Science 8 – 2.1

Fluid Introduction

Hydraulics and Pneumatics

Mass vs Weight

SCO 2.1.1 describe qualitatively the difference between mass and weight (309-1)





We often use the terms “weight” and “mass” to mean the same thing. People may say “my weight is” to mean the same as “my mass is”. When you are standing on Earth, weight and mass seem very similar. So why is it important to know how they are different?

WEIGHT is a measure of the force of gravity on an object, while the MASS of an object measures the amount of matter that is in the object. If you were to go the Moon, you would find that the force of gravity is one-sixth of that on Earth. So, what would happen to an astronaut’s weight when she travels to the Moon?

The VOLUME of an object is a measure of how much space it takes up. Volume and mass are both important measurements when determining the DENSITY of an object. Density can be measured by finding the mass of a substance in a given volume.

Mass is the amount of matter in an object; it does not depend on forces acting on it. Mass is the same no matter where the object is located as long as the object does not gain or lose any of its matter. An object that has mass can be pulled on by gravitational force. Mass is measured on a balance.

Weight is a measure of the pull of gravity on an object. Weight is related to mass but they are not the same. Weight on Earth is based on the pull of gravity toward the center of Earth. Weight can change on Earth since the pull of gravity is not the same everywhere. Weight is measured using a spring scale.

Weight can change if an object is located on another object in space, for example, the Moon or Mars. The mass of that larger object determines the pull of gravity and therefore the weight of the object.

Weight may change due to the change in gravitational force, but mass stays the same.

Different fluids not only have different viscosities, they also show a difference in their densities. Density is the amount of matter or number of particles in a given volume. The Particle Theory helps us to understand the concept of density.


• describe and explain qualitatively the relationships among pressure, volume, and temperature of fluids when compressed or heated and quantitatively the relationships of force, area, and pressure (309-3, 309-4)
• provide examples and a course of action of how science and technology affect personal and community needs (111-1, 113-2)