# DK Science & Technology: Machines

From can openers to cranes, machines make work easier. Simple machines include LEVERS, PULLEYS, INCLINED PLANES, and GEARS. They transfer force and movement from one place to another, often magnifying the force or the movement at the same time.

The mechanical advantage of a machine is the amount that it magnifies (increases) a force to overcome a load. Nutcrackers have a mechanical advantage of about five. The force you use to squeeze the handles is magnified five times?making it strong enough to crack even the hardest nut.

## HOW IS A MACHINE?S EFFICIENCY MEASURED?

The efficiency of a machine is the ratio of the work output to the work input. If the machine is perfect, then all the work (energy) that you put in (the effort) is used to move the load. It would be 100 percent efficient. In reality, a machine always wastes some energy as a result of friction between its parts.

## LEVERS

A lever is a rod or bar that turns on a pivot (the fulcrum). The effort applied at one place moves a load at another place via the fulcrum. There are three different types of levers, each with the effort, load, and fulcrum in different places.

### WHAT ARE THE DIFFERENT CLASSES OF LEVER?

The three classes of levers are suitable for different jobs. In a first-class lever, such as a seesaw, the effort and load are on opposite sides of the fulcrum. In a second-class lever, such as a bottle opener, the fulcrum is at one end, the effort at the other, and the load in between. In a third-class lever, such as chopsticks, the fulcrum and the load are at either end, with the effort in between.

These pliers are a pair of first-class levers. The fulcrum is between the load and the effort. The effort is magnified because the load is closer to the fulcrum.

These nutcrackers have the fulcrum at one end, the effort at the other, and the load between?ideal for cracking open a nut.

In chopsticks, the effort is between the fulcrum and the load. The effort is reduced by this lever, but the movement is magnified. With small movements of the hand, you can pick up anything from a grain of rice to a large nut?but you can?t crack the nut.

### WHAT IS A WHEEL AND AXLE?

The steering wheel of a car works like a circular lever?it magnifies a turning force. Your hands on the wheel move through a much larger distance than the axle (the steering column). The turning effort is magnified to produce enough force to steer the car wheels. A crank is a turning handle that works in a similar way, to raise a heavy bucket of water up a well, for example.

## PULLEYS

A pulley is a grooved wheel that a rope or belt runs over. A single pulley changes the direction of a force, without changing its size?for example, to raise a flag up a pole by pulling down on the rope.

### WHAT IS A BLOCK AND TACKLE?

A block and tackle is two sets of pulleys linked by a single rope. Pulling the rope draws the block and tackle together. This magnifies the effort put in so that you can lift or lower a heavy load?a car engine, for example. Block and tackle pulley systems have been used for centuries to raise sails and shift cargo on sailing ships.

A block and tackle is a pulley system designed to lift a large load with a smaller effort. The crane on the far right is equipped with a block and tackle to move heavy loads, such as steel plates, into position on a construction site.

## INCLINED PLANES

The first construction machine was probably the inclined plane. Early builders raised loads such as blocks of building stone by pushing them gradually up a slope. The more gradual the slope, the easier it is to raise the load, but the farther you have to travel to get to the top.

### WHY DOES A SLOPE MAKE IT EASIER TO RAISE A LOAD?

When you push a load up a slope, the weight of the load is shared between your effort and the slope. You do not have to lift the whole weight at once. However, although the effort is smaller, the distance moved is greater. If you climb a zigzag path up a mountain, each step is easier, but you take many more steps to reach the top than if you follow a steeper but more direct route to the top.

### WHAT IS A WEDGE?

A wedge is a movable inclined plane. As it is pushed forward, it pushes a load sideways. The sharper the wedge, the farther it has to be pushed to produce the same sideways movement, but the greater the sideways force it applies. The blade of an ax is a wedge that produces a splitting force as it cuts into hard materials, such as wood, rock, or ice.

## GEARS

Gears are toothed wheels that transfer turning motion and forces from one place to another, such as from a car engine to the car wheels. The gear teeth mesh (fit together) so that as one gear turns, it forces its neighbor to turn in the opposite direction.

### WHAT IS A GEAR RATIO?

If two gears have the same number of teeth, then they turn at the same rate and with the same force. If one gear has twice the number of teeth as the other, the gear with more teeth rotates at half the speed of the other gear, but with twice the force.

## FIND OUT MORE

 Engines Forces Motion