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# Section05 - Physics 204A Class Notes Section 5 Newton's...

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Physics 204A Class Notes 5-1 Section 5 – Newton’s Laws of Motion Outline 1. Newton's Laws of Motion a. Newton's First Law - The Law of Inertia b. Newton's Second Law - F=ma c. Newton's Third Law - The Law of Action/Reaction 2. Forces on Macroscopic Objects a. Weight - The Force of Gravity b. Tension in Strings c. The Normal Force from Surfaces d. The Frictional Force from Surfaces We can now thoroughly describe what do objects do? The question that remains is, “Why do they do what they do?” We will address this topic for the rest of the course. In this chapter the why is explained in terms of the concept of force. 1. Newton's Laws of Motion Here’s the thing with laws: they can’t be derived by manipulating some equations. They can only be postulated and then verified by lots of experimental tests. So, it takes a great deal of genius and creativity to produce and valid law. Newton first presented his laws of motion near the end of the 17 th century and they still are a challenge to understand. Nonetheless, Newton’s Laws do a great job of explaining why objects do what they do. Our challenge is to understand the concept of force and apply these laws. Newton's First Law - The Law of Inertia “Every object will move with a constant velocity unless a force acts on it.” The First Law establishes two key concepts associated with the idea of force. First, contrary to nearly everyone’s intuition, the “natural state” of motion of all objects is not rest, but any constant velocity. That is, objects left to their own devices don’t come to rest. They keep moving with whatever velocity they had when they were finally left alone to do what they do naturally. In our everyday life, objects tend to come to rest. Newton’s genius is that he realized that this was due to the action of a force we call friction. In space, far from the effects of gravity and air resistance, objects float along with whatever velocity they have from their previous interactions illustrating the First Law. Second, force is defined to be the agent that causes velocity to change. You can tell that a force is present because the velocity of an object will change. Conversely, if the velocity of an object changes, you can be sure a force is acting.

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Physics 204A Class Notes 5-2 Example 5.1: For each situation listed describe the motion in terms of Newton’s First Law and draw a sketch indicating the forces on the object. Ignore the force due to gravity and any forces due to surfaces. (a)A block at rest on a table. (b)The block is being pushed. (c)The block skids to rest. (d)A hockey puck sliding across frictionless ice. (e)A ball on a string twirling around in a circle. (a)Since there are no forces acting on the block, it will maintain a constant velocity. Since the velocity starts as zero, it will remain at rest.
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Section05 - Physics 204A Class Notes Section 5 Newton's...

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