Special Sub-Topic: Is Gravity Always Getting You Down? Click Here To Play: Is Gravity Always Getting You Down?

The first notable person who tried to explain gravity was Aristotle. What reason did he give for objects falling?

They were being attracted to the center of the universe, their natural place.. Scientific knowledge has progressed so much, hasn't it? Aristotle also believed that lighter elements, such as fire, rose upward because they were attracted by the more ethereal moon.

Aristotle also believed that heavier objects accelerated faster than lighter objects. Which famous scientist disproved this notion?

Galileo Galilei. Galileo's ball-rolling experiments led him to the conclusion that gravity accelerates everything at the same rate. This does NOT mean that all objects fall at the same rate though. Velocity is not only affected by acceleration but also by external forces such as air resistance. Theoretically, though, in a vacuum, all objects would fall at the same rate. Also in a vacuum, objects would never hit terminal velocity and would continue to accelerate indefinitely.

Which scientist is famous for coming up with the law of universal gravitation, namely that two point masses attract each other proportionally to the product of their masses and inversely proportional to the square of the distance between them?

Sir Isaac Newton. The equation is as follows: Force ~ M_1*M_2/r^2. This is one of Newton's most famous laws, along with his legendary three laws of motion. This law had a few problems when the gravitational force was very large (large meaning it could be divided by the speed of light squared and still hold some significance), but these problems were fixed by Einstein's general theory of relativity.

If you've taken a physics class you know that the universal gravitational constant (G) is approximately 6.67428×10-11 m3kg-1s-2. But who is responsible for discovering this number?

Henry Cavendish. Cavendish used a torsion balance (an apparatus used to measure very small forces) for this experiment in 1797. Once the value of G is obtained, it is a simple calculation to find acceleration due to gravity on Earth. Using the mass of the Earth (5.9742×10^24 kg) and the radius of the Earth (6.378×10^6 m), we can plug them into the formula to find the value for acceleration due to gravity: g = 6.67428×10^-11 * 5.9742×10^24 / 6.378×10^12 ≈ 9.8 m/s^2.

Which of the following occurs when you jump into the air?

Both are true.. Yes, both statements are true. The fact that you have mass (assuming you're not Kate Moss) means that you have a gravitational field; however, the force exerted on the Earth by this field is so infinitesimal compared to the Earth's that it's completely negligible in any calculations you might do.

What is the term used to describe the speed at which an object needs to be traveling in order to break free from a gravitational field?

escape velocity. Earth's escape velocity is about 11.2 km/s. This does not mean that if you are in a rocket ship you have to accelerate up to 11.2 km/s to leave Earth's orbit though. The 11.2 km/s refers to the velocity needed if an object were launched from the surface of the Earth with no further acceleration or applied force. In fact, as your rocket ship got further and further away from Earth, escape velocity would get smaller and smaller (i.e. it would approach 0).

Light is affected by gravity.

t. The fact that light is affected by gravity is why black holes are black. The gravitational pull of a black hole is so ridiculously strong that even light, traveling at 3×10^8 m/s, cannot break free from it. Since light cannot escape from black holes (i.e. its escape velocity is greater than the speed of light), they appear black. An interesting fact about black holes: they are so dense that a black hole with mass equal to the sun would have a radius of only 3 km!

What is the term used to describe two stars that rotate around a common center of mass?

binary star. The orbit paths of the two stars in the binary star system can sometimes cross--they do not have to be concentric. A famous example of a binary star is Sirius, the brightest star in the sky. Sirius A is the one you can see, and is very bright. Its companion star, Sirius B, is a dim white dwarf with a mass equal to that of the sun and a radius of only 6,000 km.

Black holes have ridiculously strong gravitational fields. The threshold beyond which nothing can ever affect an outside observer is called what?

event horizon. Once you pass the event horizon, you would need to travel faster than the speed of light to escape from the black hole. And we all know that's impossible. On an interesting side note, to an observer watching you pass through the event horizon, you would appear to decelerate and never actually pass through it; however, to you it would seem that nothing unusual happened and you would, in fact, pass through the event horizon unimpeded. This effect occurs because as you near the event horizon, gravity speeds you up to nearly the speed of light, and to an observer it appears as if time has stopped for you.

Major Tom: I'm going to visit the sun in my spaceship. Ground Control: Nonsense! You'll be burnt to a crisp. Major Tom: No I won't. I'm going at night! Jokes aside, even if Major Tom could land on the sun, how difficult would walking be, given the sun's gravitational field?

It would require superhuman strength.. The sun's gravitational field is 28 times stronger than Earth's, due to its enormous mass. Just to give you an idea, if you take a fairly normal, 85-kg (187-lb) man, one of his legs alone would weigh 340 kg (748 lbs) and his entire body would weigh 2380 kg (5236 lbs). Unless you're wearing tights and a cape, you're not going to be able to walk like this.