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Net Force, Acceleration and Mass Date ______ The vector sum of all the forces acting on an object is the _________ ____________. The acceleration of the object depends on the ________ _____________ on the object and the _______________ of the object. The acceleration of an object is ______________ proportional to the net force acting on the object. The acceleration of an object is ______________ proportional to the object's mass. If equal forces are applied to two objects of different masses, the object with less mass will experience a _______________ acceleration and the object with more mass will experience a _________________ acceleration. 1 Net force = ____ Net force = ____ NO ________ in motion. Object _________________ 2 This relationship can be written in equation form. a = Fnet or Fnet = ma where Fnet is a vector sum of all the forces on an m object. It call the net force. Net Force(N) Mass (kg) Acceleration (m/s2 ) 1. 10 2 2. 20 2 3. 20 4 4. 5. 2 10 5 10 When the mass is constant and the net force is doubled, the acceleration __________ When the net force is constant and the mass doubles, the acceleration ____________ 3 Two identical spring loaded dart guns are simultaneously fired straight downward. One fires a regular dart; the other a weighted dart. Which dart hits the ground first? Why? 4 In Chapter 2 acceleration was the rate of change of velocity: a = vf vi Δt In Chapter 4 acceleration is the ratio of the net force to mass: a = Fnet m The CAUSE of the acceleration is the force which produces the rate of change in velocity. Net Force, Acceleration, and Mass Practice Problems: Two people with the same mass of 50 kg are riding in a pickup truck at 20 m/s to the right. They are brought to an abrupt stop in a head on collision with a drunk driver who has crossed the divider line. One person wears a seat belt and is brought to a stop in 0.1 second. The other person is not wearing a seat belt and is brought to a stop by the dashboard in 0.01 seconds. 1) What is the acceleration of the person who comes to a stop wearing the seat belt? 5 2) What is the net force exerted by the seat belt to bring the first person to a stop? 3) What is the acceleration of the person who comes to a stop by hitting the dashboard? 4) What is the net force exerted by the dashboard to bring the second person to a stop? 5) What is the weight of the person stopped by the dashboard? 6) Compare his weight to the force of the dashboard. 6 Calculating Acceleration from Free Body Diagrams A desk is pulled along a smooth floor. FN = _______ Fa = 50 N Ff = 10 N Fg = ________ m = 8 kg Fnet = _____________ a = _____________ 7 An ball slows down as it rolls across the floor. FN = _______ Ff = 8 N Fg = 5 N m = ________ Fnet = _____________ a = _____________ 8 This object is on a level surface. What is its motion? FN = _______ Ff = 15 N m = 6 kg Fa = 15 N Fg = ________ a = 0 m/s2 Fnet = _____________ This object is ___________________________. 9 Fnet = 0 N; Fgrav = 98 N; Fnorm = 98 N; Ffrict = 19.6 N; Fapp = 19.6 N When the velocity is constant, a = 0 m/s/s and Fnet = 0 N Since the mass is known, Fgrav can be found (Fgrav = m • g = 10 kg • 9.8 m/s/s). Since there is no vertical acceleration, normal force = gravity force. Once Fnorm is known, Ffrict can be found using Ffrict = "mu" •Fnorm. Since there is no horizontal acceleration, Ffrict = Fapp = 19.6 N 10