You can't lift a car, crack a nut, or split logs just using your bare hands, however strong you might be. But you can do all these things relatively easily with the help of tools that multiply the force you can make with your own body.
In science, any device that increases force is called a simple machine. Most of the tools we use around the house are devices of this sort, including hammers, drills, and screwdrivers. Even our own bodies have the simplest of all machines - levers - built into them in the form of our arms and legs. We generally use tools because they boost our body force.
But some simple machines help us increase our speed as well. These include wheels and gears, which enable bicycles and cars to achieve amazing speeds our bodies can never hope to match.
A heavy load is hard to push across the ground because you have to work against the force of friction. If you put the load on a cart with wheels, the only friction is a tiny amount of rubbing between each wheel and its axle (the rod passing through the center of the wheel). This makes pushing a heavy load much easier.
How a wheel works: Wheels can increase force just like levers. They can also increase speed. Racing bikes have large wheels to help boost the speed from the pedals.
Ramps help you raise heavy objects. Instead of lifting something straight up, you push it up an incline, moving it a greater distance but with less effort.
Wedges, such as axe blades, are similar. When you chop wood, it splits along the ramp of the blade.
A screw behaves like a ramp wrapped around in a spiral. The large head and spiral thread make it easier to drive a screw into the wall.
Levers are rods that turn a pushing or pulling force (effort) into a bigger force (load) with the help of a pivot (fulcrum). The longer the lever, the more it multiplies force. There are three types of levers, with the effort, load, and fulcrum arranged in different ways.
Class one lever
Pliers have much longer handles than jaws, so they multiply your pressing force when you squeeze them.
Class two lever
With the fulcrum right at the end of the nutcrackers, it produces a large squeezing force in between the arms.
Class three lever
With the effort near the fulcrum, tongs and chopsticks reduce your squeezing force but give you better control.
You can lift much heavier weights using a system of ropes and wheels called pulleys. The more wheels and ropes there are, the less force you need to lift something, but the further you have to pull the rope.
Although a pulley reduces the lifting force, making it easier to lift the weight, you have to use just as much energy as you would without it (sometimes even more).
Single pulley: With one wheel and one rope, you can lift a weight with a certain amount of force.
Compound pulley: With two wheels and two ropes, you can lift the same weight with half the force, but you have to pull the rope twice as far to lift it the same distance.
Two wheels touching work like two connected levers, and are called gears. With teeth around the edge to stop them from slipping, they turn together to give an increase of either speed or force.
Gear ratio: If a large gear turns a small one, the small one spins faster but with less force. If a small gear turns a large one, the large one turns slower but with more force.
Liquids are almost impossible to squeeze into less space, so if you fill a pipe with liquid, you can use it to transmit a force. If the pipe is wider at one end than the other, the force increases, but you have to push the liquid farther. This idea is called hydraulics - and it is used to power rams, cranes, and diggers.
How a hydraulic ram works: When you push the ram, liquid flows along the pipe and up to the lift. Because the lift pipe is wider than the ram pipe, you can raise the car with less force. However, you have to pump down farther than the car lifts up.