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## Physics library

### Course: Physics library>Unit 4

Lesson 3: Newton's law of gravitation

# Would a brick or feather fall faster?

What would fall faster on the moon, a brick or a feather? Created by Sal Khan.

## Want to join the conversation?

• the last experiment was ToTaLlY cool but why is it that the paper didnt float up but fell with the book???!!!
• Because of air resistance.
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If one object is placed above another, than air resistance is blocked out by the base object. So the resistance is not on the paper (which was on top...), rather it was on the book; the paper fell with the book.
• why does the moon have no atmosphere
• The gravitational field of the moon is too weak to sustain any significant atmosphere, as the moon isn't nearly as massive as needed to prevent most gases from escaping.
• Why is r(or distance) squared? Isn't it just distance from one object to another?
• Gravitational force follows the inverse square law. It is due to the spherical geometry of the force field. Intensity of the force twice as far from the source is spread over four times the area, hence one fourth the intensity.
http://hyperphysics.phy-astr.gsu.edu/hbase/forces/isq.html
• what would happen if you filled a balloon with helium on the moon would it still float up
• Hello Victoria,

Nope, it would fall to the surface. In fact, if you dropped a brick and a balloon at the same time they would strike the surface of the moon at the same time!

The question we need to ask is why does the helium balloon float here on earth. It has to do with density. For a given amount of space, like inside the balloon, the helium gas weighs less. The helium is "lighter than air" and so the balloon rises. It will continue to rise until the balloon bursts or the helium plus weight of balloon equals weight of equivalent volume of air. Recall that the atmosphere thins as we go up.

If it helps think of the helium balloon that same way as bubbles in water. Both will rise because they are lighter than the fluid they displaced.

Finally we can answer your original question. Since there is no air on the moon the helium has not displaced anything. Because we are in a vacuum everything falls at the same rate.

Regards,

APD
• can you explain air friction to me? for example, when a skydiver is falling and reaches a terminal velocity of 120MPH after falling for 15 sec, what is the force of air friction on the skydiver as he falls at terminal speed?
• The reasoning for AndrewM's absolutely correct answer is this:

Terminal speed means that this speed no longer changes. In other words there is no accelleration happening. This means there is no net force acting on the diver. This means that all forces on the diver have to balance each other. There are only two relevant forces: friction and gravitational pull.
• what is the ratio g(earth)andg(moon)?
• The ratio would be about 6 to 1. For example if I weighed 100lb on earth and I were to travel to the moon. I would weigh about 16.6lb.
• If you would shoot a gun and drop a book would the bullet and book hit the ground at the same time
• it depends entirely on the direction you fired the gun. if you fired it perpendicular to the surface, and the bullet had enough velocity, it could potentially reach orbit. if you fired it straight into the ground, obviously it would be moving much faster than pure gravity could pull it, and therefore hit the ground before the book.
• a) We could calculate time by using kinematic formula: Vo=0 -> t=2X/g. From here we can see that the mass is irrelevant.

b) Air resistance is: F=p*A. Bigger the surface bigger the resistance force.

Is this correct?