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Seeing how simple machines work together in complex inventions.
Have you ever wondered how a pair of scissors can cut through tough cardboard so easily, or how a bicycle lets you travel faster than you can run? The secret isn't just one machine—it's a 'super-team' of machines working together!
A compound machine is a device made up of two or more simple machines working together. While a simple machine—like a lever or a wedge—does one basic job, a compound machine combines their strengths to handle much more complex tasks. Think of it like a sports team: one player is good, but a whole team can win the championship! Most of the tools we use every day, from a manual can opener to a giant crane, are compound machines. Because they combine multiple parts, they can change the direction of force and the amount of force at the same time.
Quick Check
If you combine a lever and a wedge to make a tool, what is that tool called?
Answer
A compound machine.
A pair of scissors is a classic compound machine made of two simple machines: 1. Levers: The two handles and blades act as levers that cross at a pivot point called a fulcrum (the screw in the middle). 2. Wedges: The sharp edges of the blades are wedges that push through the material you are cutting.
A bicycle is one of the most efficient compound machines ever invented! It uses a variety of simple machines to turn your leg power into fast motion. When you push down on the pedals, you are using a lever. That lever turns a wheel and axle (the front sprocket). A chain then acts as a pulley system to transfer that energy to the back wheel. Even the small bolts holding the frame together are screws, and the hand brakes are another set of levers. By working together, these parts allow you to travel much further than you could by just walking with the same amount of energy.
Quick Check
Which simple machine in a bicycle transfers the force from the pedals to the back wheel?
Answer
The pulley (the chain and gears).
Why do we bother combining machines? It’s all about Mechanical Advantage (). This is a measure of how much a machine multiplies the force you put into it. In a compound machine, the total mechanical advantage is the product (multiplication) of the mechanical advantages of all the simple machines involved.
Let's look at how a wall-mounted pencil sharpener works: 1. Wheel and Axle: You turn the handle (the wheel) which rotates the internal gears (the axle). 2. Gears (Pulleys): The gears change the direction of the force to spin the cutting blades. 3. Wedges: The metal blades inside are wedges that shave the wood off the pencil.
A fishing rod is a complex system. 1. The rod itself is a lever (Class 3) that increases the distance the tip moves. 2. The reel is a wheel and axle used to pull the line in. 3. If the line passes through a small loop at the top, that loop acts like a fixed pulley to change the direction of the line's force.
What happens to the mechanical advantage when you combine two simple machines?
Which of these is a compound machine found in a typical kitchen?
A bicycle uses a pulley system in the form of a chain and gears.
Review Tomorrow
In 24 hours, try to list the three simple machines found in a pair of scissors and a bicycle without looking at this guide.
Practice Activity
Go on a 'Machine Hunt' around your house. Find one object and try to identify at least two simple machines hidden inside it!