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Name: Fred
Status: other
Age: 30s
Location: N/A
Country: N/A
Date: 1999 


Question:
I am trying to recall my physics classes. Is friction independent of surface area? If you have two 50 pound boxes with different surface contact areas to the floor. Will both require the same amount of force to start moving the boxes and keep them moving?


Replies:
Yes, friction is independent of surface area. However, it is dependant on the types of surfaces in contact. A wooden block will move at a lower force on ice than it would on concrete. Also, friction values differ in static friction and dynamic friction. That is, it takes more force to start a block moving from rest (static friction) than it does to keep it moving (dynamic friction). Most elementary physics books will have an entire chapter on frictional forces.

Chris Murphy


Friction is usually idealized as a force independent of surface area and proportional to the normal force pressing surfaces together. In this idealization, the two boxes you describe would require the same force to move.

From the experimental data that support this idealization, you could equally well claim that the frictional force is proportional to the surface area and to the normal force per unit area pressing the surfaces together.

Clearly these idealizations break down when the surface area becomes small enough.

Tim Mooney



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