- #1
musicfairy
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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
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.
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
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/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.
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