.Wednesday, April 30, 2008 ' 3:35 PM Y
design&technology!
What is a simple machine?
Machines are used to reduce the amount of force required to do work, such as lift a load, but the trade off is that you must apply this force over a greater distance.
Man has learned to use machines to help him do work - which in our case means to move things. Some machines, such as bicycles, are complex because they involve more than one simple machine.
What are the two basic families of simple machines?
All simple machines are derived from either the inclined plane or the lever.
The six types of simple machines are:
the inclined plane
the wedge
the screw
the lever
the wheel and axle
the pulley
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Which machines belong to the inclined plane family?
The inclined plane
The word "inclined" means "at an angle". The word "plane" means "a flat surface". An inclined plane is a slope or a ramp. It can be any slanted surface used to raise a load from a lower level to a higher level. Examples of an inclined plane include: a ramp used by a workman to push a heavy load on wheels up into a truck, ramps for wheel chairs, ramps to load luggage onto a plane, an escalator. An inclined plane was used to move huge stones to build the Egyptian Pyramids.
Notice that all these inclined planes are stationary (with the exception of the stairs moving on an escalator). Inclined planes don't move! An inclined plane helps a person to move or raise heavy objects. An inclined plane enables a load to be lifted with less force, but the distance over which it moves is greater.
In other words, when an inclined plane is used, less force is required to move the resisting weight up the slanted surface than to lift it up vertically. However the amount of work remains the same since:
Work = Force x distance.
W = Fd
The wedge - two inclined planes put back to back.
It is not really a separate type of machine, as the principle involved is identical to that of the inclined plane.
Some examples of wedges include: chisels, axes, knives, saws (small wedges), a splitting wedge, a doorstop, a plow, the hull of a boat
The screw - the circular version of the inclined plane, though it must move to work.
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Which machines belong to the lever family?
The lever - a bar that turns on a point, called a fulcrum.
The wheel and axle - a circular lever, whose fulcrum (pivot point) is an axle.
The pulley - another circular lever, but the pulley wheel rotates freely on the axle.
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Why does an inclined plane make work easier?
Principle of the Inclined Plane
W = Fd (Force x distance)
As with all simple machines, the inclined plane can be used to trade increased distance for decreased force or effort. The force is applied in a different direction to that in which movement takes place.
Can you get something for nothing?
No!
The steepness of the inclined plane is a key factor. If a person uses a longer board to make the inclined plane, he or she will need less force to move the object up the ramp. If a shorter board is used (the angle of the slope is steeper), more force is required but the distance over which it must be applied is less.
For an object resting on an inclined plane, the vertical force of gravity acting on it is split into two smaller forces; one perpendicular to the plane, and one parallel to the plane. It is only the parallel force which needs to be counteracted by pushing (and, of course, friction, though we are ignoring that for this exercise). The slope provides "mechanical advantage" as a simple machine.
Principles of the lever family
With levers, the force is applied at a different point from the load.
The closer the fulcrum to the load, the less force needed to lift the load. The force will move a greater distance, and the load will move a shorter distance.
The closer the fulcrum to the force, the greater the force needed to lift the load. The force will move a shorter distance, and the load will move a greater distance.
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