# hw 15 - homework 15 – HETTIGER CHRISTOF – Due 4:00 am 1...

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Unformatted text preview: homework 15 – HETTIGER, CHRISTOF – Due: Feb 23 2008, 4:00 am 1 Question 1, chap 7, sect 3. part 1 of 3 10 points A block of mass m is pushed a distance D up an inclined plane by a horizontal force F . The plane is inclined at an angle θ with respect to the horizontal. The block starts from rest and the coefficient of kinetic friction is μ k . m D μ k F θ If N is the normal force, what is the work done by friction? 1. W = + μ k ( N - mg cos θ ) D 2. W =- μ k N D correct 3. W =- μ k ( N + mg cos θ ) D 4. W = + μ k N D 5. W = 0 6. W =- μ k ( N - mg cos θ ) D 7. W = + μ k ( N + mg cos θ ) D Explanation: The force of friction has a magnitude F friction = μ k N . Since it is in the direc- tion opposite to the motion, we get W friction =- F friction D =- μ k N D. Question 2, chap 7, sect 3. part 2 of 3 10 points What is the work done by the normal force N ? 1. W = N D cos θ 2. W = N D 3. W = ( mg cos θ + F sin θ-N ) D 4. W = ( N + mg cos θ + F sin θ ) D 5. W = ( N - mg cos θ- F sin θ ) D 6. W = 0 correct 7. W = N D sin θ 8. W =-N D Explanation: The normal force makes an angle of 90 ◦ with the displacement, so the work done by it is zero. Question 3, chap 7, sect 3. part 3 of 3 10 points What is the final speed of the block? 1. v = radicalbigg 2 m ( F cos θ + mg sin θ- μ k N ) D 2. v = radicalbigg 2 m ( F cos θ- μ k N ) D 3. v = radicalbigg 2 m ( F cos θ + mg sin θ ) D 4. v = radicalbigg 2 m ( F cos θ- mg sin θ + μ k N ) D 5. v = radicalbigg 2 m ( F cos θ- mg sin θ- μ k N ) D correct 6. v = radicalbigg 2 m ( F cos θ- mg sin θ ) D 7. v = radicalbigg 2 m ( F sin θ + μ k N ) D 8. v = radicalbigg 2 m ( F sin θ- μ k N ) D Explanation: The work done by gravity is W grav = mg D cos(90 ◦ + θ ) =- mg D sin θ . homework 15 – HETTIGER, CHRISTOF – Due: Feb 23 2008, 4:00 am 2 The work done by the force F is W F = F D cos θ . From the work-energy theorem we know that W net = Δ K , W F + W grav + W friction = 1 2 mv 2 f ....
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## This note was uploaded on 05/09/2010 for the course PHYSICS PHY 301k taught by Professor Turner during the Spring '10 term at University of Texas-Tyler.

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hw 15 - homework 15 – HETTIGER CHRISTOF – Due 4:00 am 1...

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