Forces and Motion

Differentiating between mass and the force of gravity

Teaching Guidance for 11-14 Supporting Physics Teaching

Mass vs. weight

Wrong Track: It weighs 10 kilogram. That's its mass.

Right Lines: The force of gravity acting on an object is a measure of the gravitational pull acting on that object. It is a force. Mass is a measure of how difficult it is to accelerate the object. The force of gravity acting on an object can change with location; the mass always stays the same.

Making the distinction

Thinking about the learning

Once again, here is a distinction made in the sciences that everyday conversations don't bother with. Statements such as: The bag of potatoes weighs 5 kilogram are common parlance. We all know what this means. I weigh 65 kilogram is a similar statement that only physicists might object to.

The distinction between mass and weight can often seem pernickety, unnecessary and not particularly helpful. However, in physics, the mass of an object and the force acting on an object are very different measures.

Thinking about the teaching

Unfortunately, the distinction between mass and weight is not helped by the everyday use of the verb, to weigh. Weighing has become synonymous with finding either the weight or the mass.

We'd suggest avoiding weight as a technical term, and instead associating it with the experience of being supported by the floor, or the bathroom scales. Instead use the force of gravity as a label for the gravitational force acting on an object.

There isn't really a common expression in science equivalent to massing something. If there was such a mass machine, it would work on the principle of accelerating the mass. The force required to produce a certain acceleration would depend on the mass.

Hence a read-out in kilograms would be possible. Such a machine would be making use of the inertia of the mass and would perhaps be called an accelerometer or an inertiameter. Using such a machine, a more massive object would require a bigger force to change its speed by the same amount.

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