## Newton's Laws

• Friction is a partner to the normal force -- both are aspects of contact force. The normal force is the component perpendicular to the surfaces; friction is the component parallel to the surface.
• The magnitude of the normal force depends on the details of the surface properties of the materials in contact.
• It also depends on the normal force pressing them together.
• Static friction occurs between surfaces which are at rest with respect to each other. Its magnitude can vary from zero to some maximum value:
```               0  <=  F(static)  <=  Fn * mu(s)
```
where Fn is the normal force, and mu(s) is the coefficient of static friction.
• Kinetic friction opposes the motion of surfaces which are currently moving with respect to each other. It always has the same magnitude:
```                      F(kinetic) =  Fn * mu(k)
```
where Fn is the normal force, and mu(s) is the coefficient of kinetic friction.
• Typical values for coefficients of friction are around 0.05 for smooth surfaces, 1.0 for very rough surfaces. The coefficient of kinetic friction is usually smaller than the coefficient of static friction, for any particular interface.
• Tension is the name given to the stretching force exerted along the axis of long, thin materials (ropes, strings, wires, rods). A perfect wire exerts tension of the same magnitude, but opposite directions, at its two ends.
• An object is in equilibrium if its velocity does not change; i.e., if it is not accelerating. In that case, the net external force acting on it must be exactly zero.
• On the other hand, an object out of equilibrium is accelerating. The magnitude of its acceleration is given by Newton's Second Law:
```               F  =  m * a
so
Fx =  m * ax
Fy =  m * ay
```
• To solve problems in which several forces are acting on an object, it may help to write a table in which one lists explicitly the forces on the object in each direction.

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