You first have to push/pull an object of mass m with a force F for it to have an acceleration of a. You can have an applied force with the object in a stationary position but you can't have an object accelerating if there's no resultant force applied to it. For instance, zero resultant force implies zero acceleration but can be either stationary or at a constant velocity.
Existence of a force is independent of acceleration, on the other hand, acceleration cannot exist without an applied force.

Thanks.

 

What? Newton was wrong!? F/m=a but apparently F≠ma.

I'm learning so much.

Certainly.

F=ma doesn't explicitly tell you which one comes first.

F=ma , F=am, F/a=m, F/m=a, am=F, ma=F etc What's the difference for you?

 

Existence of a force is independent of acceleration...

I sent this nonsense to a new thread.

 

Nano v Don

Personally I think you guys are arguing at cross purposes here.

Newton's laws of motion were conceived for and properly apply to rectilinear dynamic situations.

I understood the laws of statics to apply to this comment, which make it perfectly reasonable, and the reason buildings don't fall down and why my computer hasn't fallen through my desk.

You can have an applied force with the object in a stationary position

Don,
If you have a force applied by a non material method such as a magnetic field, where is the reaction?

Nano
Some forces depend upon one's (inertial) frame of reference, even in a Galilaean system of relativity.

 

What I'm saying is, first comes the force then the acceleration.

If an object is stationary, resultant force is zero and accceleration is zero, but it does not necessarily imply that there are no forces applied to it. Applied Force and Resultant Force are not exactly the same though they are both forces. There can be applied forces and no acceleration. But an acceleration present implies the presence of a resultant force, yet it's that resultant force which is a result of the vector sum of the applied forces that actually produces the acceleration. One can see something in motion, but force is invisible to the eyes.

I do agree that the magnitude and direction of forces depend on many factors, but I do not think that a force is caused by a "stand-alone source" of acceleration and that's my point.

Cheers.

 

If an object is stationary, resultant force is zero and accceleration is zero, but it does not necessarily imply that there are no forces applied to it. Applied Force and Resultant Force are not exactly the same though they are both forces.

Yes I understand where you are coming from and agree.

But remember that strictly, the simple formulation of Newton's laws only applies to particles (point masses) which can have no rotational inertia.
Rigid body mechanics introduces further considerations and non rigid body mechanics even more.

Hence my structural mechanics examples.

but I do not think that a force is caused by a "stand-alone source" of acceleration.

Centrifugal force only exists in certain frames of reference.

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

 

It sounds like everyone here understands the physics correctly, but the disagreements are about the wording.

Just as a point of interest. When an object freely falls in a gravitational field, it accelerates but feels no force. This is why the astronauts in a space station or shuttle orbiting the earth are weightless despite being well in range of the earth's gravity.

So, free-fall in a gravitational field is a case of acceleration with no net force. In effect, gravitational force and inertial force (F=ma) perfectly cancel.

So the statement that acceleration does NOT cause force is maybe not so crazy. And, the statement that you can have acceleration with NO net force is maybe not so crazy. This is a subject where speaking words puts you on a slippery slope. The math is much less confusing.

 

but the disagreements are about the wording.

It's more than just wording. Or just equations (maths).

It's all about applying the appropriate physics and maths to the situation and not setting the physics/maths from one situation in opposition to the physics/maths from another.

Forces, like systems of mass, can be point or distributed.
Different systems of physics/maths are appropriate in each case.

Gravity is a distributed force.
The feeling you get when I poke you with a stick is a point force.

If I consider a rigid body such as a ring and poke it with a stick at the centre of mass or centre of gravity what will happen? Will it accelerate?

If I now poke it at an edge what will happen? Will it accelerate?

If I now mount it in gimbals and repeat the experiment what will happen? Will it accelerate?

If instead of poking with a stick I apply a couple can I get the same effect as with my application of force?

 

So, free-fall in a gravitational field is a case of acceleration with no net force. In effect, gravitational force and inertial force (F=ma) perfectly cancel.

Wouldn't the object fall at constant velocity?

Though I do agree that a change in direction is also a change in acceleration and velocity even though if there is no change in speed (scalar) as they are vector quantities. In that sense, the speed might be constant but not the acceleration or velocity. Such is the case with constant rotational speed and yet this acceleration is due to a force.

 

It sounds like everyone here understands the physics correctly, but the disagreements are about the wording.

I agree very often the source of confusion/misinterpretation is the word choice...... unfortunately.

 

But what has all this got to do with AC current??

 

I smell hijacking

 

I smell hijacking

You bet!

 

Golly, DonQ tried to head off a hijacking! http://forum.allaboutcircuits.com/showthread.php?t=23716

 

Getting back to the original intent of the thread...

What might be the convenient and explicitly clear substitutions for:

1. AC voltage?

2. AC current?

3. DC voltage?

4. DC current?

 

I'm happy with these terms.

There are no apparent reasons for changing their definitions from what I can say even though it doesn't sound well to say Alternating Current current.

 

That's the point - nobody actually says "alternating current current", or "direct current current".

AC (or DC) current or voltage is completely unambiguous.

 

Actually, me thinks somehow this thread has diversified into a fruitless endeavor. Unfortunately, no one came up with a clear answer to the OP's original dilemma.

Now, the arguments are on the laws of mechanics and the original topic is lost.

 

Laws of semantics, you mean. Since we are talking about English, where the rules are pretty loose, I suspect it doesn't matter much.

 

Now, the arguments are on the laws of mechanics and the original topic is lost.

Maybe we can tie these topics together.

I've heard people talk about AC and DC in the context of mechanical force equations. For example with mass/spring oscillators. These problems can be analyzed the same way as circuits and you can have AC and DC analysis. How strange is it to talk about alternating and direct current in that context?

EDIT: Actually, I have to confess, I've been guilty of this sin.