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4 Newton’s Laws of Motion Lectures by James L. Pazun Goals for Chapter 4 • To understand force – either directly or as the net force of multiple components. • To study and apply Newton’s First Law. • To study and apply the concept of mass and acceleration as components of Newton’s Second Law. • To differentiate between mass and weight. • To study and apply Newton’s Third Law. • To open a new presentation of problem data in a free body diagram. Copyright © 2012 Pearson Education, Inc. publishing as Addison-Wesley Dynamics, a new frontier • Stated previously, the onset of physics separates into two distinct parts: – statics and – dynamics. • So, if something is going to be dynamic, what causes it to be so? – A force is the cause, it is either • pushing or • pulling. Copyright © 2012 Pearson Education, Inc. publishing as Addison-Wesley Types of Force Illustrated I – Figure 4.1 Copyright © 2012 Pearson Education, Inc. publishing as Addison-Wesley Types of Force II – Figure 4.2 • Single or net – Contact force – Normal force – Frictional force – Tension – Weight Copyright © 2012 Pearson Education, Inc. publishing as Addison-Wesley A force may be resolved into components – Figure 4.4 •Fx = F CosΘ •Fy = F SinΘ Copyright © 2012 Pearson Education, Inc. publishing as Addison-Wesley Components and Resultants II – Figure 4.6 • An example of component resolution. Copyright © 2012 Pearson Education, Inc. publishing as Addison-Wesley Newton’s First Law – Figure 4.7 •“Objects at rest tend to stay at rest and objects in motion tend to stay in motion.” •What that common statement of the first law often leaves out is the final phrase “until acted upon an external force”. •In one word, we say “inertia”. Copyright © 2012 Pearson Education, Inc. publishing as Addison-Wesley We determine effect with the net force. – Figure 4.8 Copyright © 2012 Pearson Education, Inc. publishing as Addison-Wesley Forces are inertial and non-inertial. – Figure 4.9 •The label depends on the position of the object and it’s observer. Copyright © 2012 Pearson Education, Inc. publishing as Addison-Wesley Mass and Newton’s Second Law I – Figure 4.11 • F=ma • We can examine the effects of changes to each component. Copyright © 2012 Pearson Education, Inc. publishing as Addison-Wesley Mass and Newton’s Second Law II – Figure 4.12 •Let’s examine some situations with more than one mass. Copyright © 2012 Pearson Education, Inc. publishing as Addison-Wesley Forces and free body diagrams I – Example 4.1 •Observe the worked example on page 108. •The forces are in equilibrium so there is no motion. •This is a good example of forces in statics. Copyright © 2012 Pearson Education, Inc. publishing as Addison-Wesley Forces and free body diagrams II – Example 4.2 •Like the previous example, we account for the forces and draw a free body diagram. •In this case, the net force is unbalanced. •This is a good example of forces in dynamics. Copyright © 2012 Pearson Education, Inc. publishing as Addison-Wesley Mass and Weight – Figure 4.17 •Perhaps saying “go weigh the object” is at fault. •Mass is a measure of “how much material do I have?” •Weight is “how hard do I push down on the floor?” Copyright © 2012 Pearson Education, Inc. publishing as Addison-Wesley We can solve for dynamic information – Example 4.4 •Knowing force and mass we can sketch a free body diagram and label it with our information. •We can solve for acceleration. Copyright © 2012 Pearson Education, Inc. publishing as Addison-Wesley Measurement of mass – Figure 4.20 •Since gravity is constant, we can compare forces to measure unknown masses. Copyright © 2012 Pearson Education, Inc. publishing as Addison-Wesley Newton’s Third Law •“For every action there is an equal and opposite reaction.” •Rifle recoil is a wonderful example. Copyright © 2012 Pearson Education, Inc. publishing as Addison-Wesley Same objects, different situations – Figure 4.22 •Making only subtle changes in the positions of the apple and the table we can observe a number of different situations. Copyright © 2012 Pearson Education, Inc. publishing as Addison-Wesley Use free body diagrams in any situation – Figure 4.24 •Find the object of the focus of your study and collect all forces acting upon it Copyright © 2012 Pearson Education, Inc. publishing as Addison-Wesley Forces transmit themselves as tension – Example 4.9 •We can solve for several outcomes using the elevator as our example. •Follow the worked problem on page 120. Copyright © 2012 Pearson Education, Inc. publishing as Addison-Wesley