2009-01-16 / Columnists

Drawing On Science For Kids

Machines

Suppose your mom told you to move your old toys to another room. You place them all in a large box. You try to pick up the box, but find that it's too heavy; your muscles are not strong enough to lift it. So how can you move the box? Perhaps a machine or two can do the work for you.

But what exactly is a machine? Before we answer the question let's take a look at force and work.

Force is what we use when we push or pull a body. If the body is at rest it moves. If the body is moving it either stops (comes to rest), changes direction, or increases/decreases speed. Force is measured in pounds, ounces, or grams; these are units of weight.

When we use force on a body and move it we are doing work. Work is also done if we cause the body to overcome

resistance. When you lift a book, or anything, up off the floor you're doing work by overcoming the resistance of gravity. If you hold the item over your head you're not doing work, but your arm sure can get tired from holding the book.

The amount of work (W) done can be measured by multiplying force (F) used times the distance (d) through which it acts: W = F x d. Work is measured in units called joules. Work is made easier by using a machine. A

machine multiplies the amount of force, changes the direction of the force, or increases the speed with which the work is done. There are two major types of machines: simple and complex.

A simple machine may be a lever, pulley, wheel and axle, inclined plane, screw, or a wedge. Just about all of these simple machines can be found in, for example, a complex machine such as an automobile. The force you apply to a machine is called the effort. The weight you are trying to lift or obstacle you're trying to overcome by using the machine is called the resistance. By combining many simple machines we can increase the amount of force overcoming resistance with the least amount of effort. This is known as mechanical advantage. For example you use a wedge and lever to lift a 150 pound cement bag by applying only 10 pounds of effort. The mechanical advantage would be 15 (150 pounds ÷ 10 pounds).

Now let's look at the simple machines. Lever: a rigid bar freely rotating on a point called a fulcrum. The distance from the effort (E) to the fulcrum (F) is the effort arm (EA) and the distance from the resistance (R) to the fulcrum is called the resistance arm (RA) There are three types of levers depending on the location of the fulcrum: a first class lever has F between E and R such as a see-saw or pair of scissors; a second class lever has R between E and F such as a wheelbarrow or nutcracker; a third class lever has E between F and R such as tweezers and your forearm. Pulley: sort of a lever made of one or more grooved wheels mounted in a frame or block. A cord runs over the wheel(s).

Wheel and Axle: a large wheel to which a smaller wheel or axle is fixed such as a doorknob or car's steering wheel. Inclined Plane: a flat surface with one end higher than the other as in a ramp. Screw: an inclined plane wrapped around a cylinder. Wedge: short, movable inclined plane used to overcome resistance such as in splitting wood. A chisel, knife, nail, and axe are different forms of the wedge used for cutting or piercing.

OK, based on what you've just read, how will you get your toys from one room to another? When you've figured it out try this:

Place enough sand in a box to bring the total weight to one (1) pound.

Set a four (4) foot board with one end resting on the edge of a table. And the other end on a pile of books about a foot in height.

Attach a spring balance to the box with a string and slowly pull the box up the incline recording the reading on the balance. Do this two more times and take the average reading of the spring balance.

Set the box on a toy truck and repeat steps 1 - 3. Summarize your results.

Questions/comments? E-mail Steve: Drawingonscience@aol.com

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