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More About Mass and Weight

We have already mentioned that the difference between mass and weight is that mass is the amount of matter in an object while weight involves the force of gravity acting on an object. As a result, you measure mass in kilograms, which is a measure of the amount of matter. Weight, on the other hand, is a force (as is gravity) and is measured in units called a Newtons. We will talk about Newtons again when we learn about the spring scale.

Mass and weight are related. Weight is equal to the mass of an object multiplied times the force of gravity at a particular location:

weight = mass · gravity

The reason the mass of an object is the same anywhere in the Universe while the weight of the same object depends on its location, is that the force of gravity is not the same everywhere in the Universe. If you compare different planets or the Moon, for example, you find that the larger, more massive the body (planet or Moon in this case), the greater the force of gravity will be near its surface. So, the larger the heavenly body, the more an object will weigh there, even though the object’s mass is exactly the same as anywhere else in the Universe.

Below is an illustration of what a person would weigh on the various planets in our solar system, as well as our own Moon. Let’s suppose the person has a mass of 45 kg (about 100 pounds) on Earth. It shows what that individual would weigh at difference places in our solar system.

When looking at this illustration, keep in mind that when you consider the force of gravity (and therefore an object’s weight), the circumference or size of the planet is not what is important, but rather its mass. Thus, while a planet like Saturn is much bigger than the Earth, it is composed largely of gas and its mass is not as much greater than the mass of the rocky Earth as you might guess. Consequently, a person would only weigh slightly more on Saturn (47.8 kg) than on Earth (45 kg).

 

Grams and Newtons on Mars

The triple-beam balance works by offsetting the mass of a sample placed on the platform with the mass and position of the poises. Therefore, the same Martian gravitational force that pushes down on the sample in the Mars photo below also pushes down on the poises.

As a result, the triple-beam balance will give us exactly the same mass reading for a  sample regardless if we perform the measurement on Earth, Mars, or anywhere else in the Universe. The same gravitational force pushes down on both the sample and poise side of the balance, no matter where the balance is located.

However, the situation is different for scales like a spring scale. There are no “balancing” poises on a scale. Instead, a spring scale measures the force gravity exerts on the mass of an object. That is why, when you calculate weight, you use an equation that takes into account both the mass of the object and the gravitational force acting on it.

Weight = mass · gravitational force

Since the force of gravity is not the same everywhere in the Universe, neither will the weight of any object be the same everywhere in the Universe either. Because we multiply the mass of an object times the gravitational force acting on it, this means that weight, unlike mass, is a force.

The Spring Scale

If you have ever been fishing, you may well have used a spring scale to measure the weight of a fish you caught. Below is a picture of the spring scale that comes with your LabLearner lab. Notice that there are four parts labeled in the illustration below. The handle provides a convenient way to hang or hold the spring scale. The adjustment nut is used to calibrate the spring scale before use. The indicator platform is pulled down by an object you are weighing and is read against a graduated (numbered) scale. Finally, at the bottom of the spring scale is a hook from which a sample can be attached for weighing.

In many cases, in your LabLearner lab, you will use the spring scale in combination with a plastic bucket that has a small hole in its handle. The small hole is used to insert the spring scale hook. When the bucket is used like this, you can place a sample into the bucket and read its weight directly on the spring scale. However, to use the bucket, you will need to calibrate your spring scale with it attached because the bucket, of course, has its own weight.

 

Take a close look at the calibrations on your spring scale. You will notice that there are different graduations on each side of the tube. One side is calibrated in Newtons (N, pictured on the left below). Notice that weight can be read up to 5 N in 0.1 N increments. The other side of the spring scale is calibrated in grams (g, pictured on the right below). Notice that mass (grams) can be read up to 500 g in 10 g increments on this spring scale.

You may notice in the pictures above that the indicator platform is not lined up with the zero graduation. Therefore, it is NOT calibrated. Before using the spring scale it must be calibrated. The three sections below will show you how to 1) calibrate the spring scale, 2) use the spring scale and bucket to measure weight and 3) give you several example spring scale readings.

LabLearner Tabs: The Spring Scale

Before using the spring scale it must be calibrated. The three sections below will show you how to 1) calibrate the digital scale, 2) use the digital scale to measure mass and 3) give you several example digital scale readings. Begin by clicking Tab One: Calibrating the Spring Scale then move to the next two tabs in order.

 

 

Calibration of the Spring Scale

Before measuring a weight, check that the spring scale is calibrated. A calibrated spring scale has an indicator platform that is positioned at the “0” graduation. Complete the instructions below for the calibration of your spring scale.

1. The spring scale is used to measure force. In the metric system, force is measured in units of Newtons.

2. When using the spring scale, hold it by the handle with the “0” graduation of the Newton (N) scale directly at eye level.

3. If the indicator platform is not lined up with the 0 N graduation, calibrate the spring scale by turning the adjustment nut at the top. Turning the nut clockwise raises the platform. Turning the nut counter clockwise lowers the platform.

In the pictures below, the indicator platform on the spring scale on the left is too high (it is above the zero line). The indicator platform on the right is too low (it is below the zero line). The Spring scale in the middle is calibrated properly.

Using the Spring Scale

1. First, make sure your spring scale is properly calibrated and set to zero (0 N).

2. To use the spring scale to measure force, place a load (sample) onto the hook of the spring scale. Your load in this example will be the white bucket itself. Measure the force exerted on the load using the Newton side of the spring scale.

3. Remember that each small line, or graduation, on the scale represents 0.1 N.

4. Also remember that the numbers on the spring scale increase as they go down the scale.

5. After adding the bucket to the spring scale, carefully examine the position of the indicator platform.

6. Read the spring scale in Newtons to the nearest 0.1 N.

Example Spring Scale Readings

Weight is a force measured in Newtons. Let’s look at several weight measurements:

Example One: 1.9 N

 

Example Two: 1.2 N

 

Example Three: 3.3 N

 

 

Example Four: 2.6 N

 

Example Five: 1.9 N

 

While mass can be more accurately measured with a triple-beam balance, there are two examples of spring scale readings for mass (in grams) below:

Example Six: 125 g

 

Example Seven: 420 g