Simple Machines
Six simple machines
1. Inclined plane
A flat slanted surface, connecting a lower level to a higher level. It reduces the force needed to raise an object by trading force for distance: mechanical advantage = ramp length ÷ height. Ramps and staircases are examples of inclined planes.
2. Wedge
Two inclined planes joined together to create a sharp edge. It is typically used to split or separate objects. Knives and chisels are examples of wedges.
3. Screw
An inclined plane wrapped around a cylinder. Screws convert rotational motion into linear motion, which can be used to fasten objects together. Bolts and jar lids are examples of screws.
4. Lever
A rigid beam resting on a pivot point, called the fulcrum.
Levers balance effort and load based on their distance from the fulcrum: effort × effort distance = load × load distance.
Example: Balancing a lever
A 120 lb load sits 2 ft from the fulcrum. How much effort is needed at a point 6 ft from the fulcrum to balance it?
effort × effort distance = load × load distance
effort × 6 = 120 × 2
effort = 240 ÷ 6 = 40 lbAnswer: 40 lb
There are three classes of levers. To identify a lever’s class, look at what is in the middle.
Class 1: Fulcrum is in the middle (classic lever)
Examples: scissors, seesaw, pry-bar (crowbar)
Class 2: Load is in the middle
Examples: wheelbarrow, bottle opener, nutcracker
Class 3: Effort is in the middle
Examples: shovel, broom, human arm
Because the effort is applied closer to the fulcrum than the load, a Class 3 lever always needs more effort than the load it moves (a mechanical advantage less than 1). In return, the load end moves farther and faster than your hand.
5. Wheel & axle
A wheel attached to a rod known as the axle. Turning the wheel applies torque to the axle. Because the wheel’s radius is larger than the axle’s, a small force on the rim produces the same turning effect (torque) as a much larger force at the axle - the force is multiplied by wheel radius ÷ axle radius. For example, a 10-inch wheel on a 2-inch axle multiplies the force by 5. Steering wheels and doorknobs are examples of a wheel and axle machine.
Gears
Gears are a specialized type of wheel-and-axle machine. They use interlocking teeth to change the speed, direction, or torque of a machine: the ratio of teeth between two meshed gears sets the tradeoff, so a large gear driving a smaller gear increases speed and reduces torque, while a small gear driving a larger gear increases torque and reduces speed.
Example: Gear speed
A 12-tooth gear turning at 90 rpm drives an externally meshed 36-tooth gear. How fast does the larger gear turn, and which way?
driver teeth × driver rpm = driven teeth × driven rpm
12 × 90 = 36 × driven rpm
driven rpm = 1,080 ÷ 36 = 30 rpmExternally meshed gears turn in opposite directions, so the larger gear turns the opposite way from the smaller one.
Answer: 30 rpm, in the opposite direction
There are four common types of gears:
-
Spur gear: straight teeth on parallel shafts, the most common type. Each gear that meshes externally with another reverses the other’s direction of rotation, so in a chain of such gears, every other gear turns the same way: the first, third and fifth turn one way, and the second and fourth turn the other. A chain of meshed gears like this, passing rotation from one shaft to the next, is called a gear train. An internal gear, with its teeth on the inside of a ring, is the exception to the reversing rule: it turns the same way as the gear meshing inside it.
-
Bevel gear: teeth cut at an angle, used to transfer rotation between shafts that meet at an angle. An example is the inside of a hand drill.
-
Rack and pinion: a round gear (the pinion) meshes with a flat, straight gear (the rack), converting rotational motion into linear motion. Used in steering systems.
-
Worm and wheel: a screw-shaped gear (the worm) turns a toothed wheel. It allows a large speed reduction, and the wheel cannot turn the worm backward, which makes it useful for locking mechanisms.
6. Pulley
A wheel with a rope around it. Pulleys can change the direction of applied force. They can also be combined to reduce the amount of force needed to drag or lift an object.