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Unformatted text preview: Lecture 8 Last lecture: Mass, momentum, and force Newtons three laws of motion Today: Inertial/Noninertial frames Analyzing forces in action Applying Newtons laws Next lecture More on Newtons law Force, mass, and momentum Force F : the action on the body that changes its velocity. Force is a vector. Mass m : the measurement on how easily the velocity of an object can be changed. Mass is a scalar. Momentum P= m V, is the product of mass and velocity. Momentum is a vector Newtons three laws of motion First Law If here is no net force acting on the body, its velocity (momentum) does not change. Second Law The rate of change in momentum is equal to the net force acting on the body. F =d P /dt, or F =m a. Third Law For every action force there is a reaction force which is opposite in direction with the same magnitude. Newtons 3 rd Law of Motion Forces Always Exist in Pairs ! Inertial vs NonInertial Reference Frames An inertial frame is a reference frame where Newtons laws are valid. a Newtons Laws apply here Newtons laws are insufficient here A noninertial frame is a reference frame which is accelerating relative to a inertial frame. a You are riding in an airplane and you are wearing a seat belt. A book lies on the seat next to you. Suddenly the plane makes a sudden lurch and for a short time the book floats above the seat. Which of the following statements is true? A) The net force on the book is zero. B) The net force on the book is downward. C) The net force on the book is upward. D) There is no gravitational force acting on the book. E) There is an upward centrifugal force acting on the book. A constant force is exerted for a short time interval on a cart that is initially at rest on an air track. This force gives the cart a certain final speed. The same force is exerted for the same length of time on another cart, also initially at rest, that has twice the mass of the first one. The final speed of the heavier cart is A) onefourth B) four times C) half D) double E) the same as that of the lighter cart. Analyzing the forces in action What is the downward acceleration? What is the tension in the wire? What is the relation between m 1 and m 2 if the blocks do not move? Weight and Forcee Weight: the magnitude of the net force required to prevent body from falling freely. W=mg Normal force: the reaction force of supporting structure on the body. Normal force can be very different from weight. Tension: the force a cord exert on the object Application of Newtons Laws The process for solving problems Identify the moving object Identify all the forces and draw them in a freebody diagram Choose a convenient coordinate system Write the individual component equations in this system Solve equations symbolically for the desired quantity Stop. Think about the result. Does it make sense?...
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This note was uploaded on 03/14/2011 for the course PHYS 116 taught by Professor Engle during the Spring '08 term at UNC.
 Spring '08
 engle
 mechanics, Force, Inertia, Mass, Momentum

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