Q: Aren't “weight” and “mass” the same?
A: Not really.
|An object has mass (say 100 kg). This makes it heavy enough to show a weight of “100 kg”.|
But the scales are only showing a guess of the mass above them!
Gravity causes Weight
An object's weight is how hard gravity is pulling on it.
We think the weight is the same everywhere … because we all live on the surface of the planet Earth!
But in orbit it would not push on the scales at all.
The scales would show … … but the mass is still 100 kg !
An object's mass doesn't change (unless you remove some!), but its weight can change.
On the Moon the scales would wrongly show 16.6for a mass of 100 kg
Because the pull of gravity on the Moonis much less than on Earth
People often use “weight” to mean “mass”, and vice versa, because Gravity is almost the same everywhere on Earth and we don't notice a difference.
But remember .. they do not mean the same thing, and they can have different measurements.
Weight is a Force
So … if weight and mass are different, why are they both in kilograms?
Well, weight should not really be in kilograms!
I have used “kilogram” so far because that is what you see on a pair of scales, but it is technically wrong to talk about weight in kilograms …
… weight is a force …
… which is measured in Newtons
The correct unit for force is the Newton (=1 kg·m/s2) which is abbreviated N.
|On the Earth's surface gravity makes a 1 kilogram mass exert about 9.8 Newtons of force|
So a 100 kg mass really weighs about 980 Newtons on Earth.
Why Do Scales Show Kilograms?
Scales show Kilograms because that is what people understand best …
… but it is really just an estimate of the mass above them.
Scales should really show Newtons, but that might confuse people!
Question: how many Newtons should the scales show when you stand on them (hint: multiply kg by 9.8)?
- So the scales show an estimate of your mass
What is the difference between mass and weight?
Shown below are two types of scales commonly used in the classroom –a spring scale (left) and a simple balance beam scale on the right.
On earth the spring scale reads 100g with an unknown mass attached at the bottom. To balance the scale on the right a 100g mass was also needed.
If we were to take both scales to the moon, what would the the spring scale read? How much mass would be needed to balance the 100g mass on the balance beam? Can you explain your answer? See if you are right by completing the questions below.
|spring scale||simple balance scale|
On the moon the spring scale on the left would read:
If you need a calculator one is available. Click on calculator in right frame.
Understand the Difference Between Weight and Mass
The terms “mass” and “weight” are used interchangeably in ordinary conversation, but the two words don't mean the same thing. The difference between mass and weight is that mass is the amount of matter in a material, while weight is a measure of how the force of gravity acts upon that mass.
- Mass is the measure of the amount of matter in a body. Mass is denoted using m or M.
- Weight is the measure of the amount of force acting on a mass due to the acceleration due to gravity. Weight usually is denoted by W. Weight is mass multiplied by the acceleration of gravity (g).
W=m∗gW = m * gW=m∗gComparing Mass and Weight
For the most part, when comparing mass and weight on Earth—without moving!—the values for mass and weight are the same. If you change your location with respect to gravity, mass will remain unchanged, but weight will not. For example, your body's mass is a set value, but your weight is different on the Moon compared with on Earth.
|Mass is a property of matter. The mass of an object is the same everywhere.||Weight depends on the effect of gravity. Weight increases or decreases with higher or lower gravity.|
|Mass can never be zero.||Weight can be zero if no gravity acts upon an object, as in space.|
|Mass does not change according to location.||Weight varies according to location.|
|Mass is a scalar quantity. It has magnitude.||Weight is a vector quantity. It has magnitude and is directed toward the center of the Earth or other gravity well.|
|Mass may be measured using an ordinary balance.||Weight is measured using a spring balance.|
|Mass usually is measured in grams and kilograms.||Weight often is measured in newtons, a unit of force.|
While a person's mass doesn't change elsewhere in the solar system, the acceleration due to gravity and weight varies dramatically. The calculation of gravity on other bodies, as on Earth, depends not just on mass but also on how far the “surface” is from the center of gravity.
On Earth, for example, your weight is slightly lower on a mountain top than at sea level. The effect becomes even more dramatic for large bodies, such as Jupiter.
While the gravity exerted by Jupiter due to its mass is 316 times greater than that of Earth, you wouldn't weigh 316 times more because its “surface” (or the cloud level we call the surface) is so far out from the center.
Other celestial bodies have different values of gravity than Earth does. To get your weight, simply multiply by the appropriate number. For example, a 150-pound person would weigh 396 pounds on Jupiter, or 2.64 times their weight on Earth.
|Body||Multiple of Earth Gravity||Surface Gravity (m/s2)|
You may be surprised by your weight on other planets. It makes sense that a person would weigh about the same on Venus, because that planet is about the same size and mass as Earth. However, it may seem odd that you'd actually weigh less on the gas giant Uranus. Your weight would be only slightly higher on Saturn or Neptune.
Although Mercury is much smaller than Mars, your weight would be about the same. The Sun is much more massive than any other body, yet you'd “only” weigh about 28 times more.
Of course, you'd die on the Sun from the massive heat and other radiation, but even if it were cold, the intense gravity on a planet that size would be deadly.
- Galili, Igal. “Weight versus Gravitational Force: Historical and Educational Perspectives.” International Journal of Science Education, vol. 23, no. 10, 2001, pp. 1073-1093.
- Gat, Uri. “The Weight of Mass and the Mess of Weight.” Standardization of Technical Terminology: Principles and Practice, edited by Richard Alan Strehlow, vol. 2, ASTM, 1988, pp. 45-48.
- Hodgman, Charles D., editor. Handbook of Chemistry and Physics. 44th ed., Chemical Rubber Co, 1961, pp. 3480-3485.
- Knight, Randall Dewey. Physics for Scientists and Engineers: a Strategic Approach. Pearson, 2004, pp 100-101.
- Morrison, Richard C. “Weight and Gravity—The Need for Consistent Definitions.” The Physics Teacher, vol. 37, no. 1, 1999.
Mass versus weight
The chains on the swing hold all the child's weight. If someone were to stand behind her at the bottom of the swing and try to stop her, they would be acting against her inertia, which comes from mass, not weight.
In the physical sciences, mass and weight are different. The mass of an object is a measure of the amount of matter in the object. Weight is a measure of the force on the object caused by a gravitational field. In other words, weight is how hard gravity pulls on an object. This means the mass of an object will remain at wherever it is on the earth's surface, but if it is moved from the equator to the North Pole, its weight will grow by 0.5% because of the increase in the earth's gravitational field.
Mass is measured in kilograms or pounds. A one-liter volume of water has a mass of one kilogram. Weight is measured in newtons, the standard unit for force. A one-kilogram mass placed on a bench presses down on the bench with almost ten newtons of force.
The weight of something is the value measured at the Earth's surface. Unfortunately the common terms used to describe the weight of an object are units of mass such as kilograms or pounds.
For almost all of human history weight has been measured on the surface of the Earth. Here, the weight is proportional to the mass. Objects which have the same mass have the same weight. An object with the twice the mass of another will also have twice the weight.
As a consequence it is common practice to use the two words, mass and weight, as if they mean the same thing, and to use kilograms or pounds as the units for both mass and weight.
Using the same terms to describe and measure the two different properties has led to confusion between these two properties, mass and weight. Mass and weight are not the same thing.
Generally, mass refers to how “heavy” something is. However, mass is really an inertial property; that is, the tendency of an object to remain at constant velocity unless acted upon by an outside force.
According to Newton's second law of motion, as expressed in the formula an object with a mass, m, of one kilogram will accelerate, a, at one meter per second per second (about one-tenth the acceleration due to Earth's gravity) when acted upon by a force, F, of one newton.
Feeling the difference
It is possible to feel the difference between weight and mass by suspending an object with a mass of over ten kilograms on a rope with a length of two or more meters.
If a person pushes upwards against the underneath of the object with one finger they will feel the weight of the object. If they push momentarily against the side of the suspended object with one finger they will feel the mass of the object.
(It is important that they do not push the object sideways far enough to begin to raise it.)
Are Mass and Weight the Same Thing?
In everyday life we use words like “variable,” “line,” “theory,” and “force” (to name just a few) rather loosely, but in math and science, these words have very specific and well-defined meanings.
In truth, the differences between the precise mathematical meanings and the everyday colloqiual uses of such words don't have a huge impact on your day-to-day life. But even so, they are nonetheless interesting things to talk about. In fact, some of the squiggly little nuances that pop-up in those discussions turn out to be extremely interesting!
Which is exactly why we're about to embrace our intrepid spirit of curiousity and ponder just such a topic. Specifically, today we're going to talk about weight and mass. Are they the same thing? If so, why do we have two words for one idea? If not, how are they different? And why are people always getting them mixed up? ;
What Is Mass?
Is your weight the same as your mass? In short (spoiler alert), no.
To see why this is, let's take a step back and think about what it is that we're trying to figure out when we “weigh” something.
For example, why does the person at the farmers' market weigh your bag of peaches before charging you? Obviously, they want to figure how much peach-stuff you have in your bag.
I don't mean the number of peaches you have (because peaches can be big or small), I mean the overall amount of peach matter you've bagged up.
Mass is a measure of how much stuff something contains.
This amount of matter is what we call the “mass” of the peaches—it's equal to the combined mass of all their atoms, and is measured in units of grams or kilograms.
What is the Difference between Mass and Weight?
Unfortunately, weight and mass are often considered to be the same quantities and misused in daily conversations. It is therefore essential for you to differentiate between these two physical quantities.
What is mass?
Mass is the amount of matter that an object has. It is a quantitative measure of a body’s resistance to acceleration. We can see it as the resistance of tableware when the tablecloth is pulled out.
Regardless of where the body is, the mass will always be the same, that is, it is an intrinsic amount of matter.
Mass is an extensive property of matter, it depends on the quantity and size of the system under study. This means that if an object of mass equal to 1 kg, we take away 0.1 kg since the mass will not be equal in the original object.
How is mass measured?
The mass is normally measured by means of a balance, making a comparison with a known standard. In the International System of Units (SI) the mass is expressed in kilogram (kg). As of May 20, 2019, the kilogram is defined in terms of the Planck constant h , a fundamental constant of quantum physics that is universal:
Difference Between Mass and Weight
Mass is the amount of matter present in a body while weight is a measure of how strongly gravity pulls on that matter. Mass is an intrinsic property of the body and remains the same wherever the body might be.
Weight is a force, and force is (Mass * Acceleration). The weight of an object is its mass times the acceleration due to gravity. The weight of the body differs by place.
For example, objects weigh less on the moon where gravity is lower compared to the Earth.
|Mass is the quantity of matter in a body regardless of its volume or of any forces acting on it.||Weight is a measurement of the gravitational force acting on an object.|
|Mass is always constant at any place and any time||The weight of an object depends on the gravity at that place|
|Mass is expressed in kilogram (kg), grams (g), and milligram (mg).||Weight is expressed in Newton (N)|
|Mass is measured using a pan balance, a triple-beam balance, lever balance or electronic balance.||Weight is measured using a spring balance.|
|Scalar and base quantity||Vector and derived quantity|
Contents: Difference Between Mass and Weight
Weight is measured using a scale which effectively measures the pull on the mass exerted by the gravity of the earth. Mass of a body is measured by balancing it equally with another known amount of mass. Mass may be measured using a pan balance while Weight may be measured using a spring balance. Methods may be interchanged if gravity is known and constant, as it is on earth.
Effect of gravity of mass and weight
The weight of an object depends on the gravity at that place while Mass is always a constant at any place and any time. Eg., If an object's mass is 60 kgs, then its Weight would be 600 Newtons but when taken to the Moon, this object will have a weight of 100 newton as the gravity of the moon is 1/6th that of the Earth. But the mass of that object will remain the same.
Mass can be a constant while Weight varies.
External Factors Affecting Weight
Mass is an intrinsic measure of an object and hence is independent of any external factors. Weight, on the other hand, depends on the mass that is attracting it and the force with which it is being attracted.
Conversion from mass to weight
Newton's second law is used to convert between weight (force) and mass:
The equation for force is F = ma (force = mass × acceleration).
Here, F is the force due to gravity (i.e. the weight), m is the mass of the object in question, and a is the acceleration due to gravity, on Earth approximately 9.8 m/s² or 32.2 ft/s²).
In this context the same equation is often written as W = mg, with W standing for weight, and g for the acceleration due to gravity.
Relative weight on Earth, moon and other planets
The following is a list of the weights of a mass on the surface of some of the bodies in the solar system, relative to its weight on Earth:
Use of weight vs. mass
In the physical sciences, the terms “mass” and “weight” are rigidly defined as separate measures in order to enforce clarity and precision.
In everyday use, given that all masses on Earth have weight and this relationship is usually highly proportional, weight often serves to describe both properties, its meaning being dependent upon context. For example, in commerce, the net weight of retail products actually refers to mass and is properly expressed in pounds (U.S.
) or kilograms. Conversely, the load index rating on automobile tires, which specifies the maximum structural load for a tire in kilograms, refers to weight; that is, the force due to gravity.
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