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Class Notes 11 November • Turn in / Pick up Homework #3: Why ? This is not a surface! #4: Force of gravity doesn’t change b) Bottom because PE is greatest #5: Use variables; Draw a sketch Start with EA + WNC = EB. • Today: Chapter 7: Impulse and Momentum Linear Momentum • We will define linear momentum as the mass times the velocity: p = mv Power and momentum are, respectively, ___ quantities. a. b. c. d. 9 Vector / Vector Scalar / Vector Vector / Scalar Scalar / Scalar 3 1 0 a. b. c. d. Linear Momentum • We will define linear momentum as the mass times the velocity: p = mv • A vector quantity! • Units: [p] = kg·m/s • Connection to force… Connection to Newton • F = ma = m(v/t) =(mv/t) = p/t • So Newton’s second law can be rewritten using momentum. • A net force causes a change in momentum! • Ex.-Impulsive Forces Impulse = Ft = p …but F is not constant! F F t t Impulse = F t = p p Impulse Ft F F F t t t Example: USC-Clemson A 400. g football resting on the ground is kicked so that it travels the maximum distance. It lands 70.0 m away, and the contact time was 48.0 ms. Estimate the force applied. What impulse is delivered? F = 218N, p = 10.5 kg·m/[email protected]º Forces and Momentum • Consider one object: F = p /t If the net external force on an object is zero then… a. The momentum must be zero b. The momentum may be zero c. The momentum must not be zero d. It must accelerate 6 6 0 a. b. c. 0 d. Forces and Momentum • Consider one object: F = p /t if F = 0 then p = constant! • Consider two objects: For any system of particles there may be a net force acting on the system… SF1=p1/t SF2=p2/t • So when the net external force acting on a system is zero the total momentum of the system does not change, which means, the total momentum (of the system) is conserved! pi = pf • So when the net external force acting on a system is zero the total momentum of the system does not change, which means, the total momentum (of the system) is conserved! pi + Jext = pf Homework • Read Chapter 7, sections 3-4 • Do problems on Worksheet #30 Use impulse and momentum ideas to solve Warning: Velocity is a vector!