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• N3 – Forces from Motion General Physics 1 The Kinematic Chain A particle moves along the x axis according to the equation x = 2.00 + 3.00 t - 1.00 t2 where x is in meters and t is in seconds. At t=3.00 s, find (a) the position of a particle, (b) its velocity, (c) its acceleration. General Physics 2 Graphical Analysis of Motion • v(t) is slope of r(t) graph • a(t) is slope of v(t) graph • Derivative is slope, so we can link r, v and a graphically as well as mathematically General Physics 3 Net Force Diagrams • • • Construct a free-body diagram. Copy the force arrows from the diagram in sequence (tip-to-tail). Draw a double-line arrow to show the vector sum. General Physics 4 Qualitative Example • Skier going down slope (Fig N3.1) FKF FD Fg General Physics FN 5 Practice Problem • N3B.1 Draw a free-body diagram and a netforce diagram for a moving motorboat whose motor has just run out of gas. General Physics 6 Third-Law Pairs • • When objects A and B interact, the force the interaction exerts on A is equal in magnitude and opposite in direction to the force it exerts of B. Force pairs connected by Newton’s 3rd Law: • • Each force in the pair must act on a different object. Each must reflect the same interaction. General Physics 7 Second-Law Pairs • Force pairs connected by Newton’s 2nd Law: • • • • • • Act on the same object Arise from different interactions Act along same axis Are = b/c a=0 along that axis Ex: Normal force and weight, when object is at rest. What if object is accelerating, like box in an elevator?? General Physics 8 Coupling Forces • Compare the forces exerted by the blocks on each other as they move on a table with some friction. Draw free-body diagram for each block. Determine if A B C General Physics block A exerts a greater force block B exerts a greater force the forces are equal 9 Coupling Forces • Compare the forces exerted by the blocks on each other as they move on a table with some friction. Draw free-body diagram for each block. Determine if A B C General Physics block A exerts a greater force block B exerts a greater force the forces are equal 10 Third Law Pair Activity • Equipment • • • Open Logger Pro • • • Wired force probe Wireless force probe Select Connect Interface, Wireless, and select the name of your wireless probe In the top right corner you will see two tabs, one for each probe. Click on each tab and select zero sensor from the calibrate menu. On one of the sensors, select change direction. Connect the hooks on the force probes together and pull and push until the values are different. Record your results. General Physics 11 Group Problems • • • • • • N3B.4 N3B.6 N3B.9 N3S.2 N3S.5 Unit IV, Worksheet 4 General Physics 12