Thursday, October 30, 2008 - Blocks 1 & 3

Collected RA 4.2

Reviewed video on Newton's 2nd Law
Factors affecting air resistance:
1) speed
2) cross sectional area

Class Lab - What Factors Affect Contact Friction
Asked class for possible variables
Tested each variable
Factors affecting friction:
1) Normal Force
2) Surface Texture

Factors that did not have a significant effect
speed, contact area

A plot of Friction force vs Weight (Normal Force) shows a linear relationship. The slope, usually given by m (as in y = m*x + b)is instead given the Greek letter m, mu, and is called the coefficient of friction. This depends only on the surfaces and removes the effect of the normal force. We get an experimentally derived relationship

Ff = mu * Fn

There are two types of contact friction, static when the objects are at rest and kinetic when there is relative motion between the surfaces. The static friction force can range from zero to some maximum value. The kinetic friction force is constant. The maximum value of the static friction force is greater than or equal to the kinetic friction force showing that it is harder to get something going than to keep it going.