Hammer Versus Feather On The Moon

First time I’ve seen this video, surprisingly, and it really is rather special. From the APOD website:

Explanation: If you drop a hammer and a feather together, which reaches the ground first? On the Earth, it’s the hammer, but is the reason only because of air resistance? Scientists even before Galileo have pondered and tested this simple experiment and felt that without air resistance, all objects would fall the same way. Galileo tested this principle himself and noted that two heavy balls of different masses reached the ground simultaneously, although many historians are skeptical that he did this experiment from Italy‘s Leaning Tower of Pisa as folklore suggests. A good place free of air resistance to test this equivalence principle is Earth’s Moon, and so in 1971, Apollo 15 astronaut David Scott dropped both a hammer and a feather together toward the surface of the Moon. Sure enough, just as scientists including Galileo and Einstein would have predicted, they reached the lunar surface at the same time. The demonstrated equivalence principle states that the acceleration an object feels due to gravity does not depend on its mass, density, composition, color, shape, or anything else. The equivalence principle is so important to modern physics that its depth and reach are still being debated and tested even today.”

Heavy stuff. Onwards!

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4 Responses to Hammer Versus Feather On The Moon

  1. Hw Looi says:

    This may not be absolutely true since every object has its own gravity which is greater if its mass is greater. So the hammer has a gravity much greater than that of the feather. Therefore the combined gravity of the hammer and that of the moon (which pulls the hammer and moon towards each other) is greater than that of the feather and the moon.
    As such the hammer should collide with the moon marginally earlier than that between the feather and the moon, though this difference is so minute that we assume that the collisions occur simultaneously.

    • hw looi says:

      However, if the hammer and feather are dropped together, then as the hammer’s gravity pulls the moon towards itself, it also pull the moon towards the feather and as such the lucky feather may get a free ride and hits the moon marginally earlier. But even with this help, the feather will still take a slightly longer time to collide with the moon as the gravity from the hammer will cause the flight path of the feather to curve towards the hammer and as such takes a longer path and hence a longer time to hit the moon.

      To be fair, the experiment should be done dropping the feather first, then the hammer and then see the different times taken.

  2. looihw88l says:

    (Corrected version) What would happen if you dropped a hammer and a feather on the earth and on the moon?
    The above experiment is supposed to prove the equivalence principle which states that the acceleration an object feels due to gravity does not depend on its mass, density, composition, colour or shape.

    Answer:

    If you drop a hammer and a feather from the same height on earth, the hammer will hit the ground first as the feather is slowed down drastically by air resistance.

    But on the moon, because it is a vacuum, and since the acceleration of an object is the same as the gravity i.e. a = g and the mass is not in the equation, all objects will have the same acceleration and hence the hammer should fall to the surface of moon at the same time as the feather but:

    “Both will hit the moon at the same time as believed by most scientists?”

    This may not be absolutely true since every object has its own gravity which is greater if its mass is greater. So the hammer has a gravity much greater than that of the feather. Therefore the combined gravity of the hammer and that of the moon (which pulls the hammer and moon towards each other) is greater than that of the feather and the moon.

    As such the hammer should collide with the moon marginally earlier than that between the feather and the moon, though this difference is so minute that we assume that the collisions occur simultaneously.

    However, if the hammer and feather are dropped together, then as the hammer’s gravity pulls the moon towards itself, it also pull the moon towards the feather and as such the lucky feather may get a free ride and hits the moon at the same time as the hammer.

    To be fair, the experiment should be done dropping the objects individually e.g. feather first, then the hammer and then see whether the times taken are the same or not.

    All the above are valid only on the assumption that the centre of gravity is the part that hits the moon but since this is not necessarily true, we also have to take into account which part of the hammer or feather is nearest to the moon before the two objects were released (assuming that the centre of gravity of both objects are at the same level on release) !

    The real answer is that there is not enough data for us to know which will hit the moon first !

    The famous experiment by Astronaut Dave Scott on the moon is not very precise and may even be misleading.

    Dr HW Looi

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