What Is Newton’s Second Law of Motion?
Lily L.
2012


Isaac Newton’s second law of motion is the most significant of all laws created by this acclaimed scientist. The second law of motion is known throughout the world and was a part of the scientific revolution, as it was created during the time period of Copernicus to Isaac Newton. It is essential to know when studying the world of physics.

Isaac Newton
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Figure 1: Isaac Newton

Isaac Newton was born on January 4, 1643, and grew up on a farm in Lincolnshire, in the eastern part of England. His father had died three months before he was born. His mother remarried and left him with his grandmother, as she was living with her second husband at the time. She did not live with Isaac until the death of her second husband eight years later. Newton clearly did not like this. He wrote in his notebook that he had threatened his "father and mother Smith to burne them and the house over them.” In his youth, Isaac Newton was incredible at making mechanical models. He created many sundials and was able to tell the time of the Sun with extraordinary precision. Because of his rare talent and interest in science, Newton attended Trinity College, Cambridge. There, Newton delved into the fields of astronomy and mathematics. The courses at the college were centered on the ideas of Aristotle, but Newton individually researched the works of more current philosophers. This most likely explains why Newton’s laws differ from the theories of Aristotle, as Aristotle did not realize what major role frictional forces played in the acceleration of objects. In 1665, the college closed because of the Great Plague. Newton journeyed home, and developed many theories about color and gravity. Several years later, he invented a remarkable telescope tremendously important to the scientific world, and continued to test his theories by performing different experiments.


The Second Law of Motion
During Isaac Newton’s scientific career, he was asked by Edmond Halley to discover what force caused a planet to travel in an elliptical orbit. Edmond Halley was a mathematician and astronomer who was also from England. Isaac’s end result was that two objects, like the Sun and the Earth, attract one another with a force that varies depending on the product of their masses, and the force decreases when there is a larger distance between each object. He turned his theory into three laws of motion. Newton’s three laws (third law) were first stated in his book, Philosophiae Naturalis Principia Mathematica, (mathematical principles of natural philosophy), also known as the Principia. This book about the three laws (first law) was published in 1687. Isaac Newton’s second law is a description that uses numbers about how force can change the motion of a body. Force acting on a body can alter the amount of momentum it has, and the direction in which it is traveling. There has to be an unbalanced force applied to a body for it to accelerate. An object also accelerates if the degree of its velocity increases or decreases, and if the direction of motion changes. The acceleration travels in the same path as the entire force, as the force alters the velocity; it does not sustain it. The equation for this is F=ma, or Force=mass x acceleration. The equation F=ma can be proven anywhere in the universe, because the mass of a body is always the same, rather than the weight of a body, which is different depending on the force of the gravitational pull. Newton’s second law of motion also describes why different objects can fall to the ground with the same acceleration. But more force is needed to accelerate a heavier object than a lighter one.

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Figure 2a: Unbalanced force acting on a lighter 75-gram ball.
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Figure 2b: Unbalanced force acting on a heavier 140-gram ball.




An Example of The Second Law
A person is pushing an empty cart. It does not take much force to accelerate the cart. The person needs more force to push a full cart than an empty cart. The cart will move faster if the person gives the cart a hard push than if he/she gently pushes the cart. This proves that the acceleration of a body increases as the force applied to it increases.


Glossary
Acceleration: A vector quantity, as it consists of a number, which is called magnitude. It also has an exact direction.

Ellipse: An oval shape.

Motion: The product of mass and velocity; momentum.

Unbalanced Force: Two forces in opposite directions. One force is greater than the other.

Velocity: A speed in a given direction.


Citations
Books:
Allaby, Michael and Gjertsen, Derek. “Sir Isaac Newton.” Makers of Science: Vol 1. New York: Oxford University Press, 2002.

Daintith, John and Gjertsen, Derek ed. “Sir Isaac Newton.” The Grolier Library of Science Biographies: Vol. 7. Danbury: Grolier Educational, 1997. Print.

Greenberger, Robert. How Do We Know the Nature of Energy. New York : Rosen, 2005. Print.

Gregersen, Erik, ed. Physics Explained: The Britannica Guide to Heat, Force, and Motion. New York: Britannica Educational Publishing, 2011. Print.

Kahan, Peter. Motion, Forces, and Energy. Needham: Prentice Hall, 2000. Print.

McGrath, Kimberley A., and Blachford, Stacey, ed. The Gale Encyclopedia of Science: Vol. 1. Detroit : Gale Group, 2001. Print.


Databases
"Matter." Compton's by Britannica. Encyclopædia Britannica Online School Edition.Encyclopædia Britannica, Inc., 2012. Web. 13 Sept. 2012.

“Newton's laws of motion." Encyclopædia Britannica Online School Edition.Encyclopædia Britannica, Inc., 2012. Web. 13 Sept. 2012.

Website
Dr. Cardall and Dr. Daunt. “Newton’s Three Laws of Motion.” Department of Physics and Astronomy. Web. Sept, 27, 2012.