Why is weight measured in newtons and mass in kilograms?

Weight is measured in newtons because it is a force, while mass is measured in kilograms as it is a quantity of matter.

Weight is a force that is the result of gravity acting on mass. In physics, force is measured in newtons, named after Sir Isaac Newton, who formulated the laws of motion and universal gravitation. The weight of an object can change depending on where it is in the universe, as the force of gravity can vary. For example, an object would weigh less on the moon than it does on Earth because the moon's gravitational pull is weaker.

On the other hand, mass is a measure of the amount of matter in an object. It is measured in kilograms, one of the base units in the International System of Units (SI). Unlike weight, mass does not change regardless of where the object is located in the universe. This is because mass is an intrinsic property of matter, independent of external factors such as gravity.

In everyday language, people often use 'weight' and 'mass' interchangeably. However, in physics, they have distinct meanings. When you step on a scale, it measures your weight, not your mass. The scale calculates the force (your weight) exerted by your mass under the influence of Earth's gravity. If you were to use the same scale on the moon, it would show a smaller number, even though your mass has not changed.

Understanding the difference between weight and mass is crucial in physics. It helps explain why objects behave differently under different gravitational forces. For instance, astronauts can float in space because there is little gravity acting on their mass, reducing their weight to almost zero. However, their mass remains the same, which is why they still have physical presence and can interact with other objects.