View Full Version : Force what is it

Peter Mason
2008-Feb-29, 07:47 PM
I am an Engineer and I thought I knew what force was. You learn all about

F =M*a

but then the problems start when you begin consider that it is a vector quantity and has a direction. Where does that come form? And what causes force?

Then because I am an Electronic Engineer you consider applying a static force, no acceleration, to the end of a longitudinally stiff body. In the instant that you do so the remote end does not know about the force being sent to it. So the force goes off but what determines the amplitude as it has no reaction force and nothing constrains it. It presumable has a propagation velocity like a transmission line and an experiment should be able to show that the force is reflected from the distant end.

Has this experiment ever been done and what is the result?

If you consider a Bowden cable coiled up how is it that the force turns round a corner and seems to be constrained by the material of the core. The core is mostly nothing and some protons with electrons suspended around them. Is it the atomic forces between the atoms that is transmitting the applied disturbance form one end to another?

Finally how does all this work on an Astronomic scale?

Steve Limpus
2008-Mar-01, 02:12 AM
Whoa. Good questions - I found this a good place to start:

"The electromagnetic force causes like-charged things to repel and oppositely-charged things to attract. Many everyday forces, such as friction, and even magnetism, are caused by the electromagnetic, or E-M force. For instance, the force that keeps you from falling through the floor is the electromagnetic force which causes the atoms making up the matter in your feet and the floor to resist being displaced.

The carrier particle of the electromagnetic force is the photon. Photons of different energies span the electromagnetic spectrum of x rays, visible light, radio waves, and so forth.

Photons have zero mass, as far as we know, and always travel at the "speed of light", c, which is about 300,000,000 meters per second, or 186,000 miles per second, in a vacuum."

You will want to consider the strong and weak nuclear forces too, and on astronomical scales you could probably argue gravity is the most important force, although scientists don't yet fully understand dark matter and dark energy.

Those are the appetizers, then for mains, if you really drill down, you start to get in to questions of what is reality, exactly? I enjoyed this book, which my local library stocks:

The fabric of the cosmos : space, time, and the texture of reality / Brian Greene.
"A foremost string theorist discusses such topics as Newton's perspectives on space, Einstein's fusion of space and time, and recent breakthroughs on multidimensional universe theory."
...and this website, play the video (all three hours are available free, on the site), I'm sure you'll love it:

Somewhere in that lot you'll find a path towards what you're looking for... and welcome to the forum. :)

2008-Mar-01, 09:06 PM
"The Force is what gives a Jedi his power."

Steve Limpus
2008-Mar-01, 09:32 PM
"The Force is what gives a Jedi his power."


...yep, and it was all so cool until the midi-chlorians came along and ruined it for everyone!

2008-Mar-02, 02:52 AM
Somebody had to make that joke, and I'm glad nobody beat me to it.

2008-Apr-25, 05:58 PM
I'll address your question regarding application of a force to a fixed solid body. Consider a horizontal bar attached to a vertical wall. Suppose you press your hand against the end away from the way. By the nature of the process, the force starts rather quickly from zero and builds up to a maximum within a second or so. The force tends to compress the bar because the far end is fixed. This tends to compress the bar increasingly as the force builds to its maximum. The bar transmits your force along its length and applies it against the wall. The bar is gradually compressed by in proportion to the instantaneous amount of the force. Because of the stiffness of the bar, the compression is very slight. The bar remains under pressure as long as your continue to press on it. The bar has a an elastic modulus, and the amount of compression is the product of the force and the elastic modulus. The latter is known as "Hooke's law", which is valid for both limited amounts of both stretching and compression.

You apply a force whenever you apply pressure against something. If you press twice as hard, you are exerting twice the amount of force.