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PHYS2001_Ch. 4(1)

# PHYS2001_Ch. 4(1) - Ch 4 Forces and Newton's Laws If I have...

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Ch. 4 Forces and Newton’s Laws If I have seen far, it is because I have stood on the shoulders of giants. - Isaac Newton When an object falls and hits the ground, it remains at rest. - Aristotle 350 B.C. It wasn’t until the 1600’s that these ideas were expanded upon: Objects continue with uniform motion if no external forces act on them. - Galileo 1609 In the previous chapters we used the concepts of displacement, velocity, and acceleration to study the motion of objects. Now we want to know how the object moves and why it moves, or, just as importantly, why an object doesn’t move? To do this, we need to develop the concept of a Force . Simply put, a Force in physics is a push or a pull on an object. We will discover that there are several types of forces that can act on an object, but they all fall into two broad categories: Contact forces and Non- contact forces . But, there is one commonality among all forces: They each have the ability to change the motion of an object.

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1687 Newton publishes his Principia , or “The Mathematical Principles of Natural Philosophy”. Newton was born on Christmas in 1642, the same year Galileo died. He was born premature and not expected to survive. He enrolled in Trinity College at Cambridge when he was 18. There he studied math and optics. After graduation he lived in seclusion in the countryside to avoid the second Plague in Europe. Over the next year and a half, he did the following: Developed the binomial theorem of mathematics. Began his study of mechanics. Developed the Universal Law of Gravitation. Analyzed the decomposition of light into its spectrum. Developed both forms of calculus, integral and differential. When he finished, he was 25! In this course, we will be interested in his studies of mechanics and gravitation.
Newton expanded on Galileo’s ideas about the motion of an object in the absences of forces: An object at rest tends to stay at rest, and an object in motion tends to stay in motion at a constant speed, moving in a straight line unless acted upon by some external force. Newton’s First Law The property of objects that make them obey Newton’s First Law is something called Inertia . Inertia is a measure of how much mass an object has. In other words, how much matter it contains. For example, a lead brick has a lot more mass (and thus inertia) than does a penny. The quantitative measure of inertia is Mass. In SI units, mass comes in kilograms [kg]. Key point: If the net force on an object is zero, then the object will remain at rest or continue on at constant velocity . Thus, if there is a net force acting on an object, then the object’s velocity WILL change. Thus, the object must be accelerating! = t v a (Newton’s First Law is often referred to as the Law of Inertia.)

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So, forces are synonymous with accelerations.
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