Science Lesson: Gravity, Relativity, Mass & Weight

If you’ve seen a baseball game, you’ve probably also experienced the sight of a ball going up and up, until you begin to wonder when it’s going to come down! Yet every time, the ball does fall back to earth. Why? Gravity is an attractive force that exerts a pull between two objects, such as a baseball and the earth.

According to Sir Isaac Newton’s Law of Universal Gravitation, all objects that have mass are attracted to each other. Mass is the measure of an object’s matter (what it’s made up of). The greater an object’s mass, the greater its gravitational force. The earth has a strong attracting force for objects with smaller mass (including the moon), and the sun has an attracting force on the earth and other planets in our solar system.

Albert Einstein’s General Theory of Relativity explains gravity in another way. Instead of being a force, Einstein theorized that gravity is the result of bending in space. Huge objects like the sun create a sort of well in space that causes planets to move in curved rather than straight paths. Although there is evidence to support this theory, it has not been tested enough to become a scientific law.

Weight is determined by the force of gravity pulling on an object. The stronger the pull of gravity on an object, the greater its weight. In physics, weight is measured in newtons (N), the common unit for measuring force. To calculate your weight in newtons, measure your mass on a scale (in pounds) and multiply it by 4.5.

Unlike mass, which remains constant, weight depends on the force of gravity that is exerted on it. What do you think would happen to your weight if you were in a place where gravitational force was less? Would the weight be less or greater? On the moon, where gravity is very low, you would weigh less than on earth. On the planet Jupiter, where gravity is stronger, you would weigh much more than you do here. On the sun, gravity is so strong that you would weigh about 27 times as much as on earth! The mass of your body, however, would remain constant in all of these situations.

In the illustration, weight differences based on stronger or weaker gravity are shown. If your weight was 150 pounds (lbs) on the earth, then your weight on the moon would be about 25 pounds and your weight on the sun would be about 4060 pounds—a little over two tons!

Science Lesson: Inertia & Newton’s First Law of Motion

Newton’s First Law of Motion, also known as the Law of Inertia, states that an object’s velocity will not change unless it is acted on by an outside force. This means that an object at rest will stay at rest until a force causes it to move. At the same time, an object in motion will stay in motion until a force acts on it and causes its velocity to change. This differs from what the Greek philosopher Aristotle taught: he said that all moving objects ‘want’ to be at rest, and it’s their natural state to be at rest. This appears logical, based on our own observations (e.g., tops that stop spinning, wheels that stop turning).

However, you can do a simple inertia experiment that shows that Aristotle was not right. You’ll need a hard-boiled egg and a raw egg for this activity. First, spin the hard-boiled egg on its side. When it’s going fast, gently put your fingers down on it to stop it and then move your hand off immediately when it stops. Next, spin the raw egg. Stop it in the same way you did with the hard-boiled egg. After you let go, what happens? The egg should start to turn again. This is because the motion of the liquid within the egg is still going; the force you exerted was not enough to stop both the inertia of the shell and the inertia of the liquid inside of it. If you held the egg longer, enough force would have been exerted to stop the egg completely.

The results of the experiment fit in with Newton’s First Law: the natural state of objects is not rest. Instead, an object will continue to remain in one state until sufficient outside force acts upon it, either to put it in motion or to bring it to rest. This is why tops and wheels eventually will stop turning—the outside force of friction is working on them.

The greater mass or velocity an object has, the greater its inertia. You can test this the next time you’re at the grocery store. It takes a strong push to get a loaded shopping cart moving, but once it gathers speed it keeps going even if you let go of the handle. When you stop a moving cart full of groceries, it takes much more force to stop it than an empty cart (one with less mass). Likewise, it takes more force to brake a fast-moving bike than a slow one (one with less velocity), even though the mass of each is equal.

Science Lesson: Newton’s Second & Third Laws of Motion

Newton’s Second Law of Motion states that ‘when an object is acted on by an outside force, the strength of the force equals the mass of the object times the resulting acceleration’. In other words, the formula to use in calculating force is force=mass x acceleration. Opposing forces such as friction can be added or subtracted from the total to find the amount of force that was really used in a situation. (Remember, force is measured in newtons.)

You can demonstrate this principle by dropping a rock or marble and a wadded-up piece of paper at the same time. They fall at an equal rate—their acceleration is constant due to the force of gravity acting on them. However, the rock has a much greater force of impact when it hits the ground, because of its greater mass. If you drop the two objects into a dish of sand or flour, you can see how different the force of impact for each object was, based on the crater made in the sand by each one.

Another way to show this is two push off two toy cars or roller skates of equal mass at the same time, giving one of them a harder push than the other. The mass is equal in both, but the acceleration is greater in the one that you exerted greater force on.

Stated simply, Newton’s Third Law of Motion is that ‘for every action, there is an equal and opposite reaction’. If your kids have roller skates, they can demonstrate how this works. What happens when they’re standing still in skates and throw a ball hard? The force of throwing the ball pushes the skater in the other direction.

You can also demonstrate this using Newton’s Swing. This apparatus consists of steel balls suspended on a frame. When the ball on one end is pulled back and then let go, it swings into the other balls. The ball on the opposite end then swings up with an equal force to the first ball, as shown in the illustration on the right. The force of the first ball causes and equal and opposite reaction in the ball at the other end.


Thrust is an important result of Newton’s Third Law. In a rocket, the pressure from the gases in the rocket pushes backward, causing a reaction that moves the rocket forward.

Science Lesson: Friction

When Aristotle theorized that all objects naturally want to be at rest, he did not realize that a force called friction is present whenever an object is in motion, and that this force opposes motion. Friction is caused by surface roughness at any point of contact with an object. Even in surfaces that appear smooth, there is roughness at an atomic level. If it’s strong enough, this frictional force can cause moving objects to slow down or stop.

You can demonstrate what friction looks like, using a board that is at least two feet long and several inches wide. Place 4-5 small objects of different shapes and textures (e.g., an ice cube, a small wood block, a plastic toy) at one end of the board. Next, gently lift the board a few inches, until any of the objects begin to slide. Which one moves first? Keep raising the board gradually, until all the objects have slid down the length of it. Which ones ‘stuck’ the longest? Less frictional force was exerted on the objects that slid first. Ask your children why those objects had less friction.

Although friction can cause us problems, it’s also a good thing—if there was no surface friction, we wouldn’t be able to move. Friction is also what holds cars on the road when they go around a curve. Wet or icy roads and sidewalks are more difficult to travel because the water and ice reduces the frictional force.

Noteworthy Scientist: Sir Isaac Newton (1643-1727)

According to an older calendar, Isaac Newton was born on Christmas Day, 1642; but when adjusted to fit the new Gregorian calendar which England adopted a decade later, the birth date ends up being January 4, 1643. Newton entered Trinity College, Cambridge in 1661, later in age than most university students. The school closed down due to a plague epidemic shortly after he received his Bachelor’s degree, so he returned to his home in Lincolnshire.

In the following two years, Newton began research in physics and in calculus, which he was apparently the first to develop. However, he did not publish until later, after Gottfried Wilhelm Leibnitz had independently discovered the same things and published his findings. When the school reopened, Newton returned for a Master’s degree and a fellowship.

There he began a brilliant career in scientific research and theory. He demonstrated that white light is made up of rays that produce a different-colored spectrum when refracted through a prism. (Earlier scientists had thought that white light was a single entity.) In 1704, after the death of his rival Robert Hooke (another famous scientist), he published Opticks. During this time, he developed his three laws of motion and in 1687 wrote his most famous work, the Principia. Among other things, the book dealt with explaining planetary motion and tides using his Universal Law of Gravitation.

Newton, although he devoted himself to studying the Bible and apparently believed it was authoritative, rejected the essential doctrine of the Trinity. He believed that at the famous church council of Nicea, the anti-trinitarian beliefs of Arius were correct, contrary to the agreement of the council that is expressed in the confession of faith called the Nicene Creed.

Newton was born under the Parliamentary Commonwealth, and lived during the very different reigns of Charles II, James II, William and Mary, Queen Anne (last of the Stuarts), and George I (of the House of Hanover). After Newton’s defense of the university against James II’s policies, Cambridge elected him to the Parliament that offered the crown to William and Mary. Afterwards he was made Master of the Mint, a position which made him rich but that he also took seriously. During the reign of Queen Anne, Newton was president of the prestigious scientific group called the Royal Society. He was also knighted by the Queen, an unprecedented honor for an English scientist.

In summing up Newton’s work, Alexander Pope says it best in this couplet that he wrote in Newton’s honor: ‘Nature and Nature’s laws lay hid in night; God said, Let Newton be! and all was light.’

The Scientific Speaker

Tribology is the study of friction, lubrication, and the wear on interacting surfaces relative to motion.

Lubricate comes from the Latin word for ‘slippery’.

Science Links

Calculate your weight on other planets at this site.