Science Education: Newton’s Law of Motion

Isaac Newton was one of the most famous and influential mathematicians and scientists who ever lived. Born in 1642, Newton studied at Trinity College in Cambridge, and he eventually went on to earn a master’s degree from there and serve as a professor of mathematics. In 1687, Newton published his famous Philosophiae Naturalis Principia Mathematica, which contained his three laws of motion. He developed his theories after studying historical works of Galileo as well as contemporaries like Robert Hooke and Edmund Halley. Newton conducted experiments with pendulums and studied the stars and moon to make key discoveries about how force and mass interact.

Newton’s First Law of Motion

Newton’s First Law of Motion is also known as the law of inertia. This law states that an object that is not moving will stay that way unless it is acted on by an outside force. The same is also true for objects in motion: Until a force interferes with that object’s motion, it will continue to move in the same manner in the same direction.

Force is a key concept in all of Newton’s laws. In the first law, unbalanced force is the focus. An unbalanced force is simply a force that is great enough to affect the object’s inertia. If two forces are equal in strength, then they will cancel each other out. A good example of this is a book sitting on a table: The book is being pulled down by the force of gravity but is also being pushed up by the table. The book will not move until one of those forces is changed.

If an object is already in motion, an unbalanced force might be strong enough to change the object’s direction or slow or stop the movement completely. Consider a bat connecting with a baseball. The bat’s force will change the direction and speed of the baseball. If the bat doesn’t connect with the ball, it continues on its path until it hits the catcher’s mitt.

A classic example of Newton’s First Law is a car crash. When a car strikes another object, the car’s motion stops suddenly, but the people inside the car continue to move on their original course until a force acts against this motion. This is why wearing a seat belt is so important: It helps to cancel out your inertia if you are in a car accident. Without it, you risk being thrown from the vehicle as your body’s inertia plays out.

Newton’s Second Law

Newton’s Second Law is built on one of the cornerstone formulas in physics: F=ma. This means that the net force is equal to the mass of an object multiplied by the acceleration of an object. Force and acceleration have a direct relationship: They will proportionally increase or decrease in response to each other as long as the mass stays constant. If the net force doubles, so will the acceleration, and vice versa. Mass and force have an inverse relationship with each other: If the mass doubles, the force will be decreased by half.

External forces can affect velocity. Depending on the direction of the force, it can alter the velocity of the object, as can forces such as gravity and friction. The basic equation of F=ma does not factor in these other forces.

Newton’s Second Law can be applied to our daily world. If a truck hits a small car and they are traveling at the same rate of acceleration, the small car will still sustain more damage because the truck’s greater mass will mean that a larger force is exerted on the car. Another example of this law can be found in analyzing the fuel economy of cars: Smaller cars take less force to move and therefore need less fuel to create that force.

Newton’s Third Law

The third of Newton’s laws states that for every action, there is an equal and opposite reaction. This means that forces always come in pairs. For example, when you jump into the air, your feet push the ground with a certain amount of downward force; exerting a force with your body downward causes your body to move upward, so you leave the ground.

Other examples of Newton’s Third Law are seen in swimming and flying. When a swimmer strokes through the water, they exert a backward force on the water. The water’s opposite reaction is what moves the swimmer forward. When a bird flies, its wings push against the air and the air pushes back. Rockets and planes fly using the exact same principle.

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