Transcript Slide 1
5 Applications of Newton’s Laws Lectures by James L. Pazun Goals for Chapter 5 • To study conditions that establish equilibrium. • To study applications of Newton’s Laws as they apply when the net force is not zero. • To consider contact forces and the effects of friction. • To study elastic forces (such as spring force). • To consider forces as they subdivide in nature (strong, electromagnetic, weak, and gravitational). Copyright © 2012 Pearson Education, Inc. publishing as Addison-Wesley The conditions for a particle to be in equilibrium • Necessary conditions for an object to settle into equilibrium – SF = 0 – SFx = 0 and SFy = 0 Copyright © 2012 Pearson Education, Inc. publishing as Addison-Wesley Equilibrium in one dimension – Figure 5.1 Follow worked example 5.1 on page 130 Copyright © 2012 Pearson Education, Inc. publishing as Addison-Wesley Two dimensional equilibrium – Example 5.2 •Both x and y forces must be considered separately. •Follow worked example 5.2 on page 130. Copyright © 2012 Pearson Education, Inc. publishing as Addison-Wesley An example involving two systems – Example 5.4 •See the worked example on page 132 and 133. •This example brings nearly every topic we have covered so far in the course. Copyright © 2012 Pearson Education, Inc. publishing as Addison-Wesley Let’s examine applications of Newton’s Second Law. Although this liquid is on a level surface, the liquid is on a slant due to different accelerations. Copyright © 2012 Pearson Education, Inc. publishing as Addison-Wesley A two-cart train – application II •Please refer to the the quantitative solution on the bottom of page 134. Copyright © 2012 Pearson Education, Inc. publishing as Addison-Wesley Application III - Example 5.5 •This experiment works in your car, a bus, or even an amusement park ride! •Please refer to the worked example on page 155. Copyright © 2012 Pearson Education, Inc. publishing as Addison-Wesley Application IV – Example 5.6 •This sled ride is worked out for you on pages 135136. Copyright © 2012 Pearson Education, Inc. publishing as Addison-Wesley Application V – Example 5.7 •This problem involves two interactive systems in a common lab experiment. •Please refer to the worked example on pages 135-136. Copyright © 2012 Pearson Education, Inc. publishing as Addison-Wesley Contact force and friction – Figure 5.10 •We need to re-examine problems we formerly did as “ideal”. •We need to be able to find frictional forces given the mass of the object and the nature of the surfaces in contact with each other. Copyright © 2012 Pearson Education, Inc. publishing as Addison-Wesley The microscopic view of friction – Figure 5.11 •A surface will always have imperfections, your perception of them depends on the magnification. •The co-efficient of friction will reveal how much force is involved. Copyright © 2012 Pearson Education, Inc. publishing as Addison-Wesley Friction changes as forces change – Figure 5.12 •Forces from static friction increase as force increases while forces from kinetic friction are relatively constant. Copyright © 2012 Pearson Education, Inc. publishing as Addison-Wesley No dependence on surface area – Figure 5.13 •The normal force determines friction and the normal force depends only on mass. Copyright © 2012 Pearson Education, Inc. publishing as Addison-Wesley How much effort to move the fridge? Figure 5.14 •Dynamics as in the last chapter with a new force. •See the worked solution on pages 141 and 142. Copyright © 2012 Pearson Education, Inc. publishing as Addison-Wesley Forces applied at an angle – Figure 5.15 •The previous example has one new step if the force is applied at an angle. •Please refer to the worked example on pages 142 and 143. Copyright © 2012 Pearson Education, Inc. publishing as Addison-Wesley A toboggan on a steep hill with friction – Example 5.12 •Refer to the worked example on pages 143 and 144. Copyright © 2012 Pearson Education, Inc. publishing as Addison-Wesley Forces in fluids – Figure 5.18 •This topic is fully developed in advanced courses. •Conceptually, observe the drag as objects fall through “thicker” liquids. Copyright © 2012 Pearson Education, Inc. publishing as Addison-Wesley Elastic forces – Figure 5.19 •Springs or other elastic material will exert force when stretched or compressed. Copyright © 2012 Pearson Education, Inc. publishing as Addison-Wesley Stretch a spring to weigh objects. Figure 5.20 •The force settings on the spring are calibrated with mass standards at normal earth gravity. •The spring scales are often calibrated in force (N) and mass (kg). Copyright © 2012 Pearson Education, Inc. publishing as Addison-Wesley There are a variety of force laws in nature. • Gravitational interactions • Electromagnetic interactions • Strong interaction • Weak interactions • A “holy grail” of physics is the unified field theory. The goal will be to find the overriding principles that give rise to each of these very similar phenomena. Copyright © 2012 Pearson Education, Inc. publishing as Addison-Wesley