22. As shown in Figure CQ5.22, student A, a 55-kg girl, sits on one chair with m

Question

22. As shown in Figure CQ5.22, student A, a 55-kg girl, sits on one chair with metal runners, at rest on a class- room floor. Student B, an 80-kg boy, sits on an identi- cal chair. Both students keep their feet off the floor. A rope runs from student A's hands around a light pul- ley and then over her shoulder to the hands of a teacher standing on the floor behind her. The low-friction axle of the pulley is attached to a second rope held by stu dent B. All ropes run parallel to the chair runners. (a) If student A pulls on her end of the rope, will her chair or will B's chair slide on the floor? Explain why. (b) If instead the teacher pulls on his rope end, which chair slides? Why this one? (c) If student B pulls on his rope, which chair slides? Why? (d) Now the teacher ties his end of the rope to student A's chair. Student A pulls on the end of the rope in her hands. Which chair slides and why? Student B Student A Teacher

Explanation / Answer

a )

student A only applies an excert T1 likely morethan 80/2 = 40 to moves the student B chair.

so student B chair moves the first.

b )

the teacher's pulling his rope developes tension T1 and T2

so student B chair moves the first.

c )

student B pulling on his rope developes the tension T1 and T2

just as in question in (a),

this is of T2 = 2 T1  

so same answer is student B chair moves the first.

d )

when teacher is went out force pulling student A chair is 2 T1  

and this is also of student B chair.

finally smaller Ff will move the first.

student A chair with FfA = 55 KN, less than 80 KN.

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