PHYS 1441 – Section 002 Lecture #9Slide 2Special Project for Extra CreditNewton’s Third Law (Law of Action and Reaction)Ex. The Accelerations Produced by Action and Reaction ForcesEx. continuedExample of Newton’s 3rd LawExample of Newton’s 3rd Law, cnt’dCategories of ForcesThe Normal ForceSome normal force exercisesSome Basic InformationFree Body Diagrams and Solving ProblemsWednesday, Oct. 6, 2010 PHYS 1441-002, Fall 2010 Dr. Jaehoon Yu1PHYS 1441 – Section 002Lecture #9Wednesday, Oct. 6, 2010Dr. Jaehoon Yu•Newton’s Third Law•Categories of forces•Application of Newton’s Laws•Force of Friction–Motion with friction•Uniform Circular MotionWednesday, Oct. 6, 2010 2PHYS 1441-002, Fall 2010 Dr. Jaehoon YuWednesday, Oct. 6, 2010 PHYS 1441-002, Fall 2010 Dr. Jaehoon Yu3Special Project for Extra CreditA 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? b) Who moves farther in the same elapsed time?•Derive fomulae for the two problems above in much more detail than what is shown in this lecture note.•Be sure to clearly define each variables used in your derivation.•Each problem is 10 points each.•Due is Wednesday, Oct. 14.Wednesday, Oct. 6, 2010 4PHYS 1441-002, Fall 2010 Dr. Jaehoon Yu4Newton’s Third Law (Law of Action and Reaction)If two objects interact, the force F21 that object 2 exerts on object 1 by object 2 is equal in magnitude and opposite in direction to the force F12 exerted on object 1 exerts on object 2. 12F21F1212F =urThe 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 table to balance the action force, the gravitational force.21F-urWednesday, Oct. 6, 2010 PHYS 1441-002, Fall 2010 Dr. Jaehoon Yu5Suppose 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 ForcesWhich one do you think will get larger acceleration?Wednesday, Oct. 6, 2010 PHYS 1441-002, Fall 2010 Dr. Jaehoon Yu6 rF∑=rPs=arA=arEx. continuedForce exerted on the space craft by the astronaut rF∑=−rPForce exerted on the astronaut by the space craftsm=Pr36 N11,000 kgi+=r20.0033m s+space craft’s accelerationastronaut’s accelerationAm-=Pr36 N92 kgi-=r20.39 m s-Wednesday, Oct. 6, 2010 7PHYS 1441-002, Fall 2010 Dr. Jaehoon Yu7Example 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?12F =urMmF12F21= - F1221F =ur12 21F F=-ur ur12 F =ur12F =ur12 xF =12 yF =21xF =21 yF =12 21F F F=- =ur ur bxma =bmarbxmabyma =021F-ur21F =urFMxMaMyMa =0SinceandEstablish the equation bxFam= =Divide by mMM ar MxMamF =MxMaWednesday, Oct. 6, 2010 8PHYS 1441-002, Fall 2010 Dr. Jaehoon Yu8b) 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.bx =bxfv =212b MxMx a tm� �= =� �� �22MxMa tmtamMMxMxfvmMMMxmbxi bxv a t+ =bxa t =Mxfv =Mxi Mxv a t+ =Mxa tSo boy’s velocity is higher than man’s, if M>m, by the ratio of the masses.Man’s velocityBoy’s velocityBoy’s displacementMan’s displacement212bxi bxv t a t+ =Example of Newton’s 3rd Law, cnt’dWednesday, Oct. 6, 2010 9Categories 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 forcesPHYS 1441-002, Fall 2010 Dr. Jaehoon YuWednesday, Oct. 6, 2010 PHYS 1441-002, Fall 2010 Dr. Jaehoon Yu10The 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 ForceWednesday, Oct. 6, 2010 PHYS 1441-002, Fall 2010 Dr. Jaehoon Yu1111 N 15 N=NF - -11 N 15 NNF + - =Some normal force exercisesCase 1: Hand pushing down on the bookCase 2: Hand pulling up the book026 NNF =04 NNF =Wednesday, Oct. 6, 2010 12Some Basic InformationNormal 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: The 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. A graphical tool which is a diagram diagram of external forces on an objectof external forces on an object and is extremely useful analyzing forces and motion!! Drawn only on an object.Free-body diagramPHYS 1441-002, Fall 2010 Dr. Jaehoon YuWednesday, Oct. 6, 2010 13Free Body Diagrams and Solving Problems•Free-body diagram: A diagram of vector forces acting on an object A great tool to solve a problem using forces or using dynamics1. Select a point on an object in the problem2. Identify all the forces acting only on the selected object3. Define a reference frame with positive and negative axes specified4. Draw arrows to represent the force vectors on the selected point5. Write down net force vector equation6. Write down the forces in components to solve the problemNo matter which one we choose to draw the diagram on, the results should be the same, as long as they are from the same motionMWhich one would you like to select to draw FBD? Let’s take box!What do you think are the forces acting on this object?Gravitational forcegMFGthe force supporting the object exerted by the floorMeWhich one would you like to select to draw FBD?What do you think are the forces acting on this elevator?NFgMFGGravitational force The force pulling the elevator (Tension)mWhat
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