Use a balance scale to explore the concept of weighing objects before introducing the idea that gravity impacts the number of grams that an object weighs. Give students a single object that is pre-weighed. This could even be a small dumbbell weight. Place this object on one side of the scale. Give students smaller objects with a uniform mass and ask them to place them on the other side of the scale. These might be small building blocks or other stackable objects. When the scale is balanced, students can determine the amount of grams in the larger object by counting the total number of grams in the smaller objects.
When weighing an object, students are really measuring a property of the mass of the object and the effect of the force of gravity on that object. The amount of material weighted is the mass. The force of gravity exerted on the mass determines the number of grams in the object's weight.
Explore the idea of mass, which is the amount of material in an object. Give students a number of everyday objects that have dimensions that are easy to measure. Students can measure the length, width and height of these objects. They can weigh these objects. Create a list that shows and compares the attributes of these objects.
Explain that although we often confuse weight and mass here on earth, that is because they are directly correlated; the force of gravity on earth is more or less constant. Have the students hold one of the objects. They can feel the dimensions or the mass of the object. They can feel the force of gravity pushing down on the object, because that is why the object feels as heavy as it does.
In space, a space shuttle is falling just like part of an amusement park ride. If the students have ever been on a roller coaster, they will understand the feeling of weightlessness that people get just as they plunge down a large hill on a roller coaster. In a space shuttle, the lack of force pushing against an astronaut's body makes him feel weightless. However, that astronaut is the same size in space as on earth, and has the same mass.
To see the effect of force on mass and weight, weigh and measure a selection of objects. Attach the objects to enough helium balloons to make them neutrally buoyant. That is, the objects should float just off the ground without moving up into the air or sinking down to the ground. Since the helium balloons are opposing the force of gravity, the objects can hover. If the students weigh them now by placing a scale under the object, the object would have no weight. It would have the same mass, however.