The Physics of a Sliding Block

A block on an inclined plane will accelerate down the plane. The amount of acceleration is determined by acceleration due to gravity, the angle of the plane, and the coefficient of friction of the block with the plane.

The coefficient of friction is a measure of the amount of friction that exists between two materials as one slides over the other. It is zero if there is no friction, and it is infinite if no motion is possible. The coefficient of friction of skis on snow is 0.01; of brass on glass, 0.1; and of two hands rubbing together, 0.5.

Here is another graphic to help acquaint you with the meaning of the vectors that will also be appearing in Lab 4 (cow sliding down incline).

In the case where you are pulling an object on a horizontal surface, your must overcome the static friction to get the object moving. That is, the applied force, must overcome the friction force:

The maximum force of static friction that exists between two surfaces is proportional to the normal force and mostly independent of area of contact. This situation is shown here: N = the total normal force (force perpendicular to the horizontal surface) which is essentially the weight of the object. The coefficient in that equation is called the coeffecient of static friction and that depends on the material:

When the object is actually moving, the friction is said to be kinetic friction which is generally less than static friction.

If we add more mass we increase the normal force (N) (because the weight has increased) and hence we have increased the total frictional force. This is shown here where it can be seen that twice as much force must be applied to move two bricks instead of one (the force meter reads twice as large).

In general frictional forces are independent of the area of contact although this is an empirical observation not a theory. Consider a metallic brick and a metallic table. The reason that friction is nearly independent of surface area is if the "microscopic" area of contact of the brick to the table is independent of the orientation of the brick. If this is not the case, then friction will have a small dependence on area. In normal circumstances, with the largest surface area of the brick in contact with the table there are a large number of "contact" points that support the load. With the smallest area in contact (brick standing on end) there are fewer contacts but as long as the area of each contact is larger due to the higher pressure (same force, smaller unit area) then there will be no difference in the amount of static friction. Over wide limits, most materials follow this and hence friction is largely independent of surface area. iF you have a situation where the microscopic contact area does not scale in accordance with the pressure, then static friction will depend upon orientation.

Frictional Energy Losses

In general, in the real world, friction is everywhere (fortunately) and almost any macroscopic system looses energy, in the form of HEAT from friction. A common example is when you apply your brakes while driving. If you are travelling at 80 mph on the freeway (and of course you are) and then decelerate to 0 mph then an awful lot of kinetic energy has been lost. That lost energy got dissipated as heat. In theory, if your velocity were high enough and you slammed on your brakes to reach velocity = 0; your tires would melt.