Ask A Scientist© Physics Archive

name         Hilary
status       student
age          7

Question -   Please explain the difference between direct force and
indirect force.  Could you give examples?
------------------------------------------------
Usually physicists (scientists that study the relationship between matter
[stuff] and energy) refer to these as contact forces and long range forces.
  A contact force is a push or a pull that is directly touching an object.
An example is the force of a chair pushing up on your body when you are
sitting in it.

A long range force is a push or a pull that acts through a distance.  You
may have played with a magnet and seen its influence on paper clips.  This
influence (force) is exerted through a distance.  Sometimes these regions
of influence are called fields.  Long range forces include magnetic forces,
electrical forces, and gravitational forces.

Great question!  I hope you enjoy learning about nature as much as I do.

---Nathan A. Unterman
========================================

First of all, I need to make sure you understand something.  Teachers
separate things into categories because they know how kids think, they
know what kids are confused by, and they know which ideas come
naturally to kids and which must be drilled in.  I don't know these
things, so my idea of the difference between direct and indirect forces
may completely miss the point.  But this never stopped me before, so
here's my best guess:

A direct force is a force between things that are touching.  If you take a
balloon and bop somebody on the head with it, the force between the balloon
and the head is direct.

An indirect force is a force conveyed, or carried, by a field.  If you rub
a balloon on your hair and then hold it over someone else's head, their
hair will stand up, attracted by the electric field of the charge you just
put on the balloon. This is an indirect force.  You still have to be
strong enough to lift both the balloon and the hair that's attracted to the
balloon--the force raising the hair is still coming from you, but it's
carried by the field.

If you look closely enough, all forces are indirect, but it's still useful
to separate the two cases so everyone knows when you're thinking about the
field and when you want to ignore the field and concentrate on the objects.

Frequently, the idea of an indirect force is extended to include forces
carried by other objects.  For example, if your car is sitting by the side
of the highway and fast-moving cars are passing by, you can usually feel a
little shudder as one goes by.  This is an indirect force from the car,
carried by the air that car was pushing out of its way as it moved.

Tim Mooney 
=========================================