Below you will find some commonly asked questions about simple machines that you can use for a discussion with your class when you begin this topic.You may also want to explore other activities related to our school program.
Why do we use machines?
We use machines to reduce the amount of effort or work we exert, and also to increase our ability to lift or move objects.
Why do we use simple machines?
Simple machines make our work easier by enabling us to use less mechanical effort to move an object. Often several simple machines are combined in complex machines.
What are the two families of simple machines?
All simple machines belong to one of two families - the inclined plane family and the lever family.
What are the six classes of simple machines?
There are six classes (types) of simple machines - the wedge, the ramp, the screw, the lever, the wheel and axle, and the pulley.
How are the classes of simple machines grouped?
The six classes of simple machines are categorized as follows:
Inclined plane family - wedge, ramp, screw
Lever family - lever, wheel and axle, pulley
What is a ramp?
A ramp (inclined plane), helps us to move heavy objects more easily, but we have to move them further to complete the task. We use less force, however, we have to apply the force over a greater distance. Friction is one problem encountered in using a ramp to move heavy objects.
We use stairs or ramps to walk up and down. If an incline is very steep, steps are cut into the incline to make it easier for us. Ramps are usedin moving vans, for wheelchair access and on loading docks.
What is friction?
Friction is the resistance produced by rubbing two objects together. To overcome friction, the surface of a ramp should be as smooth as possible. Alternatively, rollers, wheels or lubricant can be used.
The Experiments - Friction
What is a wedge?
A wedge consists of two back-to-back inclined planes. A wedge looks like an inclined plane but it works differently. It can either hold things together, as in a doorstop or nail, or it can split things apart, as in an axe or chisel. Other wedges include the cutting edge of scissors, knives and screwdrivers.
What is a screw?
A screw is an inclined plane wound about a nail. The ridges are called the thread of a screw. These threads cut a groove in the wood as you turn the screw, making it hold very tightly. To remove a screw you have to turnin theopposite direction with a screwdriver. It is very difficult to remove a screw by pulling it straight out. The distancebetween the threads depends on the slope of the inclined plane - the steeper the slope, the wider the thread. Screws with less distance between the threads are easier to turn.
As with any inclined plane used as a simple machine, the force required is less but the distance travelled is greater.
What is a lever?
The lever is a bar that turns on a point called a fulcrum.
The secret of the lever is the increased distance over which the force moves, i.e., the arm length of the lever, which is determined by the position of the fulcrum (pivot). It is the same principle as the inclined plane - the greater the distance over which the force must be applied, the smaller the force required to do the work (lift the load).
Our arm is in fact a lever. How hard it would be to use the arm if it didn't bend at the elbow! The elbow acts as a fulcrum and the muscles between the elbow and shoulder provide the force (hence the lower arm becomes the lever). Imagine that you have a caston your arm so you can't move your elbow. Now try lifting a bag of flour. Which is easier, with or without the cast?
A shovel, wheelbarrow, hockey stick, wooden bat and tweezers are also levers, and the fulcrum is located as follows:
- shovel and hockey stick - mid-point of the handle
- wheelbarrow - at the wheel
- bat - at the base of the handle
- tweezers - at the central join
What is a wheel and axle?
A wheel and axle is a lever that is able to rotate through a complete circle (3600 ). The circle turned by the wheel is much larger than the circle turned by the axle. The increased distance over which the force is applied as the wheel turns results in a more powerful force on the axle, which moves a shorter distance. For example - the steering wheel, screwdriver, faucet handle and wrench.
What is a gear?
We use the wheel and axle in gears. Gears have teeth around the outer rim. When the teeth of two gears fit together and one gear turns, it will cause the other gear to turn, but in the opposite direction. When the gears are the same size and they have the same number of teeth, they both turn at the same speed. If one gear is larger than the other, however, the smaller gear will turn faster. We use gears to regulate speed and direction of motion in complex machines and to increase/decrease the force applied.
Machines that use gears include clocks, bicycles, cars, eggbeaters and other small household appliances.
What is a crank?
A crank is the handle of a machine that is connected at right angles to an axle. It is used to transmit motion.
On a bicycle, the pedal attached to the gears is a crank. Cranks are used for turning, for example, the spinning wheel, eggbeater, apple parer, peppermill etc.
What is a pulley?
Pulleys are wheels with grooves around the rim. The pulley turns as a string moves over the wheel and a load is raised as the string is pulled. This is a fixed pulley which doesn't change position. A fixed pulley makes work easier by changing the direction of the applied force. With a fixed pulley, the force required to lift the load remains the same as lifting it by hand, but realize how much easier it is to raise a flag or sail from the ground, as opposed to climbing up the pole or mast. If a force needs to be applied around a corner, a pulley allows us to overcome friction.
With a moveable pulley , both the load and the pulley move; the load moves in the same direction as the applied force. Moveable pulleys allow you to use less force to raise an object than if you used only your hands. The amount of force required depends on the number of supporting ropes. The greater the number of pulleys and supporting ropes, the smaller the force required. However as with all simple machines, the less force required, the greater the distance the rope must be pulled further than the load actually moves .
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