Screws, levers and inclined planes are designed to increase the distance over which the reduced force acts so that we can push or pull with less effort.
Consists of a stiff beam that rotates around a fixed pivot point fulcrum located somewhere along the beam. Motion at one end of the beam results in motion at the other end in the opposite direction. The location of the fulcrum can magnify or reduce the force applied at one end at the expense or advantage of the distance over which the other end travels. It is often used to split, cut or raise heavy objects depending on the angle of the sides of the wedge.
Combines a wheel with a central fixed axle which ensures that both must rotate together.
6 Simple Machines: Making Work Easier
A small force applied at the edge of the wheel is converted by rotation to a more powerful force at the smaller axle. This effect can be reversed by applying a large force to the smaller axle resulting in a smaller force at the edge of the larger wheel with much greater rotational speed. The rotation of a threaded shaft can be converted into movement in either direction along the axis of rotation depending on the direction of its spiral thread.
They are commonly used with gears or as a fastening mechanism.
Simple Machines At Play
Is commonly used to raise or lower heavy objects. Given the friction on the ramp is small, a reduced force is needed to raise a heavy object vertically although it must be moved a greater distance along the ramp to achieve this advantage. The use of a single fixed pulley and attached cord allows for a change in the direction of the force applied to an object. Although a single overhead pulley provides no mechanical advantage it may be helpful, for example, in allowing a lifting force to be better achieved by redirecting the force down towards the ground to raise an object.
Later students can analyse more complex examples with the purpose of identifying the combination of elements they use in their design. Bring in some tools which are clearly designed with the purpose of increasing the force that can be applied to them bottle opener, crowbar, pliers, car jack and initiate a discussion about what each allows us to do more easily. Guide this discussion so students become aware of how each is able to magnify the force applied to it. Encourage students to contribute more examples from their own experiences using a screwdriver to get the lid off a paint tin is a good example of an experience many students will have had.
Use this to introduce the notion of how levers and other simple machines are used more generally in their lives.
Machines - Science Olympiad Student Center Wiki
Other simple machines can be introduced one by one by bringing in several examples of each and looking for common features. Steering wheels, screwdriver handles and windlasses are all examples of a wheel and axle; axes, log splitters, tacks and nails are all examples of wedges. The internet has many sites that provide multiple examples of different simple machines.
See the links provided at the end of this focus idea. Have students experience trying to screw the same screw into the same piece of wood with screwdrivers with handles of different diameters.
Many hardware stores sell inexpensive sets of screwdrivers that have a range of handle sizes. Consider removing the plastic handle from a screwdriver and encourage students to experience the difficulty of trying to turn a screw with the shaft alone. A downward force exerted on one end of the lever can be transferred and increased in an upward direction at the other end, allowing a small force to lift a heavy weight. All early people used the lever in some form, for example, for moving heavy stones or as digging sticks for land cultivation.
The principle of the lever was used in the swape, or shadoof , a long lever pivoted near one end with a platform or water container hanging from the short arm and counterweights attached to the long arm. A man could lift several times his own weight by pulling down on the long arm.
This device is said to have been used in Egypt and India for raising water and lifting soldiers over battlements as early as bce. A wedge is an object that tapers to a thin edge. Pushing the wedge in one direction creates a force in a sideways direction. It is usually made of metal or wood and is used for splitting, lifting, or tightening, as in securing a hammer head onto its handle. The wedge was used in prehistoric times to split logs and rocks; an ax is also a wedge, as are the teeth on a saw.
In terms of its mechanical function, the screw may be thought of as a wedge wrapped around a cylinder. A wheel and axle is made up of a circular frame the wheel that revolves on a shaft or rod the axle. In its earliest form it was probably used for raising weights or water buckets from wells. Its principle of operation is best explained by way of a device with a large gear and a small gear attached to the same shaft.
The tendency of a force, F , applied at the radius R on the large gear to turn the shaft is sufficient to overcome the larger force W at the radius r on the small gear. The force amplification, or mechanical advantage , is equal to the ratio of the two forces W : F and also equal to the ratio of the radii of the two gears R : r.
If the large and small gears are replaced with large- and small-diameter drums that are wrapped with ropes, the wheel and axle becomes capable of raising weights. The weight being lifted is attached to the rope on the small drum, and the operator pulls the rope on the large drum.
In this arrangement the mechanical advantage is the radius of the large drum divided by the radius of the small drum. An increase in the mechanical advantage can be obtained by using a small drum with two radii, r 1 and r 2 , and a pulley block. When a force is applied to the large drum, the rope on the small drum winds onto D and off of d.