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Q & A: Kinds of forces

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Most recent answer: 10/22/2007
Q:
Iím doing a project on Forces. Iíve collected all the information that i needed but i still canít find information on the tearing (also includes; twisting, pulling, pushing) force. I need URLs of some good sites about these forces.
- Rameen (age 13)
Lahore, Punjab, Pakistan
A:
You can find about different kinds of forces at , and look under the tutorial on Newton's laws.

In short, forces are really just pushes or pulls on an object. There are different kinds of forces -- reaching out and pushing things with your hands ("contact forces", but see below), gravity, electrical and magnetic forces are common, everyday kinds of forces you encounter.

What the object does when a force pushes or pulls on it (or some part of it) depends on the force and on the object. Some objects may tear, for example. To tear paper, you usually need two forces, one pulling one way and one pulling the other. Chemical bonds of varying nature and strengths hold the fibers of the paper together, and the weakest of these can be overcome easily by pulling on the edges, thus a tear. It's not the force that's different from any other kind of force, it's how the object responds to it. Other objects may break (like glass), or hold firm (like steel, until you pull really really hard).

The chemical bonds and contact forces really are different kinds of electrical and magnetic forces, and you will find out about this in your studies.

Twisting is something else entirely. In order to twist something, you need to apply an off-center force, or, better yet, two forces which point in opposite directions but which are not applied to the object in the same place. Newton's second law says that an object will accelerate if the sum of the forces on it is nonzero, and so to twist something without making it accelerate off is to apply a pair of balancing forces at different points. The force times the perpendicular distance to an axis of rotation is called the torque . A wrench (called a "spanner" in many parts of the world) is just the thing to supply a torque in order to twist something. Look around our answers to questions on forces and torques on this site to see if they help.

Tom

(published on 10/22/2007)

Follow-Up #1: kinds of forces

Q:
what are the different kinds of forces
- camille mercado (age 18)
makati japan
A:
Often you see lists of types of forces that include things like "friction" or "normal forces' or "spring forces". That sort of list can go on forever, since the fundamental forces can show up in all sorts of different ways in different materials in different circumstances. In practice, the ordinary forces you see, other than gravity, are all manifestations of electromagnetism.
Electromagnetism itself is just one aspect of a more general force called the electroweak force. There is also another type of force, the strrong nuclear force (chromodynamic). In addition there is gravity, which isn't quite a force in the usual sense but appears like one if we pretend that we live in a simple Newtonian spacetime.
It's possible that these three different effects (electroweak, chromodynamic, gravity) may all be understand in some unified theory someday, maybe soon, but we don't really know.

Mike W.

(published on 10/22/2007)

Follow-up on this answer.