Re: Why is it easier to pull than to push?

If we were talking about simple boxes being pushed on a flat surface
in a straight line, I could demonstrate that a push and pull are
identical forces and make no difference. But that contradicts what you
"know" -- often it is easier to pull. Is it the friction? A textbook
definition and explanation of friction makes it seem that that can't
be the explanation either.
 http://theory.uwinnipeg.ca/physics/force/node6.html

http://theory.uwinnipeg.ca/physics/force/node7.html


So, let's talk about what "easier" means. I argue that it doesn't mean
less force involved but more control of the final destination. 

Consider yourself on a frozen lake, with a long box.  Metal bars,
with full degrees of movement at the point of attachment (think of a
toy wagon) are on the opposite narrow sides of each box, letting you push
or pull as you like. If you were on a perfectly smooth sheet of ice (or
even perfectly smooth concrete) it wouldn't matter whether you pushed
or pulled.  But you're not. You're on a lake. If you imagine running
in your cleated shoes, pulling the boxes behind you, you know if you
look back you'll see the boxes fish-tailing. Imperfections in the
surface create forces perpendicular to the direction of motion, but
your pulling eventually corrects that disruption.

On the other hand, if you were pushing and the boxes started moving
perpendicular to your push as the angle of your bar with the box
increased, you would actually be adding to he horizontal force.
So it's easier to control where you're going with the pull. 

One final point that doesn't apply to front and rear wheel drive. In
the case of a car, the normal force is constant, independent of the
push or pull action. With the block on the ice, you can imagine that
you could have a vertical component of the applied force that would
affect the value of the normal force. It's more likely that in pulling
you would decrease the normal force, and thus decrease the force due
to friction.

Here's what looks to be a really cool collection of articles that
address the physics of cars. He's probably addressed the push/pull
difference in here somwhere, in more depth than i.
 http://www.mc68k.demon.co.uk/papers/physicsrace/index.html

Cheers,

judith

Judith E. Bush                                          jbush@fi.edu
The Franklin Institute Science Museum           New Media Specialist
222 N. 20th Street                                  1 (215) 448-1236
Philadelphia, PA 19103-1194 USA                    AOL IM judielaine