FrictionFriction

Static FrictionStatic Friction ffs - s - The force on an object that keeps The force on an object that keeps

it from slippingit from slipping ffs = s = FFpushpush

Static FrictionStatic Friction There is a limit to how big static friction can There is a limit to how big static friction can

get – fget – fsmaxsmax

ffsmaxsmax is proportional to the magnitude of the is proportional to the magnitude of the normal force between the surface and the normal force between the surface and the object. object.

FFsmax = smax = ųųssn where ųn where ųs s is called the coefficient of is called the coefficient of static friction.static friction.

The coefficient is a number that depends on The coefficient is a number that depends on the materials of the object and the surfacethe materials of the object and the surface

The higher the number, the greater the The higher the number, the greater the “stickiness” between the object and the “stickiness” between the object and the surfacesurface

Rules for Static FrictionRules for Static Friction

The direction of static friction is such The direction of static friction is such as to oppose motion.as to oppose motion.

The magnitude fThe magnitude fss of static friction of static friction adjusts itself so that the net force is adjusts itself so that the net force is zero and the object doesn’t move.zero and the object doesn’t move.

The magnitude of static friction The magnitude of static friction cannot exceed the maximum value cannot exceed the maximum value ffsmax. smax. If the friction force needed to If the friction force needed to keep the object stationary is larger keep the object stationary is larger than fthan fsmax, smax, the object slips and starts to the object slips and starts to move.move.

Kinetic FrictionKinetic Friction

Once the box starts to slide, the Once the box starts to slide, the static friction force is replaced by the static friction force is replaced by the kinetic friction force.kinetic friction force.

The direction of fThe direction of fkk is always opposite is always opposite the direction of motion.the direction of motion.

Kinetic friction has a nearly constant Kinetic friction has a nearly constant magnitude.magnitude.

FFkk = ų = ųkkn where ųn where ųk k is called the is called the coefficient of kinetic friction.coefficient of kinetic friction.

Kinetic FrictionKinetic Friction

ŲŲk k < < ųųs s

This explains why it is easier to keep a box moving than it This explains why it is easier to keep a box moving than it

was to start movingwas to start moving

Rolling FrictionRolling Friction A wheel rolling on a surface experiences rolling A wheel rolling on a surface experiences rolling

frictionfriction The portion of the wheel that contacts the surface The portion of the wheel that contacts the surface

is stationary with respect to the surface, not is stationary with respect to the surface, not sliding.sliding.

A wheel is not perfectly round. A wheel is not perfectly round. Weight causes the bottom of the wheel to flatten Weight causes the bottom of the wheel to flatten

where it touches the surface.where it touches the surface. The leading part of the tire becomes deformed.The leading part of the tire becomes deformed. This requires the road push backward on the tire. This requires the road push backward on the tire. The road causes a backward force The road causes a backward force

Rolling FrictionRolling Friction

FFrr = ų = ųrrnn ŲŲr r is the coefficient of rolling friction.is the coefficient of rolling friction.

The direction of the force opposes The direction of the force opposes the direction of motionthe direction of motion

Rolling friction acts similar to kinetic Rolling friction acts similar to kinetic friction but ųfriction but ųr r << ųųkk

This is why it is easier to roll an This is why it is easier to roll an object on wheels than to slide it. object on wheels than to slide it.

Friction EquationsFriction Equations

Friction Equations are models not Friction Equations are models not laws. laws.

We ignore surface area. It has little We ignore surface area. It has little effect.effect.

The model assumes kinetic friction The model assumes kinetic friction force is independent of the object’s force is independent of the object’s speed.speed.

Causes of FrictionCauses of Friction Surfaces are rough on the microscopic Surfaces are rough on the microscopic

level.level. High points of one surface become High points of one surface become

jammed against the high points of the jammed against the high points of the other surface.other surface.

A very small fraction (10A very small fraction (10-4-4) of the surface ) of the surface area is in actual contact.area is in actual contact.

The amount of contact depends on how The amount of contact depends on how hard the surfaces are pushed together, hard the surfaces are pushed together, which is why friction forces are which is why friction forces are proportional to n.proportional to n.