Question

As shown in figure CQ5.22, student A, a 55 kg girl, sits on one chair with metal runners, at rest on a classroom floor. Student B, an 80 kgboy, sits on an identical chair. Both students keep their feet off the floor. A rope runs from student A’s hands around a light pulley 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 student B, all ropes run parallel to the chair runners.

1. If student A pulls on her end of the rope, will her chair or will B’s chair slide on the floor? Explain why?
2. If instead, the teacher pulls on his rope end, which chair slides? Why this one?
3. If student B pulls on his rope, which chair slides? Why?
4. 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?

Short answer

(a) The rope pulls back, causing her to slide towards student B.

(b) If instead, the teacher pulls o his rope, Both chairs slide.

(c) Both chairs slide because when student B pulls on his rope.

(d) Both chairs slide because when student A pulls on the rope.

Step-by-step solution

A force:

Force can be defined as the mass of object times the acceleration of that object.

(a) If student A pulls on the rope's end:

Both students slide towards each other. When student A pulls on the rope, the rope pulls back, causing her to slide towards student B. The rope also pulls on the pulley, so student B slides because he is gripping a rope attached to the pulley.

(b) If instead, the teacher pulls on his rope end:

Both chairs slide because there is tension in the rope that pulls on both the students A and the pulley connected to Student B.

(c) If student B pulls on his rope:

Both chairs slide because as student B pulls on his rope, he also pulls on the pulley, putting force on the rope that passes through the pulley to student A.

(d) Now the teacher ties his end of the rope to student A’s chair:

Both chairs slide because when student A pulls on the rope, it pulls on her and also pulls on the pulley.