0.1 Forces  (Page 2/2)

In the lamp was represented by a simple dot. We assumed that the lamp was rigid and that adownward force applied at one particular spot on the lamp would yield the same result as a similar downward force applied at adifferent place on the lamp. Actually, in order for a force of equal magnitude and direction to affect an object's motion inthe same manner it must be applied along the same line of action as the original force (see ). If the original force had been a tug on a string tied to the lamp, thenit makes sense that grabbing the string at a different distance away from the lamp to tug should not make a difference providedthat the direction and magnitude do not change.

Friction

The normal force from the table's surface is a reaction force only . Without the downward force on the table from the object resting its weight on thesurface, the normal force does not exist. This type of behavior is also descriptive of frictional forces.

Friction is opposition to motion, so if nothing is trying to move there will be no friction. However, frictionwill be present when motion is attempted, even if the object is not yet moving. There are two different types of friction:static, which acts before the object begins to move, and dynamic, which acts after the object begins moving. Static friction is usually stronger than dynamic friction .

Friction occurs because the surfaces in contact are not smooth. The small ridges on the different surfaces catch, and in order for the objects tomove, these ridges must be broken off or the object must ramp up and over the obstructions. By adding a lubricant betweenthe two layers, it is possible to "float" one layer high enough to miss some of the obstructions to motion. At anatomic level, cold joints may form where the atoms from one object's surface may formweak bonds with the atoms on the surface of the other object. These bonds must also be brokenin order for the object to move. All of this resistance to motion is called friction . Friction is very important because it not only inhibits motion, friction alsomakes motion possible.

Most, but not all, small robots (such as those built in the Rice University course ELEC 201, Introduction to Engineering Design) will probably be wheeled vehicles, and without friction those wheels would just spin inplace without moving the robot anywhere. In order to increase the friction between the wheels and the game board one mightuse wheels made of a different material or add a rubber band around the wheel's circumference. Friction is not desirable inall cases. When it comes to axles spinning inside of holes in beams or gears rubbing up against beams or even gears pushingagainst each other, friction can cause two identically constructed gear trains to behave differently. Friction caneven render the whole assembly ineffective. For example, in one design, a worm gear in a drive train created so much friction that more of the drivemotor's effort went towards overcoming friction than actually driving the robot.