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Exploring Measurement - Weight and Mass

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WEIGHT AND MASS
Exploring Weight
What Causes Weight?
Making a Rubber Band Scale
Making a Spring Scale
Making a Homemade Balance  
Making Weights for Your Balance  
Using Your Homemade Balance
Weight Versus Mass
Investigating the English System of Weights
Exploring the Gram
Useful Measuring Devices You May Already Have


WEIGHT AND MASS

You have seen how length and volume are measured in both the English and metric systems of measurement.  In the next few experiments you will see how weight and mass are measured, and why the two are not the same.


Exploring Weight



Materials Needed: Set of analog bathroom scales.  (There are basically two types of bathroom scales - analog and digital.  Analog scales usually have a pointer that moves around a scale with numbers and lines, or a fixed line under which the number scale will move. As you put more pressure on the scales, the weight that is shown will change. The analog scale will look something like the illustration.   

A digital scale, on the other hand, will show weight using a display of numbers - digits - and in some cases it won’t display a weight until you stand still on it for a few seconds. (The digital scale may also have a switch that will allow you to see your weight in either pounds or kilograms.) Most newer scales are digital, so you may have to look around to find an analog scale.

Procedure: Step up on the scales.  How much do you weigh?

What To Look For: What do you feel when you step up on the scales? 

What Happened:   When you stepped up on the scales, you probably felt the scales go down slightly.  Most bathroom scales have a spring or other device underneath that is compressed when you step up on the scale.  You also saw the pointer move from zero to your weight.  The heavier the person or object on the scale is, the more the spring is depressed, and the farther the pointer or scale will move.

In the U.S., this type of scale will almost always be divided into pounds, but some analog scales may also have a second set of marks and numbers for kilograms as well.

Going Further:   Why is the spring compressed when you step up on the scale?


What Causes Weight?


Materials Needed: A small kitchen or diet scale; salt.

Procedure: Examine the scale you are using. Press down on the scale.  Do you feel something inside the scale compressing?

If you don’t know how to use this scale, have an adult show you how.  Measure out 55 grams (2 ounces) of salt. If you are careful to use only a clean bowl, you may return the salt to its container when you are done.

What To Look For: You should notice that the diet scale has a spring or possibly a metal strip inside that is compressed when you push down on the top of the scale.  Even if you can’t see it, you can feel it when you press down.

What Happened:  When you weighed the salt, as more salt was added to the bowl, the spring inside the scale was compressed further, and the pointer on the scale moved down.  The kitchen scale works on exactly the same principle as the bathroom scale in the last experiment.

With both scales, the object being weighed causes the spring to compress because gravity is pulling on the object being weighed.  In fact, when we measure weight, we are actually measuring how much force is being exerted on the object by gravity.  In order for an object to have weight, it must be “pulled on” by gravity.

We can use what we have learned so far about weight to make our own devices to measure weight.  In the two experiments that follow, you will make devices that will weigh small amounts.  What each device has in common is that it measures weight by measuring the pull of gravity on an object.


Making a Rubber Band Scale


CAUTION!  Always use sharp objects such as knives or scissors with adult supervision only!  Hold any sharp point away from your body, particularly your eyes.

Materials Needed:   Corrugated cardboard box (such as a copier paper box); several rubber bands; paper clips; weights (may be borrowed from school or made by using a kitchen scale); duct tape; string; scissors; plastic margarine or similar cup; strip of thin cardboard or paper.

Procedure: Turn the box on one end as shown.  Put about a 5 cm (2 in) square piece of duct tape on top of the box in the center and punch two holes about 3 cm (1 in) apart near the center. Tie a small loop of string through the holes and put another piece of tape over the holes and string for extra strength. Next, bend two paper clips into an “S” shape by folding the inside loop back, and hang one end of one of the paper clips onto the loop of string. Hang a rubber band on the other end of the paper clip.

Make a small bucket out of the margarine cup by punching two holes near the top edge and tying a piece of string through the holes.  Hang this bucket on one end of the second paper clip and hang the other end of this paper clip on the rubber band.

Cut a cardboard or paper strip about 5 cm (2 in) wide and as long as the box is tall.  Tape this strip in the back of the box directly behind the cup.  With the cup empty, mark the paper even with the top edge of the cup “0".  Next, begin adding weight to the cup, 100 grams (g) at a time.  (If you can’t borrow weights from school, use a diet scale.  Place a small container on the scale, and add salt or sand until you get the weight you need.)

Each time you add another 100 grams, make another mark even with the top edge of the cup.  Keep adding weights until the cup is near the bottom of the box.  If your rubber band doesn’t stretch enough, you may need to use a thinner rubber band.  If it stretches too much, either use a thicker rubber band or put two rubber bands together.

When your rubber band has stretched to the bottom of the box, remove the paper strip. Label every mark with the weight at that mark, and make three smaller equally spaced marks between each mark.  Each of these marks represents 25 g.  Replace the paper strip in the box.  Check your 100 gram weights again to make sure that you have put the strip back exactly where you had it the first time, and adjust it as needed.

Your rubber band scale is now complete.

Going Further: Depending on the size of your box and the size of the rubber bands you use, you can make scales to measure in almost any range you want.


Making a Spring Scale


CAUTION!  Always use sharp objects such as knives or scissors with adult supervision only!  Hold any sharp point away from your body, particularly your eyes.

A rubber band scale will be fairly accurate, but only for a short time. Eventually the rubber band will begin to stretch and wear out. A better scale may be made by substituting a metal spring for the rubber band.  Such a scale will be just as accurate, and because the spring is made of metal, it will last much longer.

Materials Needed: Three pieces of wood; nail; small spring from a hardware store; small eye screw; wire or string; small plastic container or food tin; weights; scissors or hobby knife; cardboard.
 
Procedure: The size of the items you use will depend on the size of your spring, but you should use the picture to help you put your scale together.  You will need to find a spring that stretches fairly easily.  If you can visit a hardware store, ask to see their small springs, and pick out one or two to use for this experiment.

Punch two holes in your container, one on each side near the top.  Then, using the wire or string, make a small handle for the container.

Next, make the scale support.  The back should be a little longer than the length of your spring when it is fully stretched out, plus the size of your plastic container and handle.  The top should be a little longer than the width of your container.  The bottom piece should be large enough to support you scale.  Nail these three pieces together.  Screw the eye screw into the underside of the top piece in the center.   Cut a strip of cardboard about 3 cm (1 in) wide and as long as the back support.  Tape or thumbtack this strip onto the back support.

Hang one end of the spring on the eye screw and hook the other end onto the container handle.  Your scale is now ready to calibrate using 100 gram weights just as you did with the rubber band scale in the last experiment.

Going Further: As with the rubber band scale, depending on the size of your spring, and how strong it is, you can make scales to measure many different weights.


Making a Homemade Balance
  

CAUTION!  Always use sharp objects such as knives or scissors with adult supervision only!  Hold any sharp point away from your body, particularly your eyes.

A homemade balance can be used in many experiments, and is surprisingly accurate for measuring small amounts.

Materials Needed:   Two small boards (about 12 in or 32 cm long and 3-4 in or 8-10 cm  wide - exact sizes aren’t all that important), three small nails; wooden ruler or dowel 12 to 18 in or 32 to 48 cm long; three binder clips; two paper clips; two paper cups.

Procedure:  Nail one of the boards to the other as shown above.  Use two nails so that the boards will not slip.  Nail the third nail about an inch (or 3 cm) or so from the top of the upright board.  This nail will be used to support the balance beam. 

Next, fasten a binder clip on each end of the dowel or ruler.  Fasten the third binder clip to the middle of the dowel or ruler on the opposite side of the other two clips. Now take the two paper clips and bend the inner loop of each one back to make an S-shaped hook.  Punch a small hole near the top of each paper cup and run one end of each paper clip hook through each cup.   Hang the dowel or ruler on the nail by the middle of the clip.  Then, hang each cup by the S-shaped hook onto the wire portion of the binder clip at each end of the dowel or ruler.
 
What To Look For: The balance arm will see-saw back and forth for a few seconds.  When it stops swinging, the balance should be level.  If it isn’t, move the ruler or dowel along the middle binder clip until both sides balance.

What Happened: When you adjusted the balance, you made the weight on each side of  the balance beam equal.  Now, if you place something in one of the cups, that side of the balance will go down.  If you put the same weight in the other cup, the balance will again be level.  Try it with a few coins on each side.  When we make weights in the next experiment, we will use this principle to actually weigh objects with our balance.

Going Further: You can improve your balance by using small eye screws from the hardware store instead of the binder clips on each end.  You can also substitute small metal or plastic cans for the paper cups.  In addition, you might also want to make a pointer for your balance by using a small straw or coffee stirrer.  You can glue or tape it to the middle binder clip at right angles to the ruler or dowel.  This pointer will make it easier for you to see small changes in the balance arm.


Making Weights for Your Balance
 


CAUTION!  Always use sharp objects such as knives or scissors with adult supervision only!  Hold any sharp point away from your body, particularly your eyes.

Materials Needed: Aluminum foil; scissors; small film canisters or plastic bottles; sand; weights (1, 5, 10, 25, and 50 grams) or a triple beam balance.

Procedure: Most people don’t have balance weights or a triple beam balance at home, so you’ll probably need to borrow weights from your teacher, or make your weights at school.

If you use a triple beam balance, ask your teacher to show you how to use it.  Once you have learned how to measure using this balance, cut a square of aluminum foil about 16 cm (6 ½ in) on each side.  The foil square should weigh a little more than a gram, depending on how thick it is.  If it doesn’t weigh at least a gram, cut a larger piece. Once you have a piece that weighs a little more than a gram, make a note of the size.  Now, carefully trim away foil until the piece weighs exactly one gram.  Fold the foil into a small square and label it 1 g.  Make four more 1 g weights the same way.


To make the five gram weight, you’ll need to cut a piece of foil about five times larger than the one gram sheet.  If you cut a piece about the same width and a little more than five times longer, it should weigh a little more than 5 grams.  Once you have the right size, again carefully weigh and trim the foil until it weighs exactly five grams.  Fold the foil as with the 1 g weights and label it 5 g.  Make at least two more 5 g weights.  Repeat this to make at least two 10 gram weights.

For the 25 and 50 gram weights, you will need to use film canisters (or other small plastic containers) and sand.  Place the canister and the lid on the balance.  Add sand to bring the weight up to 25 grams.  Put the lid on the canister and label it 25 grams.  Do the same thing to make at least one 50 gram weight.

If you use your homemade balance and weights borrowed from school, follow the same procedure as above.  Make sure that the arm of your balance is level.  Then, place the standard weight on one side of the balance, and your foil on the other. Trim the foil as instructed above.  For the larger weights, put the weight on one side and the bottle with lid on the other.  Add sand or salt until the arm balances. When it does, you have the right weight.

What To Look For: You probably noticed that it takes some patience to make the weights just right.  However, the more careful you are in making your weights, the more accurate your balance will be.

What Happened:  If you made all the weights, you will be able to weigh a small object up to 100 grams on your balance.

Going Further: Many other materials can be used to make weights.  You might try paper or plastic instead of foil.  For larger weighs, you could use modeling clay, BB’s, or small nails, screws, bolts, nuts, etc., instead of sand. Also, if you don’t have film canisters, you can use most any small plastic bottles with lids. You can also make larger weights, either by following the procedure above, or by using your homemade weights and balance.  Just be careful not to make weights heavier than your balance can support.

Youn can also go online to find out how much the coins in your country weigh.  The weights are usually accurate enough that you can use the coins as weights.


Using Your Homemade Balance


Materials Needed: Several small objects, such as coins, keys, nuts, bolts, checkers; table salt or sand; balance and weights.

Procedure:   First, make sure your empty balance is level.  To find out how much one of your small objects weighs, place it in one of the balance cups.  Then, begin placing weights in the other cup until the balance is level.  The weight of the object will be equal to the total of the weights you added. 

If it is not exactly level, (and it probably won’t be) add weights until the weight side is slightly higher than the side with the object.  When this happens, adding one more gram weight should tip the weight side just below the side with the object.  If 6 grams places the weight side just above the object side, and 7 grams places it just below, the object weighs between 6 and 7 grams.

Try weighing all the objects you have gathered to within a gram.

To weigh out a certain amount of sand, salt, or other substance, again make sure the empty balance is level. Then to weigh, say 12 grams of salt, place weights equal to 12 grams in the weight cup.  Slowly add salt to the cup on the other side until the balance is level.  You will then have 12 grams of  salt.

Try weighing out different amounts of sand or salt.

What To Look For: Most objects won’t weigh exactly a certain number of grams, and your balance is accurate only to the nearest gram.  However, when you use your balance to weigh out a certain amount of a chemical such as salt, it is possible to get much closer to the exact weight.

What Happened: When you weighed objects, you probably noticed that some, if not most, of the objects wouldn’t balance exactly.  In other words, the weight side would still be a little above or below the side with the object on it.  When that happens, the weights that most nearly make the balance level is closest to the actual weight of the object.  In other words, you can weigh to the nearest gram.

Going Further: Try making ½ gram or smaller weights. 


Weight Versus Mass


Materials Needed: Your mind.

Procedure: You have seen pictures of astronauts as they appear to float and tumble around in space. Imagine that you are one of these astronauts aboard the International Space Station.   You’re having a great time, aren’t you?

As an astronaut, you are “weightless” because gravity doesn’t appear to have any effect on you.  It actually does, but a spacecraft in orbit moves at exactly the right speed and in the right direction to just balance out the force of gravity, and so you appear to be weightless and able to “float” from one place to another.  Believe it or not, you are not actually floating.  Instead, when you are in orbit, you are constantly falling - in a circle.  This is because you are going forward fast enough (about 17,500 miles per hour) that although gravity is pulling you back to earth, you can never actually fall back to earth.  This is so because you are also moving forward at a fast enough speed so that even though you are falling, you will not reenter the atmosphere and hit the earth.   Not only are you constantly falling, but everything else around you is as well.  Because of this, you, and everything around you, appears to be weightless.

If you attempt to step up on a bathroom scale aboard the Space Station, you will not register any weight on the scale.  This is because the force of gravity is being canceled out by the falling of both you and the scale, so gravity cannot “pull” you down. 

Yet you are not any smaller.  Even though you are weightless, the amount of “stuff” you are made of is unchanged.  Living in space will cause you lose all of your weight, but it won’t make you one bit smaller!

Now suppose you are leaving the Space Station on an experimental spacecraft to go to the moon.  The gravity of the moon is only 1/6 that of the earth, so  you would only weigh 1/6 as much on your bathroom scales.  Still, how much there is of you, as well as your size, would not change.

When we measure weight here on earth, what we are really measuring is the “mass” of an object, or how much “stuff” it has.  We can use weight here on earth because everywhere you go, gravity is almost the same.  The farther away you go from the center of the earth, the less the force of gravity affects you.  You would weigh slightly less on the top of Mount Everest than you would at the beach because you are slightly farther from the center of the earth there.  However, the difference between your weight at the beach and Mount Everest doesn’t amount to all that much.  So long as you stay on earth, weight is a pretty good indicator of your mass.

That’s why, when we talk of measuring weight, we are usually actually talking about measuring mass, and we often mix units for measuring the two.  Even though the difference between weight and mass isn’t all that important as long as you are on earth, you should keep in mind that there is a difference.  Weight depends on the pull of gravity, and is a measure of the force of gravity on an object. It changes as the force of gravity changes.  Mass, on the other hand, does not change with a change in gravity.  It is an accurate measure of how much there is of an object, regardless of the how little or great the force of gravity is.


Investigating the English System of Weights

You have already seen that the English system of measurements for length and volume can be pretty confusing.  The system for measuring weights can be as well.  It may not seem quite as confusing because there are only three units for weight that we use regularly, although there are many more.

Materials Needed: Diet scale; salt or clean sand; paper; small light plastic bowl (such as a whipped topping container).

Procedure: Measure out one ounce of salt or sand.  Pour it onto a piece of paper.  How heavy does it feel? 

Next, measure out 16 ounces of salt or sand and place it into the plastic bowl.  How heavy does it feel?  If the scales won’t measure 16 ounces at one time, measure a few ounces at the time until you have measured 16 ounces.
 
What Happened: You almost certainly realize that you have just weighed out a pound of salt or sand, since there are 16 ounces in a pound.  Pounds and ounces are the basic units we use to measure smaller weights in the English system. 

For larger weights, we use tons, and there are two kinds - the short ton and the long ton.  A short ton is most often used and it equals 2,000 pounds.  The long ton is 2,240 pounds.

For weights smaller than an ounce, the English system uses units like grains, scruples and drams, although you don’t see them used much anymore. Can you find out how much each of them is equal to?

All of these units are actually units of weight.  There are also units to measure mass in the English system, but we will not bother with them just now.

If you look at a table of English measures for weight, you can see that there is no easy to remember pattern for keeping track of them.  The metric system makes measuring weight much easier, as we will see.


Exploring the Gram

The basic unit of mass in the metric system is the gram.  It is not a very large quantity, but as we will see, just like the basic unit for volume - the liter - it is based on the meter

Materials Needed: Homemade balance or school balance; film canister or pill bottle (two of the same kind if you are using the homemade balance); medicine dropper; water.

Procedure: If you have visited the Measuring Volume page, you may have learned how to measure small amounts of water by counting the number of drops in 1 milliliter.  If not, go there and take a look at the Measuring Small Amounts With a Medicine Dropper experiment.  Determine how many drops of water are in a ml and write it down. You will need that information for this experiment.

If you are using the homemade balance, place an empty film canister or pill bottle in each  cup, and adjust the scale so it is balanced.  In one container, measure out 1 milliliter of water by drops.  Next, add a 1 gram weight to the other side of the balance.  What happens?

If you are using a school balance, you may need a teacher to show you how to use it.  First, weigh an empty canister or bottle and write down the weight.  Next, add 1 milliliter of water to the canister and weigh it again.  How much does the weight increase?

What Happened: In each case, 1 ml of water should have weighed one gram.  Depending on how accurate the balance and your measurements were, you may have been off just a little, but the weight should be very close.  Remember that the milliliter is one cubic centimeter, and that the centimeter is 1/1000 of a meter.  Since the milliliter is based on the meter, and since one milliliter of water weighs one gram, you should see that the basic unit for weight - the gram - is also based on the meter.

Everything you weigh in the metric system may be weighed in grams, fractions of grams, or multiples of grams.  Very small objects are weighed in milligrams (1/1000 of a gram), while larger objects, such as people may be weighed in kilograms.  1 kilogram is equal to 1000 grams and is about 2.2 pounds.  You’ve seen the two prefixes, milli- and kilo-, before.  Using the metric system to weigh things only requires you to know the one unit, grams, and these two prefixes.  You don’t have to constantly convert from ounces to pounds, or from pounds to ounces.  Aren’t you glad?

Going Further: Since 1 ml weighs 1 g, how much would 1 liter of water weigh?  Try weighing a liter of water to see if you are correct.  Don’t forget to weigh the empty container first!


Useful Measuring Devices You May Already Have


There are many devices used around the home for measuring that may be very useful to you as you do your experiments.  Some of these may be things that you may collect for your own use, while others may be things you will need to use and return.  Here are some things you might look for.

Thermometers: Household thermometers, candy thermometer (for measuring high temperatures).

Kitchen measuring cups - Many of these are marked in milliliters as well as cups.

Measuring spoons - Again, many of these are marked in milliliters.

Kitchen scales, diet scales, bathroom scales.

Small medicine cups such as those used in hospitals or those which come with liquid over the counter medicines.  Especially look for ones that are marked in milliliters or cubic centimeters.

Rain gauge.

Barometer.

Clocks.

Canning jars (with graduations).

Stop watch. 

Rulers.

Tape measures.

Can you think of others?


If you have already checked out the Measuring Length and Measuring Volume pages, good for you!  Now check out some of the many experiments offered here by visiting the Experiments page.

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