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.(adsbygoogle = window.adsbygoogle || []).push({});

For 1 & 2

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/s^{2}

(B) 4/3 m/s^{2}

(C) 2 m/s^{2}

(D) 8 m/s^{2}

(E) 12 m/s^{2}

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 = t^{2}/3, v(2) = 4/3

For 3.

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

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

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# Homework Help: Some force problems

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