Definisi : Definisi The resistance encountered by one body in moving over another. W F W F
Slide 4: The friction force is proportional to the normal load.
Amounts to the statement that the coeficient of friction μ, is independent of the normal load.
2. The friction force is independent of the apparent area of contact.
3. The friction force is independent of the sliding velocity. The laws of friction
Solid Friction : Solid Friction Resistance force for sliding
Surface roughness (asperities)
Adhesion (bonding between dissimilar materials)
Factors influencing friction
Frictional drag lower when body is in motion
Sliding friction depends on the normal force and frictional coefficient, independent of the sliding speed and contact area Effect of Friction
Frictional heat (burns out the bearings, ignites a match)
Wear (loss of material due to cutting action of opposing
Engineers control friction
Increase friction when needed (using rougher surfaces)
Reduce friction when not needed (lubrication)
Friction : Friction Based on these observations we can conclude :
There are two different friction forces: the static friction force (no motion) and the kinetic friction force (motion).
The static friction force increases with the applied force but has a maximum value.
The kinetic friction force is independent of the applied force, and has a magnitude that is less than the maximum static friction force. 10
Friction and Braking : Friction and Braking Consider how you stop in your car:
The contact force between the tires and the road is the static friction force (for most normal drivers). It is this force that provides the acceleration required to reduce the speed of your car.
The maximum static friction force is larger than the kinetic friction force. As a result, your are much more effective stopping your car when you can use static friction instead of kinetic friction (e.g. when your wheels lock up). 11
Friction and Normal Forces : Friction and Normal Forces The maximum static friction force and the kinetic friction force are proportional to the normal force.
Changes in the normal force will thus result in changes in the friction forces.
The normal force will be always perpendicular to the surface.
The friction force will be always opposite to the direction of (potential) motion. 12
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