Science: An Elementary Teacher’s Guide/Simple machines

What are Machines?

Machines are tools containing one or more parts that use energy to perform an intended action. They are also used to help make work easier. A simple machine is a mechanical device that changes the direction or magnitude of a force. They can be defined as the simplest mechanisms that use mechanical advantage (also called leverage) to multiply force. The term usually refers to the six classical simple machines which were defined by Renaissance scientists. We still use all these machines in everyday use without us even knowing. The construction, design, operation and application of robots are becoming more popular at school. The robotics programs used some of these simple machines for the robots construction.

Lever
Wheel and axle
Pulley
Inclined plane
Wedge
Screw

How do machines help us?

Simple machines make work easier for us by allowing us to push or pull over increased distances.

Increase the amount of force applied to an object
increase the distance and speed an object moves
Transfer a force from one place to another
Change the direction of a force

No machine can increase the force and distance at at the same time

Mechanical Advantage

When a machine is used to make work easier it produces mechanical advantage by amplifying our force. To figure out the mechanical advantage one divides the load by the effort, this is where one's math skills kick in! Because of friction (which effects how fast a object will go) the mechanical advantage is less than the real mechanical advantage.

Load/Effort = Mechanical Advantage

Work is moving an object some distance, must use force

For example, think of how much more force you can apply to a screw with a screwdriver compared to just using your fingers.

Also, Levers, gears, and pulleys are all machines that give mechanical advantages. If a lever lets you lift a 100 pound weight by applying 20 pounds of force, the mechanical advantage is 5:1.

Friction

Is a resistance that happens because two unlike surfaces are rubbed together it produces heat. Due to inertia, the initial starting friction is greater than the friction once you get an object moving.

Starting Friction: The amount of force required to start an object sliding across a surface
Sliding Friction: The amount of force required to keep an object sliding once it has started
Rolling Friction: The amount of force required for the irregularities of two surfaces to rollover each other as opposed to sliding over each other

Sliding friction is greater than rolling friction (think of pushing a stack of boxes compared to putting them on a dolly and rolling the stack)

Friction can be reduced as well by smoothing out the surfaces. Or one can use some type of lubrication such as oil, grease, wax. and graphite, and water.

Group 1:The Inclined Plane Family

An inclined plane is a slope or slanting surface used to connect a lower level with a higher level. An inclined plane will help reduce work, but it will increase in distance.

Ex. A barrel that is to heavy to lift up to a higher platform can use a ramp (inclined plane) and be rolled up. This helped the person put less force in moving the barrel, but the distance the barrel had to be moved increased.

The Wedge Is an inclined plane that was designed to move into or under something.

Ex. The woodcutter's wedge(in form of a nail) is used to split wood.

The Screw Has a thread(ridge) which winds round and round the center in the form of a spiral.

Ex. The base of a light bulb.

Group 2: The Lever Family

Levers are classified by the position of the fulcrum, effort and resistance(load). It is common to mention the input force as the effort and the output force as the load/resistance. This allows the identification of the three classes of levers by the relative locations of the fulcrum, the resistance and the effort.

Three Classes of Levers

Class 1:

Fulcrum in the middle: the effort is applied on one side of the fulcrum and the resistance (or load) on the other side. Ex. A seesaw, a crowbar, or a pair of scissors. Mechanical advantage may be greater or less than 1.

Class 2:

Resistance (load) is in the middle and the effort is applied on one side of the resistance and the fulcrum is located on the other side. Ex. A wheelbarrow, a nutcracker, and a bottle opener or the brake pedal of a car. Mechanical advantage is always greater than 1.

Class 3: Effort is in the middle and the resistance (load) is on one side of the effort and the fulcrum is located on the other side. Ex. A pair of tweezers or the human mandible. Mechanical advantage is always less than 1.

Group 3: The screw Family

A screw is a usually circular cylindrical member with a continuous helical rib, used either as a fastener or as a force and motion modifier.You can actually consider it is a special case of an inclined plane in which something is moved from a higher position to a lower position or conversely, but in a series of circles, thereby conserving horizontal space.

One of the more known examples of screw's simple machine is invention of the screw press. It is probably invented in Greece in the 1st or 2nd century B.C , has been used since the days of the Roman Empire for pressing clothes. In the 1st century A.D, wooden screws were used in wine and olive-oil presses, and cutters (taps) for cutting internal threads were in use.

Group 4: The Wheel and Axle Family

The simple machine refers to the assembly formed by two disks, or cylinders, of different diameters mounted so they rotate together around the same axis. The thin rod which needs to be turned is called the axle and the wider object fixed to the axle, on which we apply force is called the wheel. Wheels reduce friction because there is much less surface contact with the ground compared to sliding without wheels. The forces applied to the edges of the two disks(cylinders) provide the mechanical advantage. When used as the wheel of a cart the smaller cylinder is the axle of the wheel, but when used in a windlass, winch, and other similar applications then the smaller cylinder may be separate from the axle mounted in the bearings. It cannot be used separately! This system is a version of the lever with loads applied tangentially to the perimeter of the wheel and axle, respectively, should be balanced around the hinge(fulcrum).The mechanical advantage of the wheel and axle is the ratio of the distances from the fulcrum to the applied loads, or what is the same thing the ratio of the radial dimensions of the wheel and axle.

There are a lot of advantages for the wheel and axle and without this machine we would not be able to do some of the simple task we do throughout our everyday life. For example, lifting a bucket of water from a well with a windlass, a door handle, wagon, Ferris Wheel, fan, pizza cutter and most important to our society is the gears and tire motion of vehicles. The wheel and axle help to use these different objects more easily to get into a building, have fun at a carnival or simply get from point A to point B quickly.

Group 5: The Pulley Family

It consists of a wheel on an axle or shaft that is designed to support movement and change of direction of a taut cable or belt along its circumference. Pulleys are used in a variety of ways to lift loads, apply forces, or transmit power. Pulleys transfer direction of the force and can give mechanical advantage if there is more than one force. One or more independently rotating pulleys can be used to gain mechanical advantage, especially for lifting weights.

The Block and Tackle Is a system of two or more pulleys with a rope or cable threaded between them, usually used to lift or pull heavy loads. The more rope involved, the greater the mechanical advantage. The pulleys are assembled together to form blocks and the blocks are paired so that one is fixed and one moves with the load. The rope is threaded, or rove, through the pulleys to provide mechanical advantage that amplifies the force applied to the rope.

In the figure to the right, there are two freely rotating pulleys , one is on the upper block, which remains fixed, and one is on the lower block, which moves up as the load is lifted; one end of the rope is anchored to the upper block

Group 6: The Compound Machines Family

A compound machine is a tool when two or more simple machines are combined. Most of the machines we use today are compound machines, for example an automobile engine, it contains many simple machines; such as pulleys. Another example would be an ax, the handle is a lever and the blade is a wedge. There are many household items that are classified as compound machines such as eggbeaters, screwdrivers, and the list goes on and on.

External Link:

https://www.youtube.com/watch?v=UtfVZtuyuHU Simple Machine Video

Quiz

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Science: An Elementary Teacher’s Guide
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