Tuesday, June 14, 2011 PHYS 1443-001, Spring 2011 Dr. Jaehoon Yu 1 PHYS 1443 – Section 001 Lecture #6 Tuesday, June 14, 2011 Dr. Jaehoon Yu • Newton’s Laws of Motion – Newton’s third law of motion – Categories of Forces • Free Body Diagram • Application of Newton’s Laws • Force of frictionTuesday, June 14, 2011 PHYS 1443-001, Spring 2011 Dr. Jaehoon Yu 2 Announcements • Mid-term exam – In the class on Tuesday, June 21, 2011 – Covers: CH 1.1 – what we finish Monday, June 20 plus Appendices A and B – Mixture of free response problems and multiple choice problems • Bring your special project #2 during the intermissionTuesday, June 14, 2011 PHYS 1443-001, Spring 2011 Dr. Jaehoon Yu 3 Special Project for Extra Credit A large man and a small boy stand facing each other on frictionless ice. They put their hands together and push against each other so that they move apart. a) Who moves away with the higher speed, by how much and why? b) Who moves farther in the same elapsed time, by how much and why? • Derive the formulae for the two problems above in much more detail and explain your logic in a greater detail than what is in this lecture note. • Be sure to clearly define each variables used in your derivation. • Each problem is 10 points. • Due is Monday, June 20.Tuesday, June 14, 2011 PHYS 1443-001, Spring 2011 Dr. Jaehoon Yu 4 Special Project for Extra Credit A 92kg astronaut tied to an 11000kg space craft with a 100m bungee cord pushes the space craft with a force P=36N in space. Assuming there is no loss of energy at the end of the cord, and the cord does not extend beyond its original length, the astronaut and the space craft get pulled back toward each other by the cord toward a head-on collision. • What are the speeds of the astronaut and the space craft just before they collide? (10 points) • What are the magnitudes of the accelerations of the astronaut and the space craft if they come to a full stop in 0.5m from the point of initial contact? (10 points) • What are the magnitudes of the forces exerting on the astronaut and the space craft when they come to a full stop? 6 points) • Due is Wednesday, June 22.Tuesday, June 14, 2011 5 PHYS 1443-001, Spring 2011 Dr. Jaehoon Yu 5 Newton’s Third Law (Law of Action and Reaction) If two objects interact, the force F21 that object 2 exerts on object 1 is equal in magnitude and opposite in direction to the force F12 that object 1 exerts on object 2. 1 2 F21 F12 The reaction force is equal in magnitude to the action force but in opposite direction. These two forces always act on different objects. What is the reaction force to the force of a free falling object? The gravitational force exerted by the object to the Earth! Stationary objects on top of a table has a reaction force (called the normal force) from the table to balance the action force, the gravitational force.Tuesday, June 14, 2011 PHYS 1443-001, Spring 2011 Dr. Jaehoon Yu 6 Suppose that the magnitude of the force P is 36 N. If the mass of the spacecraft is 11,000 kg and the mass of the astronaut is 92 kg, what are the accelerations? Ex. The Accelerations Produced by Action and Reaction Forces Which one do you think will get larger acceleration? Yes, you are right! The astronaut!Tuesday, June 14, 2011 PHYS 1443-001, Spring 2011 Dr. Jaehoon Yu 7 F∑=PEx. continued Force exerted on the space craft by the astronaut F∑= −PForce exerted on the astronaut by the space craft +0.0033i m s2space craft’s acceleration astronaut’s acceleration −0.39 i m s2Tuesday, June 14, 2011 8 PHYS 1443-001, Spring 2011 Dr. Jaehoon Yu 8 Example of Newton’s 3rd Law A large man and a small boy stand facing each other on frictionless ice. They put their hands together and push against each other so that they move apart. a) Who moves away with the higher speed and by how much? M m F12 F21= - F12 Since and Establish the equation Divide by mTuesday, June 14, 2011 9 PHYS 1443-001, Spring 2011 Dr. Jaehoon Yu 9 b) Who moves farther while their hands are in contact? Given in the same time interval, since the boy has higher acceleration and thereby higher speed, he moves farther than the man. So boy’s velocity is higher than man’s, if M>m, by the ratio of the masses. Man’s velocity Boy’s velocity Boy’s displacement Man’s displacement Example of Newton’s 3rd Law, cnt’dTuesday, June 14, 2011 10 Categories of Forces • Fundamental Forces: Truly unique forces that cannot be derived from any other forces – Total of three fundamental forces • Gravitational Force • Electro-Weak Force • Strong Nuclear Force • Non-fundamental forces: Forces that can be derived from fundamental forces – Friction – Tension in a rope – Normal or support forces PHYS 1443-001, Spring 2011 Dr. Jaehoon YuTuesday, June 14, 2011 11 PHYS 1443-001, Spring 2011 Dr. Jaehoon Yu Gravitational Force and Weight Since weight depends on the magnitude of gravitational acceleration, g, it varies depending on geographical location. The attractive force exerted on an object by the Earth Gravitational Force, Fg FG= ma =Weight of an object with mass M is By measuring the forces one can determine masses. This is why you can measure mass using the spring scale. mg FG= Mg =Tuesday, June 14, 2011 PHYS 1443-001, Spring 2011 Dr. Jaehoon Yu 12 The normal force is one component of the force that a surface exerts on an object with which it is in contact – namely, the component that is perpendicular to the surface. The Normal ForceTuesday, June 14, 2011 PHYS 1443-001, Spring 2011 Dr. Jaehoon Yu 13 Some normal force exercises Case 1: Hand pushing down on the book Case 2: Hand pulling up the bookTuesday, June 14, 2011 14 Some Basic Information Normal Force, n: When Newton’s laws are applied, external forces are only of interest!! Why? Because, as described in Newton’s first law, an object will keep its current motion unless non-zero net external force is applied. Tension, T: Reaction force that reacts to action forces due to the surface structure of an object. Its direction is perpendicular to the surface. The reactionary force by a stringy object against an external force exerted on it. Free-body diagram PHYS 1443-001, Spring 2011 Dr. Jaehoon YuTuesday, June 14, 2011 15 Free Body Diagrams and Solving
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