# Application of Newton's Laws Answers - Physics 125...

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Physics 125 Discussion #7 Solution 1 Problem 1: Push a Box on an Incline Plane When a force of 500 N pushes on a 25-kg box as shown in the figure below, the acceleration of the box up the incline is 0.75 m/s 2 . Friction can NOT be neglected. (1) First, let’s define our coordinate system as shown in the figure above. (2) Draw a free-body diagram for box. (Hint: there are four forces acting on the box.) (3) Find the x- and y- components of each force. (Use symbols. Let 40 0 = θ .) Symbol x-component y-component Force #1 F N 0 N F + Force #2 F push cos push F + sin push F - Force #3 W sin W - cos W - Force #4 f k -f k 0 40 0 40 0 a=0.75 m/s 2 m=25kg F push =500 N x y F N f k F push W

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Physics 125 Discussion #7 Solution 2 (4) Write out Newton’s second law in the y-direction for the box. (Use symbols!) 0 0 cos sin = + - - = θ W F F F push N y (5) Find the normal force on the box. (N) 509 ) 40 )(cos 8 . 9 )( 25 ( ) 40 )(sin 500 ( cos sin 0 0 cos sin 0 0 = + = + = = + - - = N push N push N y F W F F W F F F (6) Write out Newton’s second law in the x-direction for the box. (Use symbols!) ma f W F F k push y = - - + = sin cos 0 (7) What is the kinetic coefficient of friction, μ k ? 41 . 0 509 ) 75 . 0 )( 25 ( ) 40 )(sin 8 . 9 )( 25 ( ) 40 )(cos 500 ( 509 sin cos sin cos sin cos 0 0 0 = - - = - - = = - - = = - - + = k push N k k push k k push y ma W F F f ma W F f ma f W F F μ
Physics 125 Discussion #7 Solution 3 Problem 2: Two-mass Pulley Consider the following Atwood's machine with m 2 > m 1 , and with a very light (or massless), frictionless ideal pulley. (1)

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Application of Newton's Laws Answers - Physics 125...

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