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Weight and Mass

People sometimes think the words weight and mass mean the same thing. But for scientists, they mean different things. Weight is the force of gravity on a person or object at the surface of a planet. When you stand on a scale, the scale measures the force with which Earth pulls on you. Mass is something different. It is a measure of the amount of matter in an object. Far out in space, far from the pull of Earth’s gravity, your weight might go down to just about zero, but you would still have the same mass.

The gravitational pull of an object depends on the amount of mass it has. The greater the mass, the stronger the pull. When you fall off your skateboard, you pull Earth to you at the same time Earth pulls you toward its center. But your mass is tiny compared to that of Earth. So the pull you exert on Earth is much, much weaker than the pull of Earth’s gravity on you. Friction is a force that can affect the motion of an object. Friction occurs when two surfaces rub together. Think of the wheels of a skateboard on pavement. It may seem that the wheels and the pavement are both smooth. But actually both have bumps and ridges. Friction is created when the bumps and ridges of the two surfaces come into contact with each other. If a moving object meets continuous friction, sooner or later it will be brought to a stop. Without friction, the object would keep moving at a constant speed forever. With friction, the only way the object can keep moving is if it gets a push (or a pull) from some other force. For the skateboard, you supply the push. How strong the force of friction will depend on a couple of factors. One is the type of surfaces involved. For example, the rougher the surfaces, the greater the friction. Another factor is how hard the surfaces push together. There is more friction if you rub your hands together with some force than if you rub your hands together lightly.

Mass and Payload

Imagine an empty cardboard box. It has very little mass. It is very easy to push. Suppose you fill it with rocks. Now the mass is much greater, and you have to use a lot more force to push it. This fact is explained by Isaac Newton’s second law of motion. This science principle says that the amount of force needed to move an object—that is, change its speed or direction—depends on the size of the object’s mass. The greater the mass, the greater the amount of force required. The law also says that for a given mass, a greater force will produce a greater change in speed or direction. The change in speed or in direction will occur in the same direction as the force. The cardboard box will move in the direction you push.

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