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7-momentum-and-impulse

# 7-momentum-and-impulse - Chapter 7 Momentum and Impulse...

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Chapter 7 Momentum and Impulse Chapter 7 IMPULSE AND MOMENTUM PREVIEW The momentum of an object is the product of its mass and velocity. If you want to change the momentum of an object, you must apply an impulse , which is the product of force and the time during which the force acts. If there are no external forces acting on a system of objects, the momentum is said to be conserved, that is, the total momentum of the system before some event (like a collision) is equal to the total momentum after that event. In this chapter, we will discuss examples of both one- and two-dimensional collisions. The content contained in all sections of chapter 7 of the textbook is included on the AP Physics B exam. QUICK REFERENCE Important Terms impulse The product of the average force acting on an object and the time during which it acts. Impulse is a vector quantity, and can also be calculated by finding the area under a force versus time curve. linear momentum The product of the mass of an object and its velocity. Momentum is a vector quantity, and thus the total linear momentum of a system of objects is the vector sum of the individual momenta of the objects in the system. internal forces The forces which act between the objects of a system external forces The forces which act on the objects of a system from outside the system, that is by an agent which is not a part of the system of objects which are being studied. inelastic collision A collision between two or more objects in which momentum is conserved but kinetic energy is not conserved, such as two railroad cars which collide and lock together. elastic collision A collision between two or more objects in which both momentum and kinetic energy are conserved, such as in the collision between two steel balls. center of mass The point at which the total mass of a system of masses can be considered to be concentrated. 96

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Chapter 7 Momentum and Impulse Equations and Symbols p = m v J = F t = p = 0 v v m m f - 2 1 2 2 1 1 m m x m x m x cm + + = where p = momentum m = mass v = velocity J = impulse F = force t = time interval during which a force acts x cm = position of the center of mass of a system of particles x 1 = position of a mass relative to a chosen origin Ten Homework Problems Chapter 7 Problems 5, 9, 17, 22, 28, 29, 32, 39, 41, 55 DISCUSSION OF SELECTED SECTIONS 7.1 The Impulse – Momentum Theorem The momentum p of an object is the product of the mass m of the object and its velocity v : p = m v The momentum of a moving mass is a vector which has a direction that is the same as the velocity of the mass. Thus, the momentum of an object can be broken down into its components: p = p x + p y where p x = pcos θ and p y = psin θ . The magnitude of the momentum vector can be found by the Pythagorean theorem: 97 p x p y p 2 2 y x p p p + =
Chapter 7 Momentum and Impulse Newton’s second law states that an unbalanced (net) force acting on a mass will accelerate the mass in the direction of the force. Another way of saying this is that a net force acting on a mass will cause the mass to change its momentum. We can rearrange

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