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# Lecture7

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Unformatted text preview: Lecture 7 Linear momentum and collisions Chapter 9 Ques6on: Which is harder to stop in each of the following sets? A 5 m/s 4kg 10 m/s 4kg B 10 m/s 4kg 2kg 10 m/s C 5 m/s 4kg 2kg 10 m/s It appears that the we may need a quan6ty which is deﬁned the product of mass and velocity How about mass 6mes velocity? Consider two par6cles 1 and 2 that can interact with each other but are otherwise isolated from the surroundings. This means the par6cles may exert force on each other but no external forces are present. The only forces ac6ng on the system of two par6cles are the F12, the force on 1 by 2 and F21, the force on 2 by 1. (note: my order of the subscripts are opposite to the one used in your textbook.) By Newton’s third law, F12 = − F21 ⇒ F12 + F21 = 0 dv1 dv2 ⇒ m1a1 + m2 a2 = 0 ⇒ m1 + m2 =0 dt dt d ⇒ ( m1v1 + m2 v2 ) = 0..............(1) dt m1 F12 F21 m2 Here we have assumed m1 and m2 are constants (not changing with 6me)...
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