Apparent Weightlessness in an Elevator
Astronauts have to contend with a number of unusual issues while living on the space station. Possibly the biggest difference between life up there and life down here is the apparent “weightlessness”: Nothing stays put, not even yourself! There’s no guarantee that water flows down, and if you don’t strap yourself in at bedtime, you might wake up in a storage closet.
Many people think that astronauts are weightless because there’s no gravity up there. But as it turns out, the space station is subject to quite a bit of gravity (How else would it stay in orbit?). In fact, the Earth’s gravitational force at the space station’s altitude is only about 10% less than what it is at the earth’s surface. So what’s going on?
When something is in orbit, it is said to be in free fall—it is falling through space with no force pushing back. But it falls at the exact same rate that the surface of the Earth curves away from it. If it falls a hundred feet vertically, it also moves far enough horizontally so that the Earth’s surface drops off by the same amount. A satellite basically falls without ever hitting the ground.
In this experiment, you’re going to play around with how falling—and any kind of vertical motion—affects an object’s apparent weight.
Purpose: Examine how falling affects an object’s weight.
- Paper or plastic cup
- Bucket or sink
- Spring scale
- Small mass
- Poke a couple of holes in the bottom of the cup and fill it with water. Observe what happens.
- Now, drop the cup into the bucket. How does the behavior of the leaking water change as the cup falls through the air?
- Hang the mass on the spring scale and note the weight.
- Hold the scale low to the ground then quickly (but steadily) raise it. What happens to the weight?
- Hold the scale high in air then quickly (but steadily) lower it. What happens to the weight? How does this compare to when you raised the scale?
- Step into the elevator and ride it up and down for a few floors while holding your spring scale with the weight attached. Keep track of how the weight changes as the elevator moves up and down.
For the first experiment, as soon as you pour water into the cup, water starts leaking out the bottom. When you drop it, however, the water stops leaking while the cup is falling.
For the second experiment, the weight apparently increases as you lift the scale up and decreases as you lower it. Similarly, the weight will appear higher when the elevator is ascending and lower as it descends.
In 1589, Galileo reportedly dropped two balls with different masses off the Leaning Tower of Pisa and noticed that they hit the ground at the same time. When astronaut David Scott stood on the Moon in 1971, he famously replicated Galileo’s experiment in the near vacuum of the Moon’s environment using a feather and a hammer. Sure enough, they hit the lunar surface at the same time!
This probably sounds completely counterintuitive. You might think that gravity would pull more strongly on the hammer and make it fall more quickly. It’s true that the hammer feels a stronger gravitational pull than the feather; however, because the hammer has more mass, it’s also more difficult for any force (including gravity) to move it! Try pushing on a shopping cart and a car with the same amount of force and see which one moves more easily. The hammer’s resistance to movement cancels the stronger gravitational force it feels.
When you drop the cup, both the water and the cup fall at the same rate. The water drips out, but the cup catches up! Weirdly, the water is momentarily suspended within the leaky cup.
The same thing happens to astronauts onboard the space station. They don’t appear weightless because there’s no gravity; they appear weightless because, as mentioned earlier, they are always falling. And so is everything else onboard: tools, food, and clothing. The astronauts in the space station are like the water in your cup. They are falling along with the station, free to drift around as they please.
The second experiment drives this home further. Weight is just the force on your body from gravity. When you pull the scale up, the force from your hand has to overcome the gravitational force. Because the mass feels more force, its weight appears to increase. The weight while moving is actually called the apparent weight; it takes into account other forces besides gravity. When you move the scale down, the force from your arms removes some of the gravitational force and the apparent weight goes down. What happens to the weight if you lift up the scale and then drop it? How is this like the first experiment?
The experiment in the elevator is no different, except this time the elevator motors are doing the work instead of your arm. What do you think would happen to the weight if the elevator cable broke?
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