# Question from a HS Drop Out (Gravity)

1. Jul 9, 2004

### hillbilly

1. What is the force that makes an airplane/SR-71 compress as it goes faster? Is this same force what gravity is? Does the counter clockwise spin of the earth create gravity? If we spun faster would gravity be greater? If we stopped spinning would the earth expand like a fish being pulled up from the depths of the ocean? If I spun a plate really fast the acceleration would want to pull it apart, what is the reaction that occurs causing the plate to compress? Whatever it is it must be greater then the acceleration force.

2. Does the sun rotate?

2. Jul 9, 2004

### Staff: Mentor

No. Even though I don't know about "airplane compression", I'm sure it's not gravity causing it.
No. In Newtonian physics, the earth's gravity is caused by the mass of the earth.
No.
No.
The bonds between the molecules are elastic and able to create tension, like little springs. To spin, each molecule of plate must be pulled towards the center; these bonds do the pulling. The faster the plate spins, the harder the bonds must pull. There's a limit: go too fast and the plate will come apart.

Here's an analogy: Hold a hammer by the handle and spin in a circle with your arm outstretched. The faster you spin, the harder you must grip that handle. Spin too fast and your grip will not be strong enough and that hammer will go flying.

Yes. But since it's hot gas, not solid, different latitudes rotate at different speeds.

3. Jul 9, 2004

### Chi Meson

This "compression" isn't so much that an airplane gets measurably shorter, it is that the material is "under compression." It is not a mysterious force, it's simply du to the fact that the engines are at the back of the airplane.

The engines push the rest of the airplane forward. In front of the plane is air. As the plane gets going faster, the air resistance pushes back on the front of the plane. Engine pushes forward, air resistance pushes back. If anything has two forces pushing it from both sides, it is "under compression." Again, this doesn't mean that you can measure a difference in length of the plane.

4. Jul 9, 2004

### Zorodius

No, but our apparent weight would be less.

Every particle in the universe attracts every other particle with a force that is proportional to the mass of the two particles and inversely proportional to the square of the radius between them. That means that, if you're thinking about two particles, or two objects that can be treated as particles - say, a person and the Earth - the gravitational force between them can change only if the mass of either object changes (like, say, the person gets fatter), or the separation between the two objects changes (like if we launch the person into space).

Last edited: Jul 9, 2004
5. Jul 10, 2004

### Staff: Mentor

"apparent" weight vs weight

Let me explain what Zorodius refers to when he says apparent weight.

As explained, the term "weight" refers to the force that the earth pulls on an object. That depends on the mass of the earth and the mass of the object.

Apparent weight is what a scale (bathroom scale) would read if you stepped on it. What that scale is actually reading is the force that presses against it: the so-called "normal" force that is supporting you. Imagine being in an elevator standing on that scale. If the elevator is moving at a constant speed, that scale reads your usual weight. But if the elevator accelerates upward, the scale reads higher as you are pressed against it: you feel heavier and thus your "apparent" weight is greater.

What if the elevator cable is cut and it starts falling? Assume it freefalls, the scale reading drops to zero. This is what the term "weightlessness" means: your apparent weight is zero (not your real weight). Want to experience weightlessness? Jump out the window! (Space shuttle astronauts experience weightlessness because in orbit they are actually falling around the earth.) Gravity and weight don't magically disappear, only your apparent weight does.

After all that, back to Zorodius's comment. If the earth didn't spin your apparent weight would equal your real weight. But since it's spinning, you are "pulled away" from the earth a bit, so your apparent weight is less.

6. Jul 10, 2004

### kuenmao

Just a note: I remember calculating the difference between the weight of a 50 kg person at the equator(ie. affected by Earth's rotation most) and the weight of the same person at the North and South poles(ie. not affected by Earth's rotation). The difference was around 1.8 N.

So, ladies...consider moving towards the equator a good idea of losing weight quickly.

7. Jul 10, 2004

### Zorodius

Of course, that's less than one percent of their weight, so don't quit your diet just yet.

You're closer to the core of the Earth at the north and south poles as well, because the Earth is somewhat flattened instead of totally spherical, so the gravitational pull on you will be marginally greater, in addition to the difference in radial acceleration.

8. Jul 10, 2004

### Bob3141592

By the way, an SR-71 flying at speed (over Mach 3) actually lengthens. This is because air friction makes it get hotter, and so the metal superstructure expands.

At that speed, the titanium skin gets significantly hot, and anneals itself in flight. This makes the metal stronger. An SR-71 lands structurally stronger than it was at takeoff.

Pilots in the SR-71 have the luxury of eating hot meals on their long missions, even though there is no oven in the cockpit. They just hold their sandwiches against the canopy for a minute or two and it gets nice and hot.

This thermal expansion was a problem the plane's designers had to deal with. The piping and other internal components also expand, and if the plumbing was made too rigid, it'd crack under the forces of expansion. So it was designed to be loose at normal temperatures, and when it heats up in flight the expansion makes everything nice and tight. I've heard the plane leaks like a stuck pig on the ground. Partialy for this reason, it carries a minimum fuel load on takeoff. Standard procedure is that once getting airborne, it refuels in flight from a tanker, then kicks in the afterburners to go supersonic. allowing the skin heat to soak into the plane and sealing all the joints.

"Cool" plane.