Physics Stack Exchange is a question and answer site for active researchers, academics and students of physics. Join them; it only takes a minute:

Sign up
Here's how it works:
  1. Anybody can ask a question
  2. Anybody can answer
  3. The best answers are voted up and rise to the top

My science teacher is always saying the words "weight of an object" and "mass of an object," but then my physics book (that I read on my own) tells me completely different definitions from the way these words are used in my science class... so which is right?

What is the difference between the weight of an object and the mass of an object?

share|cite|improve this question

10 Answers 10

up vote 24 down vote accepted

Weight is the force with which gravity pulls on a mass.

Maybe the simplest way to explain the difference is that on the Moon or on Mars, your weight is reduced because gravity is weaker there, but your mass is still the same.

share|cite|improve this answer

The mass, strictly the inertial mass, relates the acceleration of a body to the applied force via Newton's law:

$$ F = ma $$

So if you apply a force of 1 Newton to a mass of 1kg it will accelerate at 1m/s$^2$. This is true whether the object is floating in space or in a gravity field e.g. at the Earth's surface.

The weight is the force a body exerts when it is in a gravitational field. The weight depends on the gravitational field. For example the weight of a 1kg mass at the Earth's surface is 9.81 Newtons, while at the surface of Mars it's about 3.5 Newtons.

This is possibly a bit too much info: if so ignore this last paragraph. Although weight specifically means the force exerted in a gravitational field, Einstein told us that sitting stationary in a gravitational field is equivalent to being accelerated in the absence of gravity. The inertial mass defined using Newton's laws is the same as the gravitational mass defined by the force a body exerts in a gravitational field. So if you take a 1kg mass at the Earth's surface, the weight of 9.81 Newtons it exerts is exactly the same as the force you'd need to accelerate the 1kg mass at 9.81m/s$^2$.

share|cite|improve this answer
just to clarify, when most people say "weight" they are really referring to mass right? So when I say my weight is 84kgs, that's my mass and my weight is really 84 x 9.81 = 824.04 Newtons. So 824 Newtons is my weight. Right ? – gideon Aug 21 '15 at 8:55
Yes. Though to be honest in everyday life it makes little difference. – John Rennie Aug 21 '15 at 9:20

Yes of course, According to physics the Mass and Weight are different from each other. Following is their main difference,


  • Mass is the amount of matter contained in a body.

  • Mass of the body is the constant quantity and does not change with the change of position or location.


  • Weight is the force by which the earth attracts a body toward its centre, Or we can say it is the force on the object which is offered by the gravity
  • Weight of the body is the variable quantity and changes with the change in position and location due to the acceleration of the gravity acting on it. Yes they are used at different places and time.

So these are the basic difference between Weight and Mass of the object

share|cite|improve this answer

Mass is a constant for each object, meaning that the mass won't change unless the object changes.

On the other hand Weight changes due to the gravity; any object's weight is the force pulling it down to the ground. The force can be determined like so: $F=MA$, where $F$ is the force, $M$ is the mass and $A$ is the acceleration (the gravity in case of weight). So $W=MG$, where $W$ is the weight, $M$ is the mass and $G$ is the gravity.

Btw, weight is in Newtons, while Mass is in kilograms.


share|cite|improve this answer

To elaborate on John Rennie's answer - As he said, the mass is the inertial mass of the body, which isn't the same as the weight. The weight is typically defined in context to a gravitational field.

No doubt you know that the acceleration due to gravity on the earth's surface is $9.8 m/s^{2}$ (on average). So if your mass on earth is say, 5 kilograms, your weight on earth would be $9.8 \times 5 = 49 N$. So the weight that your weighing machine registers is your weight, not your mass. You'd have to divide by the acceleration due to gravity at that point to calculate the mass.

So basically your weight basically measures the force that you exert on the weighing scale, not the actual mass of your body. Which is why if you stand on a weighing scale in free fall it'll register zero, since you aren't exerting any additional force on it.

share|cite|improve this answer

There is a basic difference, because mass is the actual amount of material contained in a body and is measured in kg, gm, etc.

Whereas weight is the force exerted by the gravity on that object mg.

Here m=mass in kg,gm,.. $g=9.8m/s^2$ (acceleration due to gravity)

Note that mass is independent of everything but weight is different on the earth, moon, etc.

share|cite|improve this answer

Mass is the amount of matter in an is also does not change with ur position.While weight is the effect of gravity on a given is also a variable quantity and changes with the change in position and location due to the acceleration of the gravity acting on it

share|cite|improve this answer

Weight is the force that the body exerts onto its support. Mass is an abstract property of the body invariant of the support and defining what acceleration the body gets when being affected by a force.

This is why weightlessness is possible inside an airplane in freefall (or a space ship rotating an orbit) - both experience the same acceleration and so both move with the same speed and so the body exerts zero force onto its support and so it becomes weightless.

share|cite|improve this answer

Mass is not variable while weight depends on the force of gravity wich is the body. Per example a body on the Earth is drawn to the Earth with a force of 9.8 m/s^2 while the same body with the same mass is drawn 2,5 if it was on the Jupiter´s surface. In other words the same body with the same mass is dr

share|cite|improve this answer

The major difference between weight and mass --weight is dependent on surrounding and mass is not. e.g. If you travel to space where you will be weightless because you have no surrounding mass exerting on you. But in space your mass will be constant as it was on earth.

share|cite|improve this answer

protected by Qmechanic Aug 21 '13 at 21:50

Thank you for your interest in this question. Because it has attracted low-quality or spam answers that had to be removed, posting an answer now requires 10 reputation on this site (the association bonus does not count).

Would you like to answer one of these unanswered questions instead?

Not the answer you're looking for? Browse other questions tagged or ask your own question.