READ: Gravity
READ: Gravity
Long, long ago, when the universe was still young, an incredible force caused dust and gas particles to pull together to form the objects in our solar system. From the smallest moon to our enormous sun, this force created not only our solar system, but all the solar systems in all the galaxies of the universe. The force is gravity.
Gravity has traditionally been defined as a force of attraction between things that have mass. According to this concept of gravity, anything that has mass, no matter how small, exerts gravity on other matter. Gravity can act between objects that are not even touching. In fact, gravity can act over very long distances. However, the farther two objects are from each other, the weaker is the force of gravity between them. Less massive objects also have less gravity than more massive objects.
Earth’s Gravity
You are already very familiar with Earth’s gravity. It constantly pulls you toward the center of the planet. It prevents you and everything else on Earth from being flung out into space as the planet spins on its axis. It also pulls objects that are above the surface—from meteors to skydivers—down to the ground. Gravity between Earth and the moon and between Earth and artificial satellites keeps all these objects circling around Earth. Gravity also keeps Earth and the other planets moving around the much more massive sun.
Q: There is a force of gravity between Earth and you and also between you and all the objects around you. When you drop a paper clip, why doesn’t it fall toward you instead of toward Earth?
A: Earth is so much more massive than you that its gravitational pull on the paper clip is immensely greater.
Gravity and Weight
Weight measures the force of gravity pulling downward on an object. The SI unit for weight, like other forces, is the Newton (N). On Earth, a mass of 1 kilogram has a weight of about 10 Newtons because of the pull of Earth’s gravity. On the moon, which has less gravity, the same mass would weigh less. Weight is measured with a scale, like the spring scale shown in the Figure below. The scale measures the force with which gravity pulls an object downward.
Newton’s Law of Universal Gravitation
You may have heard a story about Isaac Newton coming up with the idea of gravity when an apple fell out of a tree and hit him in the head. The story isn’t true, but seeing how things like apples fall to Earth helped Newton form his ideas about gravity, the force of attraction between things that have mass.
Newton was the first one to suggest that gravity is universal and affects all objects in the universe. That’s why Newton’s law of gravity is called the law of universal gravitation. Universal gravitation means that the force that causes an apple to fall from a tree to the ground is the same force that causes the moon to keep moving around Earth. Universal gravitation also means that while Earth exerts a pull on you, you exert a pull on Earth. In fact, there is gravity between you and every mass around you—your desk, your book, your pen. Even tiny molecules of gas are attracted to one another by the force of gravity.
Factors That Influence the Strength of Gravity
Newton’s law also states that the strength of gravity between any two objects depends on two factors: the masses of the objects and the distance between them.
- Objects with greater mass have a stronger force of gravity between them. For example, because Earth is so massive, it attracts you and your desk more strongly that you and your desk attract each other. That’s why you and the desk remain in place on the floor rather than moving toward one another.
- Objects that are closer together have a stronger force of gravity between them. For example, the moon is closer to Earth than it is to the more massive sun, so the force of gravity is greater between the moon and Earth than between the moon and the sun. That’s why the moon circles around Earth rather than the sun. You can see this in the Figure below.
Einsteins Concept of Gravity
In the late 1600s, Isaac Newton introduced his law of gravity, which identifies gravity as a force of attraction between all objects with mass in the universe. The law also states that the strength of gravity between two objects depends on their mass and distance apart. Newton’s law of gravity was accepted for more than two centuries. It can predict the motion of most objects and was even used by NASA to land astronauts on the moon. It’s still used for most practical purposes. However, Newton’s law doesn’t explain why gravity occurs. It only describes how gravity seems to affect objects. There are also some cases in which Newton’s law doesn’t even describe what happens.
Q: Newton expressed his ideas about gravity as a law. A law in science is a description of what always occurs in nature. For example, according to Newton’s law, objects on Earth always fall down, not up. What is needed to explain gravity?
A: A theory is needed to explain gravity. In science, a theory is a broad explanation that is supported by a great deal of evidence.
Einstein Explained It All
In the early 1900s, Albert Einstein came up with a theory of gravity that actually explains gravity rather than simply describing its effects. Einstein showed mathematically that gravity is not really a force that of attraction between all objects with mass, as Newton thought. Instead, Einstein showed that gravity is a result of the warping, or curving, of space and time, which made up the same space-time “fabric.” These ideas about space-time and gravity became known as Einstein’s theory of general relativity.
Visualizing Einstein’s Ideas
Einstein derived his theory using mathematics. However, you can get a good grasp of it with the help of a simple visual analogy. Imagine a bowling ball pressing down on a trampoline. The surface of the trampoline would curve downward instead of being flat. Now imagine placing a lighter ball at the edge of the trampoline. What will happen? It will roll down toward the bowling ball. This apparent attraction to the bowling ball occurs because the trampoline curves downward, not because the two balls are actually attracted to one another by an invisible force called gravity.
Einstein theorized that the sun and other very massive bodies affect space and time around them in a way that is similar to the effect of the bowling ball on the trampoline. The more massive a body is, the more it causes space-time to curve. This idea is represented by the Figure below. According to Einstein, objects move toward one another because of the curves in space-time, not because they are pulling on each other with a force of attraction. Einstein’s theory is supported by evidence and widely accepted today, although Newton’s law is still used for many calculations.
This diagram shows how Earth’s mass bends the “fabric” of space and time around it, causing smaller objects such as satellites to move toward Earth.
Summary
- Gravity has traditionally been defined as a force of attraction between things that have mass. The strength of gravity between two objects depends on their mass and their distance apart.
- Earth’s gravity constantly pulls matter toward the center of the planet. It also keeps moons and satellites orbiting Earth and Earth orbiting the sun.
- Weight measures the force of gravity pulling on an object. The SI unit for weight is the Newton (N).
- Newton’s law of universal gravitation states that the force of gravity affects everything with mass in the universe.
- Newton’s law also states that the strength of gravity between any two objects depends on the masses of the objects and the distance between them.
- Newton’s law of gravity can predict the motion of most but not all objects. It also doesn’t explain gravity. It only describes its effects on the motion of objects.
- Einstein showed mathematically that gravity is not really a force of attraction between all objects with mass, as Newton thought. Instead, gravity is a result of the warping of space-time.
- Einstein’s ideas have been supported by evidence and are widely accepted today.