Eventually, he will slow down to the point where the air resistance force and the gravitational force are the same. Now that the force is in the opposite direction as his speed, he slows down. At some point the air resistance force becomes larger than the gravitational force like this.
Eventually, his speed will get large and the density of air will increase as he gets lower. Oh, this is the part that he could go faster than the speed of sound. The key point is that he is STILL speeding up and getting faster. This means that he will still speed up, but the rate that he speeds up will be less. Since there is an air resistance force in the opposite direction to the gravitational force, it essentially makes the total force smaller (but still down). So, at the begging of the jump, the forces might look like this: However it also depends on the density of air. You have probably felt this force when you put your hand out of a moving car window. Since this force is down, it causes him to start moving faster and faster as he travels down.Īs he starting going faster, there is an air resistance force. Since he isn't really moving (yet) and there isn't much air anyway, there is just the gravitational force on him. However, there is one model for the speed of sound that says it is proportional to the temperature (this is just a model - but it works fairly well). But sound is an interaction between air molecules - so it really depends on what they are doing (and it really isn't so simple). This is the value for the speed of sound at normal temperatures and pressures (like near the surface of the Earth). If you think about introductory physics, the speed of sound is often stated as being around 340 m/s or 760 mph. So, what is the speed of sound? I guess you could ask "what is sound?" - but maybe I will look at that later. One of the cool things about the Red Bull Stratos jump is that it will be a chance for a falling human to fall faster than the speed of sound. Then why do astronauts float around in space? I am glad you asked (your answer). Oh, maybe I should add that the gravitational force on astronauts in the International Space Station is about 91% the value at the surface. So the answer is that the gravitational force at 120,000 feet is pretty much the same as on Earth. At an altitude of 120,000 feet the gravitational force would be 9.68 Newtons (2.18 pounds).
But what about 120,000 feet? Well, a 1 kg mass has a gravitational force of about 9.8 Newtons (2.2 pounds) on the surface of the Earth. The gravitational force doesn't change too much near the surface of the Earth.
This is because your calculator rounds off the actual value.įine. Put 2.09 x 10 7 + 100 in your calculator. It has a radius of about 6.38 x 10 6 meters (or 2.09 x 10 7 feet). So, if I am 10 feet above the surface of the Earth and I double this height to 20 feet, how far did I move from the center of the Earth? The answer: not at all (to first approximation). If you double the distance between the center of a planet and a spaceship, the gravitational force will only be one fourth as much. On a side note, it is fairly common for people to think there ISN'T any gravity in space.Īnyway, the gravitational force depends on the distance between the objects (for spherical objects at least). This is the force that causes the moon to orbit the Earth and the Earth to orbit the Sun. Ok, so the air is pretty thin up there, but what about gravity? Clearly there IS gravity in space. How much less will the gravity be at 120,000 feet? So, in the end, you need a balloon about 80 meters (over 250 feet) across when fully inflated to lift a jumper and life support capsule to that height. The result is you need a bigger balloon (which unfortunately has more mass). With a low density, there not as many collisions between the air and the balloon. Still, as you can see above, the density of air at 120,000 feet is really low.
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