Friction: a force that opposes motion
قوة الاحتكاك تكون فى اتحاة معاكس لاتجاه الحركة
- Depends texture of each surface
- Forces holding the objects together (example: gravity and weight)
Types of friction أنواع الاحتكاك
Rolling friction أحتكاك الدحرجة : on wheels - riding a bike or skateboard
- Rolling friction has less resistive force than sliding friction
When
the Great Pyramids were being build in ancient Egypt, they used logs as
rollers under the giant blocks of granite instead of trying to slide the
rocks along the ground.
[b]Causes of rolling friction
[/b]
- Molecular attraction or adhesion between the materials
- Surface roughness of the materials
- Deformation resistance in the case of soft materials.
Sliding Friction احتكاك انزلاقى : one surface rubbing against another surface: cartilage against your bone, box against the floor
- Two solid objects are in contact
- A force is applied to slide one object against the other
- Sliding friction force resists the motion
Pushing force greater than friction force
Static Frictionالاحتكاك الساكن :
Pushing force is equal to the resisting force, then there will be no
motion and the objects remain static or still when compared to each
other
- once you start an object moving, the friction decreases from the static friction holding the object in place.
- You
- have seen this in trying to slide a heavy box across the floor. It may
- be very difficult to move, but once it starts sliding, it is easier to
- push.
Fluid Friction:
- When
- a solid object is in contact with a fluid, such as a liquid or gas, and
- a force is applied to either the object or to the fluid, there is a
- friction force that resists the motion
- water flowing through a hose, an airplane flying through the atmosphere
