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Mass and Weight

The terms mass and weight are often used by non-scientists to mean the same thing – that is, how heavy an object is. However, this is not correct. The big difference between mass and weight is that mass is a measure of the amount of a substance or an object, while weight is the force that object has because of gravity. Weight takes gravity into account – mass doesn’t. As a result, the mass of an object is the same anywhere in the Universe. The object’s weight, on the other hand, depends on its location. For example, an object on Earth weights about six times as much as it weighs on the Moon! That is why old videos of astronauts walking on the Moon look so peculiar – they seem to bounce along rather than walk. Imagine how high you could jump if your whole body weighed the amount of a two year old toddler! That’s how you would feel on the Moon.

Even though there is this important difference between mass and weight, on Earth, they are very nearly identical so we may use either term without being incorrect for most purposes. As you become better at science, you will want to think of certain things in terms of force and weight. For now, let’s simply focus of accurately determining the mass or weight of an object.

The basic unit of mass in the metric system is the gram. There are 1,000 grams in one kilogram (kg). You recall that the prefix kilo– is Greek for 1,000. So there are 1,000 grams in 1 kilogram. A paperclip and piece of chewing gum each have a mass of about one gram. A kilogram, on the other hand is the approximate mass of a small laptop computer or a pretty thick book. It is also very close to a the mass of a one liter-size plastic bottle of water. You will understand why this is so later when we learn more about metric volume in relation to mass.

Mass is the amount of a substance in an object. An example is the mass of a person, an apple, or a book. Can you think of five other examples of mass?

The Triple-Beam Balance

There are a number of different scientific instruments that can be used to determine mass and weight. We will discuss the spring scale and the triple-beam balance. Let’s begin with the triple-beam balance. Below is a picture of a typical triple-beam balance.

 

 

All balances work pretty much the same. In its simplest form, a balance requires only a stiff beam and a fulcrum. A fulcrum is the point on which the beam is balanced. Look at the balances below made from nothing but a meter stick and a block of wood.

Notice that the fulcrum is located near the 50 cm mark on the meter stick. As you may have guessed, this is because 50 cm is at the exact mid-point of the meter stick. When the fulcrum is placed here, half the mass of the meter stick is on the left side of the fulcrum and half the mass of the meter stick is on the right side of the fulcrum. When balanced, the meter stick (beam) is level, neither the left nor the right side is “tipped”. If however, we place a mass of some kind on one side of the balance beam (the small blue bear in the illustration above), that side tips and the balance becomes “unbalanced“. If, we next place a similar bear of the same mass (the small green bear in the illustration above) on the right side of the balance at the exact same distance from the fulcrum, the balance beam comes back into “balance” and neither side is tipped.

You have a meter stick, wooden block, and plastic bears in your LabLearner lab. You should try to reproduce the pictures in the illustration above. Work on a flat, level table. Be careful to keep the fulcrum at the 50 cm mark and to place the bears at the same distance from the fulcrum on either side of the beam. Once you have set this up and reproduced the pictures above, feel free to add more bears to one side or another and play with putting the bears at different distances from the fulcrum.

The triple-beam balance works very much the same as this simple balance beam. However, you will notice that the fulcrum is NOT in the center of the balance. Tip or rock the beam up and down to determine where the fulcrum is. This is the point where the beam pivots.

Use the three toggles below. They will show you how to 1) calibrate the triple-beam balance, 2) make mass measurements with the triple-beam balance and 3) provide several examples of triple-beam balance readings for you to examine.

 

The Triple-Beam Balance

Learn more about the triple-beam balance by clicking on one of the tabs below. Start with Tab One: Equilibrating the Triple-Beam Balance. Then move on to the next tabs in order

 

 

Equilibrating the Triple-Beam Balance

1. Equilibrate the triple beam balance before using it to obtain the mass of a sample.

 

2. Turn the balance so the scales are facing you.

3. Move all 3 poises (front, rear and center) to the “0” position and be sure the platform is clean and empty.

 

4. Turn the adjustment wheel to the rear of the balance to move the balance indicator up.

 

5. Turn the adjustment wheel toward the front of the balance to move the indicator line down.

 

6. The balance is equilibrated (calibrated) when the balance indicator and the center graduation are even.

Using the Triple-Beam Balance

1. Never pick up a triple beam balance by the beam or platform. To move a triple beam balance, pick it up by the base, using one hand on each side.

 

2. Place the triple beam balance on a flat surface before using.

3. Turn the balance so the scales are facing you.

4. Make sure the platform is clean and empty.

5. Move all 3 poises to the “0” position.

 

6. Make sure the triple beam balance is equilibrated.

 

7.  Place the object you want to measure the mass of on the platform.

 

8. Move the poises either left or right until the balance indicator lines up with the center graduation. Notice that the rear and center poise must fit into a groove or notch on the beam. This is very important! You will feel and even hear when the poise drops correctly into the beam notch.

 

9. The mass of the object is equal to the sum of the numbers indicated by the arrows on the poises. For example, if the rear poise points to the 10 g mark, the middle poise points to the 200 g mark, and the front poise to 2.3 g mark, the total mass of the object on the platform would be 212.3 g:

10 g + 200 g + 2.3 g = 212.3 g

 

10. After you have finished measuring the mass of the object, remove the object from the platform and return each of the poises to the 0 position.

Example Triple-Beam Balance Readings

Below are a number of triple-beam balance readings for you to examine.

Example One: 295.0 g

 

Example Two: 171.1 g

 

Example Three: 142.9 g

 

Example Four: 517.2 g

 

Example Five: 23.7 g

 

Example Six: 222.1 g

 

Example Seven: 74.9 g

 

Weighing Dishes

When using the triple-beam balance to weigh many types of objects (a cell phone, rock, or pencil) you can usually simply place the object directly on the platform. But what if you want to determine the mass of a powder or a liquid? This can be done by first placing a container on the platform and then putting the sample into the container.

Any container can be used to do this. It depends on the sample. A beaker may work well for liquids, for example. There is also a special container called a weighing dish. These are very light-weight containers that are good for powders and granular chemicals like salt or samples of dirt and other items that would be difficult to remove from the platform after weighing.

Regardless of the container that you use to hold your samples in, you must always remember to subtract the mass of the empty container from the mass of the container plus the sample. This will give you the mass of the sample alone. You will use containers and weighing dishes all the time in your LabLearner lab.