IB Design Technology Topic A3.3.3: Mechanical Advantage Notes

Mechanical Advantage: Trading Distance for Force

1. Mechanical advantage (MA) is the factor by which a machine multiplies the force you put in.
2. An MA greater than 1 means the machine gives out more force than you put in, and an MA less than 1 means it gives out more speed or distance instead.

What Mechanical Advantage Buys You

1. It reduces the effort needed to move or lift something.
2. It increases the output force you get for a given input.
3. It lets a person do work that their own muscles could never do unaided, which is why a bottle jack lets one hand lift a car.

The Simple Systems That Deliver It

1. A handful of simple systems are the building blocks every complex machine is assembled from.
2. Each one improves function and efficiency in the same way: it lets a smaller effort do a bigger job.

Inclined Plane

1. A ramp lets you raise a load with less force by pushing it along a longer path.
2. The gentler the slope, the higher the mechanical advantage and the further you have to push.

Wedge

1. A wedge is an inclined plane that moves, driving into a material and pushing it apart sideways.
2. Axes, knives and chisels all convert a modest push into a large splitting force.

Screw

1. A screw is an inclined plane wrapped around a cylinder, so turning it converts rotary motion into a powerful linear push.
2. The finer the thread, the greater the mechanical advantage and the more turns you need.

Levers and Pulleys

1. The other two simple systems are the lever and the pulley, and both are important enough to get a lesson of their own.
2. A3.3.7 defines the pulley and its arrangements, and A3.3.9 defines the lever and its three classes.
3. When you meet them, the mechanical advantage defined here is the measure you will use to judge them.

Simple Systems Inside Real Products

1. Real products rarely use one simple system on its own, they stack several together.
2. Learning to spot which ones are present, and what each contributes, is the analysis skill this lesson is really training.

The Trade-Off to Remember

1. Every gain in force is paid for with distance, and every gain in speed is paid for with force.
2. A designer chooses which side of that bargain the user needs, and that single decision explains most of the mechanisms in this topic.