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Homework Help: Some force problems

  1. Sep 26, 2008 #1
    Here's a couple of multiple choice problems that deal with force, friction, and the like. I need someone to check my answers. Of course, I checked them, but my reasoning could be wrong.

    For 1 & 2

    0456.jpg

    A block of mass 3 kg, initially at rest, is pulled along a frictionless, horizontal surface with a force shown as a function of time t by the graph above.

    1. The acceleration of the block at t = 2 s is
    (A) 3/4 m/s2
    (B) 4/3 m/s2
    (C) 2 m/s2
    (D) 8 m/s2
    (E) 12 m/s2

    My answer: B

    2. The speed of the block at t = 2s is
    (A) 4/3 m/s
    (B) 8/3 m/s
    (C) 4 m/s
    (D) 8 m/s
    (E) 24 m/s

    My answer: A
    how I got it: a = (2t)/3, v = t2/3, v(2) = 4/3

    For 3.

    049.jpg

    3. An object weighing 300 N is suspended by means of two cords, as shown above. The tension in the horizontal cord is
    (A) 0N
    (B) 150N
    (C) 210N
    (D) 300N
    (E) 400N

    My answer: D because it's 45 degrees, so the x and y components are equal.

    For 4, 5 & 6

    04101112.jpg

    4. Which figure best represents the free-body diagram for the box if it is accelerating up the ramp?
    (A) Figure A
    (B) Figure B
    (C) Figure C
    (D) Figure D
    (E) Figure E

    my answer: E

    5. Which figure best represents the free-body diagram for the box if it is at rest on the ramp?
    (A) Figure A
    (B) Figure B
    (C) Figure C
    (D) Figure D
    (E) Figure E

    my answer: C

    6. Which figure best represents the free-body diagram for the box if it is sliding down the ramp at constant speed?
    (A) Figure A
    (B) Figure B
    (C) Figure C
    (D) Figure D
    (E) Figure E

    my answer: C

    7. Two blocks of masses M and m, with M > m, are connected by a light string. The string passes over a frictionless pulley of negligible mass so that the blocks hang vertically. The blocks are then released from rest. What is the acceleration of the block of mass M?

    A) g
    B) (M - m)g/M
    C) (M + m)g/M
    D)(M + m)g/(M - m)
    E) (M - m)g/(M + m)

    my answer: E

    Mg - mg = (M + m)a
    a = (M - m)g / (M + m)

    8. A horizontal force F pushes a block of mass m against a vertical wall. The coefficient of friction between the block and the wall is μ. What value of F is necessary to keep the block from slipping down the wall?
    (A) mg
    (B) μmg
    (C) mg /μ
    (D) mg(1 - μ)
    (E) mg(1 + μ)

    My answer: C

    μF = mg
    F = mg/μ
     
  2. jcsd
  3. Sep 26, 2008 #2

    PhanthomJay

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    Looks like you really know your stuff! I agree with all your answers.
     
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