eloquent logic - dynamics

Legend

F_net	=	net force
m	=	mass
a	=	acceleration
F_g	=	force of gravity
F_N	=	normal force
F_f	=	force of friction

This skier moves ahead by pushing back on the ground with the pole. The ground pushes forward on the pole, which transmits the force to the person. Without friction, it would be hard to move around or stop.

Friction

Most of the time we ignore or assume many things, for example, we ignore air resistance, assume F_g is constant. However, friction is too significant to completely ignore, and is a factor in many problems.

There are two types of friction, and for good reason, they are different. For example, when you brake hard and the tires do not skid, friction helps you to stop. However, if you brake hard and the tires skid, you still eventually stop, even though it takes longer.

These are the two frictions:

Static Friction	Force that acts as long as surfaces in contact are not slipping.

Kinetic Friction	When surfaces in contact are slipping, this is the force acting on it.

Direction of friction force is always parallel to the surface of contact, and in the direction opposite to applied forces.
Size of friction force depends on normal force (F_N) and force of gravity (F_g).

F_f	=		F_N	= coefficient of static friction
F_f	=		F_N	= coefficient of static friction

The coefficients of friction are a measure of the amount of cling a surface has. You may notice that when pushing something heavy the toughest part is getting it to move; once it's sliding along, it is easier to push. This is reflected in

and

. For most surfaces,

(surfaces stick to each other better when they aren't sliding).

Example:


A horizontal force F acting through a spring scale is applied to a motionless block of weight F_g. Static friction opposes the impending motion with a force F_f that can be as much as
F_{f (max)}= F_N
The free-body diagram indicates that no horizontal acceleration will occur until F > F_f.

Weight

Inclined Plane