Contrary to popular belief, weight and mass do not mean the same thing. One measures the matter that we consist of, i.e., the atoms we're made up of. The other measures the force of gravity exerted on that matter.
Both words are used interchangeably in everyday life, but that's because we all live on Earth, where the force of gravity is the same for everyone.
With the 'Race to Mars' well underway and the first commercial flight to the planet only two years away, we may have to get more specific with how we use the terms weight and mass.
This article will explain what they both mean, their differences, how they're measured, and more.
Simply put, mass is the amount of matter in a person or object, while weight measures the effect of the gravitational force exerted on that mass.
To dive a little deeper, mass is the measure of everything that makes up an object or person, such as its protons, electrons, and neutrons. The mass is always constant, regardless of where you go. For instance, your mass remains unchanged whether you are on Earth or the International Space Station since the very essence of what you're made of is the same.
When it comes to weight, since it's a measurement of the force of gravity exerted on a mass, it can change. Using the example above, your weight will be zero in space, also known as being weightless. This is because there is no gravity acting on your body.
How do you convert mass to weight?
Converting between mass and weight is incredibly easy. You just have to use Newton's second law, which states that the force of an object equals its mass multiplied by its acceleration. The formula for Newton's second law is as follows:
Force (F) = Mass (m) x Acceleration (a)
In the case of mass and weight, the force of acceleration is due to gravity (g). The measurement of gravity multiplied by mass results in an object's Weight (W). Therefore, we can substitute both W and g into the formula to give us:
Weight (W) = Mass (m) x Gravity (g)
Using this equation, you can determine the mass, weight, and force of gravity exerted on an object or person.
For example, if there is an unknown mass, but you know the weight of the object measured and the amount of gravity being exerted on it, you can rearrange the equation to find its mass:
Mass (m) = Weight (W) / Gravity (g)
What is mass?
To get into the nitty gritty of what mass is, we have to look at it through the lens of science.
Mass is referred to as the quantitative measure of inertia. This just means that it's a measurement of an object's willingness to remain in a constant state of motion.
For instance, think of coasting on a bike along a straight path. Without interference from wind, friction from the ground, or gravity, you will coast on that same path at the same speed forever. Mass is simply the measurement of how much force it would take to change your speed or path.
How do you measure mass?
Mass is typically measured using grams, kilograms, or pounds, but scientists stick to grams and kilograms. Scientists can determine the true mass of an object using an ordinary balance and trying to balance it equally against another known amount of mass.
What is weight?
Weight is a measurement of the force of gravity on an object or person. The weight of an object will increase or decrease alongside changes in mass and/or gravitational pull. Therefore it's never constant. Also, as the mass of an object increases, so does the force of gravity since it pulls harder on heavier things, which makes its weight even higher.
How do you measure weight?
In everyday life, weight is measured using grams, kilograms, or pounds. But, in the realm of science, weight is measured using Newtons (N). Since weight depends on gravity's force on an object, a spring balance is used to measure weight accurately.
On Earth, the force of gravity is a little over 9.8 Newtons per kilogram. So when we say someone weighs 80 kg, what we're really referring to is their mass. Their weight would be 784 Newtons (80 kg x 9.8 N).
The same calculation is made when you step on a scale. But, to make it easier for us to understand, the weight is displayed as kilograms, not Newtons.
Side-by-side comparison of mass and weight
| Mass | Weight | |
| Definition | Mass is the quantity of matter, regardless of volume or external forces acting on an object. | Weight is a measurement of the external gravitational force acting on an object. |
| Effect of gravity | The mass of an object remains the same, regardless of location and time. | The weight of an object will increase or decrease depending on the gravitational force at that location. |
| Can it be zero? | An object's mass can never be zero. | If no gravity is acting on an object, the object’s weight will be zero, as in space. |
| Unit of measurement | Mass is typically measured in grams (g) or kilograms (kg). | Weight is typically measured in Newtons (N), which is a unit of force. |
| The balance used for measurement | An ordinary balance is used to measure the mass of an object. | A spring balance is used to measure the weight of an object. |
| Type of quantity | Mass is a base and scalar quantity. It has magnitude but no direction associated with it. | Weight is a derived and vector quantity. It has magnitude and direction (direction is toward the center of the gravity well). |
How much do you weigh on other planets?
To highlight the difference between mass and weight, let's take a look at how a person's weight would change depending on what planet they were standing on.
As we've discussed earlier, an object's or person's weight varies depending on the gravitational forces acting on it. Each planet has a different gravitational force. Thus, a person's weight will change accordingly.
For instance, if we use Earth as our benchmark, there is a gravitational force of 9.8226 m/s2 on the Earth's surface. By comparison, Mars has 3.727 m/s2 of surface gravity, which is 0.3895 times smaller than Earth. This means that when standing on Mars, you will weigh 0.3895 times less than when you stand on Earth.
On the other end is the Sun, which has a surface gravity of 274m/s2. This means that you would weigh 27.90 times more on the Sun's surface compared to Earth, even though you would have the same mass.
| Body | Multiple of Earth's Gravity | Surface Gravity (m/s2) |
| Sun | 27.90 | 274.1 |
| Mercury | 0.3770 | 3.703 |
| Venus | 0.9032 | 8.872 |
| Earth | 1 | 9.8226 |
| Moon | 0.165 | 1.625 |
| Mars | 0.3895 | 3.728 |
| Jupiter | 2.640 | 25.93 |
| Saturn | 1.139 | 11.19 |
| Uranus | 0.917 | 9.01 |
| Neptune | 1.148 | 11.28 |
| Planet | 40kg | 50kg | 60kg | 70kg | 80kg | 90kg |
| Sun | 1082.8N | 1353.6N | 1624.3N | 1895N | 2165.7N | 2436.4N |
| Moon | 6.6N | 8.3N | 9.9N | 11.6N | 13.2N | 14.9N |
| Mercury | 15.1N | 18.9N | 22.6N | 26.4N | 30.2N | 34N |
| Venus | 36.2N | 45.3N | 54.4N | 63.4N | 72.5N | 81.6N |
| Mars | 15N | 18.8N | 22.6N | 26.3N | 30.1N | 33.9N |
| Jupiter | 101.1N | 126.4N | 151.6N | 176.9N | 202.2N | 227.5N |
| Saturn | 42.5N | 53.2N | 63.8N | 74.4N | 85.1N | 95.7N |
| Uranus | 35.5N | 44.4N | 53.3N | 62.2N | 71.1N | 80N |
| Neptune | 45N | 56.2N | 67.5N | 78.7N | 90N | 101.2N |
| Pluto | 2.6N | 3.3N | 4N | 4.6N | 5.3N | 6N |
