.

[Heavenguard]

i have no idea what FC is, i'm just in chemistry (i'm a sophomore). But just applying logic, since the distance of an object orbiting around another larger object, perfect centripetal force in which the object moves neither closer nor further from the center object, the amount of force would be 0 since inertia and gravity cancel eachother out... a positive centripetal force would result in a move closer to the center object, and a negative centripetal force would mean the opposite; this goes against the definition of what centripetal force is, yes? So it's not centripetal force?

again, though, that was just some vomit from my uneducated-in-physics mind, so don't expect any degree of scientific accuracy.

-> http://en.wikipedia.org/wiki/Centrifugal_force

That makes so much sense, since the forces always should cancel them out. But you can't use Inertia to cancel something, its just not taken into account for newton's laws (well not for me yet). It would make sense that since the mass wants to resist motion, that it would cancel that force, but I have no clue how all that would go into matter with FC XD, but what you say does make a lot of sense to me. I don't think Inertia can actually cancel gravity out, but idk lol. I just wish there was some stupid diagram or something that could just show me Lmao.

Also its good to take into matter that this is not going at constant velocity, and its not orbiting anything, its simply being spun by a persons finger from the tension of a string. Should have mentioned that earlier -facepalm- my bad.

[TomTuff]

-> http://en.wikipedia.org/wiki/Centrifugal_force

That makes so much sense, since the forces always should cancel them out. But you can't use Inertia to cancel something, its just not taken into account for newton's laws (well not for me yet). It would make sense that since the mass wants to resist motion, that it would cancel that force, but I have no clue how all that would go into matter with FC XD, but what you say does make a lot of sense to me. I don't think Inertia can actually cancel gravity out, but idk lol. I just wish there was some stupid diagram or something that could just show me Lmao.

Also its good to take into matter that this is not going at constant velocity, and its not orbiting anything, its simply being spun by a persons finger from the tension of a string. Should have mentioned that earlier -facepalm- my bad.

ahhh okay then i'm not sure, whenever i think of centripetal force, planetary orbits come to mind.

[Heavenguard]

I just found out that FC at the top is negative because the acceleration on the top is pointing down, simple as that. So it was pretty much what everyone said xD, Thanks for the help all, this thread is finished.

Top
∑F = -FC
Ft + Fw = -FC
Ft + mg = -m(v₂/r)
Ft = -m(v₂/r) - mg

Bot
∑F = FC
Ft – Fw = FC
Ft – mg = m(v₂/r)
Ft = m(v₂/r) + mg

[marpis]

Actually any value can be negative. Negative velocity means that the object is moving to left on a regular x, y-plane, negative force that the force is implied to push the object to the negative direction on our plane. Usually when something is dropped, we say that going up is negative and going down is positive. That's why g=9,8, not -9,8.

Frame of reference.

[Main]

I am assuming your in physics B?
It can't be negative for an object at constant velocity, but when you are in a centrifugal force and then if you were to integrate an decelerating object (you'll get to this in physics c) it can be negative.

[Method]

Actually any value can be negative. Negative velocity means that the object is moving to left on a regular x, y-plane, negative force that the force is implied to push the object to the negative direction on our plane. Usually when something is dropped, we say that going up is negative and going down is positive. That's why g=9,8, not -9,8.

Frame of reference.

That doesn't seem very consistent to me, unless for some reason you think the positive y-axis is below the x-axis.

[Heavenguard]

He means Frame of Reference is all. In physics we consider gravity is 9.8, but since its always acting as a force of weight, it point downwards, thus why its really negative. We do use it as a negative in individual solving sometimes, but if your doing a sum of all the forces (∑F) then you must take into the account what your solving for and which forces are acting together. Im not sure if its Physics C or B, but its not the high level AP, so which letter is that one xD

[Main]

K you must be B then.
In B, you always assume acceleration is constant when force is involved. So for you, cetripetal force can never be negative, unless they are in a centrifugal condition which then you are not in an inertial frame.Once you get to C, they'll pretty much teach you AP Calculus AB in like 2 days (LoL) and you'll be using differential and integral form of equations (which makes more sense).

[Heavenguard]

:X yeah its B i believe I'm in. <.< Calculus <.< I really wish I took that lmao. Trigonometry was very new to me when I went into the class, but I understood that pretty easily lol, but that's probably because its very basic trigonometry. So I can imagine using Calculus with Physics being amazing lol, with all the graphs and wonderful equations <.<.. Thanks for explaining that to me . Much appreciated.