Inclined planes, wedges, and screws

 An inclined plane is the simplest of the simple machines because there are no moving parts.  A plane is a flat surface, and if it is inclined, one end of it is raised.  Examples include ramps, sloped roads or driveways, and the up or down sections of a roller coaster.

 The inclined plane produces a mechanical advantage by increasing the distance a force must move.  The box below is moved 15 feet along the slope in order to increase the vertical distance (height) by 3 feet.

 The mechanical advantage of an inclined plane is equal to the length of the slope divided by the height of the inclined plane.  MA = Slope/Height

 As the angle of incline increases, the force required to move the blocks at constant speed increases too.

 Imagine that you need to move a block to the top of a table.  Block C illustrates that you must exert the greatest force to lift the block to the table top when the angle in incline is the greatest.  For block B, the distance through which you exert the force is the shortest.  For block A, you would exert less force in the first example, but would need to exert that force over a greater distance. AB C

 Examples of wedges are everywhere in your life: knives, nails, fork tines, flat head screwdrivers, ax heads, your front teeth (incisors)

Now imagine a wedge (perhaps an axe) on the top of the block of wood. You hit the top of the wedge with a hammer. Imagine holding a block of wood and trying to pull it apart into two pieces. That would be difficult…

 A wedge is two inclined planes placed back-to-back.  A wedge allows you to push through and cut apart substances with less force than you would need to push through or pull the substance apart without the wedge. Inclined Plane #1 Inclined Plane #2

 The force of the hammer is in a downward direction. The wedge changes the direction of the force outward, pushing sections of the block apart.

 A knife cutting butter functions the same way. You push downward on top of the putter with a knife.  The butter is not crushed under the edge of the knife, it is pushed apart into two pieces as the knife moves through it.

 The mechanical advantage of a wedge is determined by dividing the length of the slope (S) by the thickness (T) of the widest end.

 In the example below, the length of the slope of the wedge is 6 inches and the thickness is 2 inches.  The mechanical advantage is equal to 6/2, or 3.

 A screw is a combination of simple machines: it’s an inclined plane that wraps around a shaft with a wedge at the end of the shaft.  Some screws are used to lower and raise things. They are also used to hold objects together.

 Jar Lids  Light Bulbs  Stools  Clamps  Jacks  Wrenches  Key Rings  Spiral Staircase

 The total mechanical advantage of a screw is equal to the circumference of the screwdriver handle divided by the pitch of the screw. (The pitch is the distance from one thread to the next.)

 In this example, a screw with 12 threads per inch is turned by a screwdriver having a handle with a diameter of 1.5 inches.  MA = Handle circumference/Pitch  Handle circumference = 1.5 x Pi = 1.5 x 3.14 = 4.71”  Pitch = 1/12 of an inch = 0.083”  MA = 4.71”/0.083” = 56.7

 The screw with more space between thread (higher pitch) will take more force and less turns to be turned into the wood.  The screw with less space between threads (lower pitch) will take less force and more turns to be turned into the wood.