Review Forces Homework: 3 Boxes, F=ma, Drawings & Equations

[RJLiberator]

Homework Statement

Three Boxes, A, B, and C, are connected to each other using light vertical ropes, with box B hanging below box A, and C hanging below B. Call the tension in the rope connecting A and B "T_1" and that between B and C "T_2". A constant upward force F is applied to box A. Together, the boxes have a downward acceleration a. Gravity, as usual, pulls downwards, and take the positive direction as upwards. a) Make a labeled drawing. B) draw correct and complete free-body diagrams. c) give the equation(s) that come from the free body diagrams. You do not need to solve for anything.

Homework Equations

F=ma

The Attempt at a Solution

I am most concerned with C.

For box a: F-Ft_1-F_g=m_a*a
For box b: Ft_1-Ft_2-F_g=m_b*a
For box c: Ft_2-F_g=m_c*a

1) my acceleration, a, is the same for all boxes
2) There is no normal force as the problem does not state that the boxes are on the ground or anything(?)
3) Should the mass for box c include the mass's of A+B?

Thank you.

 

[Nathanael]

1) my acceleration, a, is the same for all boxes

Right

2) There is no normal force as the problem does not state that the boxes are on the ground or anything(?)

Right

3) Should the mass for box c include the mass's of A+B?

Why?

 

[RJLiberator]

Hm. Since there is no normal force, and these boxes are accelerating upwards, it does not make sense to include all mass's for box c.

 

[Nathanael]

When it says something like "make a free body diagram for box C" or "evaluate all the forces on box C" then you are taking box C as it's own system. That means the mass of the system is the mass of box C, regardless of any normal forces.
Normal forces would just mean that different forces are acting on box C, but it wouldn't change the mass of box C.

 

[RJLiberator]

I see.

Thank you for the clarification. That makes sense.