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lecture 8

# lecture 8 - Lecture 8 Examples using N2L 1 Example A box...

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1 Lecture 8 Examples using N2L

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2 Example: A box slides across a rough surface. If the coefficient of kinetic friction is 0.3, what is the acceleration of the box? f k;sb w eb N sb x y FBD for box: 0 eb sb k;sb = - = = - = w N F ma f F y x Apply Newton’s 2 nd Law: a w f N F m = + + = eb k;sb sb
3 mg w N w N ma f = = = - = - eb sb eb sb k;sb 0 ma mg N f = - = - = - k sb k k;sb μ μ (1) (2) From (1): ( 29 ( 29 2 2 k m/s 94 . 2 m/s 8 . 9 3 . 0 - = - = - = g a μ Solving for a: Example continued

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4 Example: In the previous example, a box sliding across a rough surface was found to have an acceleration of -2.94 m/s 2 . If the initial speed of the box is 10.0 m/s, how long does it take for the box to come to rest? Know: a x = –2.94 m/s 2 , v 0x =10.0 m/s, v x = 0.0 m/s Want: t. sec 40 . 3 m/s 94 2 m/s 0 . 10 0 2 0 0 = - + - = - = = + = . a v t t a v v x x x x x
5 Example: A 1.00 kg mass is at rest an on ramp that makes an angle of 20 ° with respect to the horizontal. For this situation μ s = 0.400. What is the magnitude of the static friction force?

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