Weight and Mass

The weight and mass of an object are two different things. The weight of an object is the force or push of that object against its support mechanism, which can change from time to time depending on its location or motion, as described in the previous page. The mass of an object is its inertia, or resistance to change in motion, which does not vary.

Pushing a car

Have you ever tried to push-start an automobile? If so, you know that it takes a very strong push to get the car moving, even on level ground with the car's transmission in neutral. The car is very massive, so it takes a strong push to get it moving.

Imagine that we take the same car into outer space and bring it into Skylab or the International Space Station. The car is weightless and floats inside the space station cabin.

Suppose you want to get the car moving. Is it any easier to push than on Earth? The answer is no. You need to plant your feet against the wall of the space station cabin and push very hard to get the car moving, just as hard as you push the same car on level ground on Earth. A car in the space ship has no weight, but it has the same mass as on Earth, so it resists any change in motion just as much in space as on Earth.

Is a weightless moving car dangerous? In space, you cannot get "run over" because there is no roadway underneath the wheels. However, if you get caught between the front of the moving car and the wall of the space station, you could be crushed, just like getting caught between a brick wall and the front of a moving car on Earth.

The astronauts aboard the International Space Station must get exercise to stay healthy. You might think that exercise weights such as dumbbells would not be useful for exercise in a weightless environment.

Weightless weights
The author with (briefly) weightless dumbbells of different masses

Although the dumbbells have no weight, they still have mass, or resistance to change in motion. You cannot "lift" the weights against gravity, but you can still move them back and forth, and your muscles still feel their resistance to change in motion. Heavier dumbbells, being more massive, are harder to move than light ones.

The following experiments demonstrate the properties of "heavy" and "light" objects under weightless conditions.

Next: Weightlessness at Home

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2009 Gray Chang