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Kinds of forces

Page history last edited by Joe Redish 12 years, 5 months ago

Class content > Newton's Laws > Newton's Laws as Foothold Principles > Quantifying impulse and force




In Quantifying impulse and force, we decided that a law of motion should work with a force -- what a stretched spring is doing to whatever it is attached to -- and that we could use the stretch of a standard spring as a standard force.  Now what we need to do is decide what kinds of forces an object might feel.


The reason we want to do this is to predict the future -- to understand how things move -- and what's going on when they don't move.  Here's the way it works.  If we know an object's starting position and velocity, Newton's second law will tell us how that velocity will change if we know all the forces acting on the object. If we know all the forces the object will encounter as it moves, we can do more than just get it started.  Since Newton's second law can be written as a differential equation, we can use it as a stepping rule to predict the future motion of the object.  Knowing what controls an object's motion can be quite useful, whether you are trying to throw a large rock into a castle, to put a rocket in orbit around Saturn, to understand how a fish has to evolve to slurp up food, or why certain ions pass through a membrane and others don't.


For working with macroscopic objects, we want to build on our experience.  We know of two very different ways that object's velocities can be changed; the first is to touch it (hit it with a hammer), the second is to let some action-at-a-distance force act on it (drop it).  This suggests that we consider two broad classes of macroscopic forces: touching and non-touching.


Touching forces

To think about how objects interact to change each other's velocities, we have to keep in mind object egotism -- that we have decided that objects are in general dumb as posts (indeed, they may be posts) and only can respond to things that act on them directly at the instant that interaction takes place.  So: thinking like an object, what are the different ways that an object can interact with you by touching?  The key -- to the object -- is in what direction does the force act?  We'll define three kinds of touching forces: normal, tension, and friction.


  1. Normal forces -- The word "normal" is an old word for "perpendicular".  We call a touching force on an object "normal" if it is perpendicular to the surface of the object and pushing in. Think of pushing on the object with your hand -- or a hammer. 
  2. Tension forces -- We call a force a tension force if it pulls out on the surface of the object.  Think of somebody gluing a string to the object and pulling on the string.
  3. Resistive forces -- When two object rub surfaces, they exert forces on each other that resist the sliding.  The result is a force on each object that is parallel to the surface of interaction and that opposes the sliding.  (NOT "opposes the motion".) When the two surfaces are solids, the force is called friction.  When one (or both) of the surfaces are fluids, the force is called viscosity.  If a solid moves through a gas, it also experiences drag.  Friction is independent of the relative velocity of the two surfaces, viscosity is proportional to the relative velocity of the two surfaces, and drag is proportional to the relative velocity squared.


There are subtle issues about each of these forces.  We consider each of these in more detail on separate pages ( available from the links connected to the force name above).


Non-touching forces

You've had experience with the fact that things can be made to move without anything touching it directly since you were an infant.  Everywhere on the earth every object feels a downward pull that we give the name "weight". (We're going to save the name "gravity" for something else.)  When you take a sock out of the dryer (if you haven't used dryer sheets) it might jump out of the basket and stick to your shirt. You probably have seen magnets pushing other magnets around at a distance from the time you were in kindergarten or before.  These three forces are more fundamental than touching forces.  At the microscopic level, touching forces can be seen to be the result of electric forces (like the sock in the dryer) combined with quantum mechanics.  The non-touching forces we'll be concerned with here are


  1. Weight -- An object's weight is the result of the gravitational force that acts between the object and the earth.  Gravitational forces act between all objects, but gravity is such a weak force that it's only really relevant when planet sized bodies are around.  Since we only have one, we only have to worry about the earth.  (We might consider some properties of gravity and the implications for planets, moons, solar systems, and galaxies later down the line.)
  2. Electric forces -- As you know, all objects are made of atoms and all atoms contain two kinds of charged particles -- electrons and protons.  Charged particles exert electric forces on each other at a distance.  Since the kinds of charges are nearly perfectly balanced in everyday matter, these forces were not thought to be important for thousands of years.  But they underlie all of the fundamental biological processes that make life possible.
  3. Magnetic forces -- The basic charged particles also exert another kind of long-range touching force on each other -- magnetic forces.  Every electron and proton is not only a separate electric charge, it is a little bar magnet.  But the magnetic force tends to be significantly weaker than the electric force in most biological cases.  It is, however, of great importance in engineering -- where it makes possible the entire structure of electrical generation and transmission that has an immense impact on our everyday lives.  Some chemical and biological measurement tools (such as NMR, fMRI, and magnetic spectrometers) depend strongly on magnetic forces.


Labeling forces

In many examples in this class we will have LOTS of forces.  It's therefore really important to have a way to label each force carefully so that we know what we are talking about.  We'll use three conventions. We will call every force "F" to remind ourselves that the symbol represents a force; we will put a subscript on the F to tell us what object is causing the force and what object is feeling it; and we will put a superscript from the following set of labels: (N, T, f, W, E, M) to tell us whether the force is (Normal, Tension, friction, Weight, Electric, Magnetic).  Thus, if block A is pushing up against block B, we will write the force felt by block B as NA→B.


Joe Redish 9/18/11


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