How do Things Fall?

The collapse of certain very large stars creates a black hole, shown in this artist's conception. The gravitational force caused by a black hole is so great that even light cannot escape. Anything that enters a black hole disappears from the observable universe. A black hole has an infinite density and collapses space around it.

Engineering Connection

We experience gravity so pervasively in our day-to-day lives that we may take it for granted. But, engineers must understand gravitational attraction when they design scales, elevators, airplanes, bridges and dams. And, since gravitational constants are different for places other than Earth, engineers must especially take gravity and weight into consideration when they design spacecraft, and moon or planetary vehicles.

Introduction/Motivation

The effects of the force of gravity are far-reaching and dramatic. All matter in the universe is pulled towards all other matter by gravitational attraction. Objects that have more mass pull more than objects with less mass. That's why we say we are pulled towards the surface of the Earth, rather than saying the Earth is pulled to us, when, in fact, both statements are true!

Gravity is the dominant force in the universe. Gravity keeps planets in their orbits around the sun, and the moon in orbit around the Earth. Gravity is why the planets are round, and gravity pulled enough mass together to make the Sun hot. On a smaller scale, gravity is why baseballs don't always fly out of the ball park when they're hit.

It is useful to know about the force of gravitational attraction to make predictions about how things fall or stay up. If scientists and engineers didn't know how the force of gravity behaves, they couldn't build spaceships, airplanes or buildings! Luckily, since gravity is everywhere, there are some good ways to learn more about how it works. People can measure how strong gravity is, and what direction in which it pulls. We can learn how things fall with some experiments that we can do anywhere in the world, or even at out-of-the-world places like the moon!

The two satellites in the GRACE project — Tom and Jerry —are used to precisely measure small changes in Earth's gravitational field.

Understanding gravity is so important that scientists have launched two satellites for the purpose of measuring the force of gravity all over the Earth. The gravity field of the Earth is fairly uniform and differences are not easily perceptible. However, if you have a very sensitive gravity meter, you can measure small variations in the Earth's gravity field. Aerospace engineers in the US and Germany designed and built two satellites, nicknamed Tom and Jerry, and launched them into space in March 2002. Tom followed Jerry in the same orbit, with Tom always about 200 kilometers behind Jerry. Small changes in gravity under the satellites caused small changes in the distance between them. Other forces like drag from the atmosphere also changed the distance between Tom and Jerry. Scientists and engineers developed instruments that allowed them to measure only the movements in Tom and Jerry caused by gravity. This allowed the satellites to monitor movement of mass in the ocean, atmosphere, land surface and polar ice until about 2015. In 2018, two satellites were sent to replace the original Tom and Jerry, which had been retired. These satellites are collectively called GRACE (Gravity Recovery and Climate Experiment)

Lesson Background and Concepts for Teachers

On the Shoulders of Giants: The Law of Universal Gravitation

Gravity is responsible for much of the structure of the universe. Indeed, it is the dominant force in the universe. The planets continuously travel around the sun in elliptical orbits. Many moons orbit the various planets. Saturn's rings are composed of orbiting bodies ranging from huge ice boulders to tiny ice particles. The asteroid belt consists of countless chunks of material, all orbiting the sun. Satellites are held in their orbit by Earth's gravitational pull. Even Earth's nearly spherical shape is caused by gravity. And, any object you drop on Earth will fall toward the center of the Earth.

Many important scientific developments preceded Sir Isaac Newton and enabled him to understand the force of gravity. Galileo developed the ideas of acceleration and inertia through careful observation and experimentation with inclined planes and falling objects. Tycho Brahe built the first astronomical observatory capable of precise measurement and compiled 20 years of data showing the planets' motions and positions throughout the year. Johannes Kepler used Brahe's data to determine the laws of planetary motion (Kepler's laws of planetary motion). He was the first to show that the planets move in elliptical orbits with the sun at one focus of the ellipse (Kepler's first law). Kepler also determined that the speed of a planet changes as it moves through its orbit, with the planet moving faster when it is nearer the sun (Kepler's second law). Finally, Kepler determined that the square of the time for a planet to complete one orbit is proportional to the cube of its average distance from the sun (Kepler's third law). Newton was the first person to understand how the work of Galileo and Kepler fit together.

First, Newton recognized that an object moves in a straight line unless a force acts upon it (Newton's first law of motion). Using Kepler's second law, Newton was able to show that the force acting on the planets was toward the sun, because the orbits curved toward the sun. Based on Kepler's third law, Newton was able to determine that the force from the sun is inversely proportional to the square of the distance between the sun and a planet. Newton had also observed that there is a force between a falling object and the Earth that makes the object fall. He conceived that the force between the Earth and a falling object was the same as the force between the sun and a planet.

So, although Newton didn't discover gravity, he realized that gravity was a universal force and determined its magnitude. He was the first to understand that every object in the universe is gravitationally attracted to every other object in the universe. Newton said that he was able to comprehend gravity because he "stood on the shoulders of giants," crediting the important work of such thinkers, astronomers and scientists as Galileo, Brahe and Kepler.

Legend has it that a young Isaac Newton was sitting under an apple tree when he was bonked on the head by a falling piece of fruit, a 17th-century “aha moment” that prompted him to suddenly come up with his law of gravity. In reality, things didn’t go down quite like that. Newton, the son of a farmer, was born in 1642 near Grantham, England, and entered Cambridge University in 1661. Four years later, following an outbreak of the bubonic plague, the school temporarily closed, forcing Newton to move back to his childhood home, Woolsthorpe Manor. It was during this period at Woolsthorpe (Newton returned to Cambridge in 1667) that he was in the orchard there and witnessed an apple drop from a tree. There’s no evidence to suggest the fruit actually landed on his head, but Newton’s observation caused him to ponder why apples always fall straight to the ground (rather than sideways or upward) and helped inspired him to eventually develop his law of universal gravitation. In 1687, Newton first published this principle, which states that every body in the universe is attracted to every other body with a force that is directly proportional to the product of their masses and inversely proportional to the square of the distance between them, in his landmark work the “Principia,” which also features his three laws of motion.

In 1726, Newton shared the apple anecdote with William Stukeley, who included it in a biography, “Memoirs of Sir Isaac Newton’s Life,” published in 1752. According to Stukeley, “After dinner, the weather being warm, we went into the garden, & drank thea under the shade of some apple trees… he told me, he was just in the same situation, as when formerly, the notion of gravitation came into his mind…. occasion’d by the fall of an apple, as he sat in a contemplative mood.”

The esteemed mathematician and physicist died in 1727 and was buried at Westminster Abbey. His famous apple tree continues to grow at Woolsthorpe Manor.

What is the Law of Universal Gravitation? How Does It Describe Gravity?

The Law of Universal Gravitation, one of Newton's great achievements, states that the gravitational force between two objects is proportional to the masses of the objects and inversely proportional the square of the distance between them:

The gravitational force between any two objects is always attractive. That is, any two objects will accelerate toward each other due to their gravitational attraction. When an apple falls from a tree, the Earth pulls on the apple, but the apple also pulls on the Earth.

How Does Gravity Compare with the Other Forces?

There are four fundamental forces in nature: the gravitational force, the electromagnetic forces, the strong nuclear force and the weak nuclear force. Gravity is the weakest of the four fundamental forces. Gravity is always attractive, while the electromagnetic force can be either attractive or repulsive. The strong and weak nuclear forces dominate at distances smaller than the size of the nucleus of an atom. The electromagnetic force dominates at the atomic level. But gravity dominates in the universe, even though it is the weakest force, because there is so much matter in the universe and much of it is aggregated into sizeable lumps. So, beyond the atomic level, gravity is the dominant force.

What Is Weight? How Can an Object Be Weightless?

A force produces acceleration of the body on which the force is applied. Any two objects will accelerate toward each other due to their gravitational attraction. If they are already touching, they still exert a gravitational force on each other. Weight is the force an object exerts against some supporting structure such as a floor or a scale. An object's weight is equal to the product of its mass and the gravitational acceleration constant:

So, an object's weight varies depending on the gravitational acceleration it is experiencing, whereas its mass is always the same. You would weigh much more on Jupiter and much less on the moon than you do on Earth. You even weigh slightly more on the top of a mountain compared to your weight in the deepest ocean trench because there is more mass between you and the center of the Earth, and therefore a greater gravitational acceleration, when you are on top of a mountain.

An object is weightless when it is in free fall — its acceleration is equal to the gravitational acceleration. Even though there is still gravitational force acting on an astronaut in free fall the astronaut feels weightless because there is no supporting structure against which to feel weight. Even if a scale were held at the astronaut's feet, it wouldn't read a weight for the astronaut because it would be falling with the same acceleration as the astronaut. Of course, the astronaut still has weight, but cannot sense it due to the lack of a supporting force. This type of weightlessness isn't "true weightlessness." True weightlessness can only occur when all the gravitational forces on a body are exactly canceled such that the body experiences no gravitational force at all and, therefore, travels in a straight line.

G-Force One -- the "Vomit Comet"

Aboard a specially modified Boeing 727-200, G-FORCE ONE®, weightlessness is achieved by doing aerobatic maneuvers known as parabolas. Specially trained pilots perform these aerobatic maneuvers which are not simulated in any way. ZERO-G passengers experience true weightlessness.

Before starting a parabola, G-FORCE ONE® flies level to the horizon at an altitude of 24,000 feet. The pilots then begins to pull up, gradually increasing the angle of the aircraft to about 45° to the horizon reaching an altitude of 34,000 feet. During this pull-up, passengers will feel the pull of 1.8 Gs. Next the plane is “pushed over” to create the zero gravity segment of the parabola. For the next 20-30 seconds everything in the plane is weightless. Next a gentle pull-out is started which allows the flyers to stabilize on the aircraft floor. This maneuver is repeated 12-15 times, each taking about ten miles of airspace to perform.

In addition to achieving zero gravity, G-FORCE ONE® also flies a parabola designed to offer Lunar gravity (one sixth your weight) and Martian gravity (one third your weight). This is created by flying a larger arc over the top of the parabola.

G-FORCE ONE® flies in a FAA designated airspace that is approximately 100 miles long and ten miles wide. Usually three to five parabolas are flown consecutively with short periods of level flight between each set.

Cost and What you Get

$5,400 per person
~ 30 seconds of reduced gravity
Flight Suit

What zero G's looks like

What 5 G's looks like.

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