If two objects are in contact and at
rest and a force applied to one object does not cause motion, then
this force must be balanced by an opposing and equal size frictional
force so that the resultant force is zero.
If the applied force is increased a
point is reached when motion just starts. When this occurs there must
be a resultant force acting on the object and thus the applied force
must have become greater than the frictional force.
The value of the frictional force that
has to be overcome before motion starts is called the limiting
frictional force. The following are the basic laws of friction:
# Law 1. The frictional force is always
in such a direction as to oppose relative motion and is always
tangential to the surfaces in contact.
# Law 2. The frictional force is
independent of the apparent areas of the surfaces in contact.
# Law 3. The frictional force depends
on the surfaces in contact and its limiting value is directly
proportional to the normal reaction between the surfaces.
The coefficient of static friction Us
is the ratio of the limiting frictional force F to the normal
reaction N:
Us = F/N
The coefficient of kinetic friction Uk
is the ratio of the kinetic frictional force F to the normal reaction
N:
Uk = F/N
Typical values for steel sliding on
steel are 0.7 for the static coefficient and 0.6 for the kinetic
coefficient.
Example
If a block of steel of mass 2 kg rests
on a horizontal surface, the coefficient of static friction being
0.6, what horizontal force is needed to start the block in horizontal
motion? Take g to be 9.8 m/s2.
The normal reaction force N = mg. The
maximum, i.e. limiting, frictional force is F = uN = 0.6 × 2 × 9.8
= 11.8 N. Any larger force will give a resultant force on the block
and cause it to accelerate while a smaller force will be cancelled
out by the frictional force and give no resultant force and hence no
acceleration. Thus the force which will just be on the point of
starting the block in motion is 11.8 N.
No comments:
Post a Comment