Laws of Motion
Sir Isaac Newton was an English physicist and mathematician, who in the 17th and 18th centuries was able to solve several mysteries that have stumped humans for centuries. His remarkable work created modern day physics as we know it.
Newton’s First Law of Motion: The Law of Inertia
Newton observed that a ball rolling on a smooth surface will roll farther than a ball rolling on a rough surface such as grass. He realized that unless another force acted upon it, the ball would keep on rolling. This led him to his first law of motion.
Inertia is a property of matter that allows an object in motion to remain in motion. So if you push a ball, its inertia will naturally continue it in that motion. So, a roller coaster moving down a hill due to gravity can climb another hill due to inertia. Of course, as it climbs, the force of gravity is still pushing it back to Earth, and friction slows the coaster too. Thus, the coaster will not be able to climb higher than the previous hill. You can also see inertia at work if you have seen children playing on a “Slip n’ Slide.” By just throwing themselves onto the slick yellow pad, they are able to travel long distances.
Likewise, inertia describes how an object at rest will remain motionless. A cup sitting on a table will not move unless something causes it to move. Inertia is what allows it to stay at rest. Every object has inertia, whether it is moving or still.
The Force of Friction
Common sense tells us that if you roll a ball along a smooth, flat surface, even with inertia, it will not roll forever. Eventually, the ball will come to a stop. Yet, if Newton’s Law of Inertia is true, what force stops it?
Friction is the force that will stop a moving object rolling on a flat surface. Friction is everywhere, since there are very few truly “frictionless” surfaces, even in the air. Have you ever stuck your hand out the window of a moving car? If you have, you would have felt air friction, more commonly known as air resistance. Friction, from the air and from the contact of the coaster and the tracks, slows a coaster down.
What causes friction you may ask. Friction is caused by the electric forces between the atoms and molecules of the two objects in contact. So the atoms and molecules in your hand are actually meeting resistance from the atoms and molecules in the air. Likewise, the molecules of a ball will interact with the molecules on a flat surface, changing the kinetic energy of the ball into heat energy. Friction always produces heat energy, and if you have ever received a “rug burn,” you have first-hand experience with friction and heat energy. This change from kinetic energy to heat energy demonstrates the Law of Conservation of Energy.
Without friction, what would our lives be like? Cars would be useless! You would not be able to write with a pen on paper! None of your possession would stay in place! Imagine trying to walk anywhere.
Newton’s Second Law of Motion: The Law of Acceleration
The Law of Acceleration describes how, if an unequal force is applied to an object, the object will change speed, either by going faster, accelerate, or slowing down, decelerate. Again, let us use the roller coaster as a model. First, we must apply an upward force stronger than the downward force of gravity. This will allow the roller coaster to climb. Once it reaches the top, it is filled with gravitational potential energy, and as we stop the upward force, gravity will take over. Since there is not an equal force pushing up on the coaster, this unbalanced force causes the coaster to accelerate downward. In other words, as an object falls, its speed will increase
Newton’s Law of Acceleration is often broken into two parts, because it is actually based on two observations made by Newton. The first observation was that an object, such as a stick, could be thrown farther and faster simply by throwing it harder or with more force.
Newton’s Third Law of Motion: The Law of Interaction
This law is extremely important, but can be more difficult to envision. Try this example: When a bat hits a baseball, the baseball will fly through the air. Right? The ball will do so because the bat exerts a force onto the baseball, but also because the baseball exerts a force back onto the bat. If it did not do this, the bat would just sail right through the ball.
Here’s another example: a cup is sitting on a table. If there were not an equal and opposite force pushing against the downward force of gravity, the cup would fall through the table. Thus, the table is actually pushing back, with an equal force, in the opposite direction of gravity.
Newton’s Law of Universal Gravitation
Earlier, we introduced the idea of gravity. Newton discovered the properties of gravity, not by sitting underneath an apple tree and being hit on the head by a falling apple, but rather by using his laws of motion in a quest to answer several mysteries.
Before Newton’s time, people did not know the answer to the following questions:
- If the Earth is round, why do we not fall off it? How and why does the Moon circle the Earth?
- Why do all objects thrown up into the sky always eventually come back down?
Newton eventually described a universal force, called gravity, which applies to all objects. His law states three things:
The Law of Universal Gravitation
Gravity is an attractive force that pulls two objects toward one another.
Gravity is stronger coming from heavy (or more mass) objects when compared to light objects. The force of gravity weakens as the distance between objects increases.
All objects in the universe feel the effects of gravity; we commonly feel the Earth’s gravitational pull towards its center. The Sun, being much larger than the planets, pulls all of them toward itself, and each planet pulls back against the Sun. Since they are much smaller than the Sun, their pull is much weaker.
Even humans have a gravitational pull toward other humans, except that it is relatively weak and not noticeable. So even if you find someone “unattractive,” you are each pulling yourselves toward one another. We might not notice it because the only gravitational force we are aware of is from the Earth itself. Therefore, the force of gravity diminishes with the decrease in size. Likewise, the force of gravity diminishes as you draw away from an object. Gravity can lose ¼ of its strength each time the distance between two objects doubles.