Simple machines. Law of Simple Machines. Resistance Force x resistance distance = effort force x effort distance

Transcription

1 Simple machines A simple machine is a evice that requires only the force of a human to perform work. One of the properties of a simple machine is that it can be use to increase the force that can be applie to a task. Law of Simple Machines esistance orce x resistance istance effort force x effort istance The Mechanical Avantage is the ratio of the resistance force to the effort force resistance force MA effort force There are four types of simple machines: the lever, the incline plane (the wege an the screw are special cases of the incline plane), the pulley, the wheel an axle (incluing the gear). The Lever The lever is a simple machine consisting of a rigi bar that is free to pivot on a fulcrum. Depening on the position of the force (), the loa or resistance () an the fulcrum, there are three classes of levers: irst Class Lever ulcrum Secon Class Lever Thir Class Lever ulcrum ulcrum Simple Machines.oc Physics ~ ASC 005

2 irst Class Level orce (*) ulcrum The fulcrum is between the force an the loa. This is the most common arrangement. The mechanical avantage of this lever epens upon where we place the fulcrum. If the fulcrum is closer to the loa, the mechanical avantage is higher. Examples of this class are the seesaw, the rows in a boat, etc. Secon Class Level orce ulcrum (*) Here the loa ( ) is between the force an the fulcrum. The mechanical avantage of this type of lever epens upon the placement of the loa. It is greater when the loa is closer to the fulcrum. When the loa is closer to the force, the mechanical avantage approaches to one, so no mechanical avantage at all. Examples of this class are: the wheelbarrow, the stapler, the nutcracker, etc (*) Notice that the istance is always measure from the force (or loa) to the fulcrum no matter where it is locate. - - Simple Machines.oc Physics ~ ASC 005

3 Thir Class Level Loa ( ) orce ulcrum In this class the force is between the fulcrum an the loa ( ). The human forearm is a thir class lever. The elbow is the fulcrum, an the forearm muscles apply the effort between the elbow an han. Tweezers, tongs, an the fishing ro are examples of this type. Levers of this class are use less often because their mechanical avantage is less than one; this means that the force neee to use them is greater than the force they can move. The Incline Plane Loa() Loa Distance orce orce Distance lenght of height of plane plane MA incline plane The incline plane is a simple machine, consisting of a sloping surface, which has some angle above or below the horizontal use to raise objects that are too heavy to lift vertically. Gangways, chutes, an ramps are all examples of the incline plane. Switchbacks on mountain roas are also examples of incline planes that reuce the effort of an automobile engine but increase the istance a car must travel to ascen the mountain. The incline plane has been moifie in many ways. The screw an wege are applications of the principle behin the incline plane but o not require that the loa be move vertically for their successful operation. The screw consists essentially of a soli cyliner, usually of metal, aroun which an incline plane wins spirally, either clockwise or counterclockwise. It is use Simple Machines.oc Physics ~ ASC 005

4 to fasten one object to another, to lift a heavy object, or to move an object by a precise amount. The wege shape has a triangular cross-section. It may be use to lever, split, or tighten. The Pulley The pulley is a simple machine, consisting of a wheel that rotates aroun a stationary axle. The outer rim of the pulley is groove to accommoate a rope or chain. Pulleys are use for lifting by attaching one en of the rope to the object, threaing the rope through the pulley (or system of pulleys), an pulling on the other en of the rope. A single, fixe pulley just changes the irection of the applie force an make it easier to lift the loa, since a person can pull own on a rope, rather than simply lifting the loa. A common example of a pulley can be foun at the top of a flagpole.. Pulleys reuce the effort neee to lift an object by increasing the istance over which the effort is applie orce orce Loa Loa The law of simple machines as applie to pulleys:.. Where refers to the istance move, not the iameter of the pulley So, we can say MA pulley When one continuous cor is use, this ratio reuces to the number of strans holing the resistant in the pulley system, Simple Machines.oc Physics ~ ASC 005

5 Number of strans holing the resistance The resistance force () is sprea equally among the supporting strans. Therefore, n T, where n is the number of strans holing the resistance an T is the tension in each supporting stran. The effort force () is equal to the tension in each supporting stran, so nt n T The wheel-an-axle This simple machine is a wheel attache rigily upon an axle or rum of smaller iameter; the wheel an the axle have the same axis, so that both can turn together. The law of simple machines as applie to wheel-an-axle is r where:. r. r r resistance force r raius of resistance wheel effort force r raius of resistance wheel raius of effort force MA wheel an axle raius of resistance force r r Examples are the steering wheel of an automobile, the oorknob, the tires an the casters Simple Machines.oc Physics ~ ASC 005