In science, the word work has a different meaning than you may be familiar with. The scientific definition of work is: using a force to move an object a distance when both the force and the motion of the object are in the same direction.
Types of Machines
There are two kinds of mechanical machines: Simple and compound. As you might guess, a simple machine is a single mechanical device. A compound machine uses more than one simple machine in it.
Machines usually multiply force. The amount by which a machine can multiply a force is called the machine's mechanical advantage. Whether a machine is useful depends on whether it gives us more mechanical ‘strength’ (mechanical advantage, or M.A.) than doing the work ourselves. Can we raise a heavier load up in the air better with a machine? If so, then it is useful and has an advantage of some value times more than if we did it ourselves.
Mechanical advantage is the ratio of force output to force input. Since both forces are in Newtons, the units cancel (unit less).
The work a machine does is called work output. The energy or work put in the machine is calledwork input. The work output of a machine can never exceed the work input.
Efficiency is the percentage of input work that is converted to usable output of work.
Efficiency = work output/work input
Machines help us by taking the input work (energy/forces) we provide and get some more force or energy out than we supplied. Uh, doesn’t that violate the law of conservation of energy? No, because the extra force comes at a price. Remember that work is a function of force AND distance. If we get more force out, it comes at a sacrifice of some distance during which the force will be applied. In actual fact, the work in and the work out of the machine are equal. But the forces in and out, and distances applied in and out, will not be. One increases, the other decreases to compensate. We have to decide if we want more force out or greater distance to apply a smaller force.
Levers work by having an input force applied at some distance (called the rigid arm) from a pivoting point, called the fulcrum, and some force comes out at another distance from the fulcrum (a second, rigid arm value).
Pulleys are like levers but the forces in and out are changed by rotation around the center of a pulley wheel, which acts like a fulcrum. For pulleys, the mechanical advantage is calculated by asking how many of the ropes on all sides of all pulleys in the system are pulling up against gravity.
Wheel and Axle
A wheel and axle is a pulley connected to shaft. The wheel just rotates.
The input force can be on the outside rim of the wheel, while the output can be the shaft itself or a rope attached to it. You get more force out this way.
It is like a bicycle pedal. You turn with plain old foot power around the outside of the wheel, a long lever arm connected to the shaft. The small shaft turns a smaller circumferential gear. This gear has more power to turn the chain and the big wheel than if you were to pedal power the rear wheel all by yourself. It trades rigid arm length for increased power. The mechanical advantage can be calculated by the ratio of the outer wheel rim where force is applied divided by the smaller shaft diameter where force comes out.
The Inclined Plane
An incline plane redirects a horizontal pushing force into a vertical force that raises a mass upwards. The longer the base of the incline compared to the vertical height the mass is being raised, the more mechanically advantageous. You still need the same amount of work it takes to raise the mass upwards (a box, you, a wheelchair) but you do it with less force over a longer horizontal distance than just back-breaking lifting!
A wedge is TWO inclined planes back to back. Instead of pushing an object up the incline, you push the inclines into an object!
Nails, knives, razors, axes, hatchets, shovels, the points of a fork, even a shark's tooth -- are all wedges. Like simple planes, the advantage increases with ‘sharpness’. (You knew that; it is why you make sure your knives are sharpened when you cut the turkey.)
Screws are inclined planes wrapped around a cylinder. They also act like a wedge, albeit with just one edge to separate things. The more gentle the curving, the less force needed, but it is needed over a longer distance. If you have tighter curving, you need more force and move less distance as the screw turns.