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Kinds of forces

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Saved by Joe Redish
on September 18, 2011 at 9:06:01 pm
 

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

 

Prerequisites

 

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

 

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