Homework Help: Physics

Posted by Physics on Tuesday, August 25, 2009 at 8:36pm.

I have a question
26. A box weighing 85 N rests on a table. A rope tied to the box runs vertically upward over a pully and a weight is hung form the other end. Detrmine the force that the table exerts on the box if the weight hanging on the other side of the pully weighs (a) 30 N, (b) 60 N, and (c) 90 N.

Ok well we know that the Fn of the first block is 85 N and the Fn of the other block changes so for (a) I\'m not exactly sure how to do this problem...

Do I take the Fn of the first block and the Fn of the second block and then find the Fn exerted by the table

or do I take the Fn of the first block and the Fg of the second block...

not exactly sure how to do this thanks...

Physics - bobpursley, Tuesday, August 25, 2009 at 9:07pm
look at the forces acting on the rope (make clockwise the + direction)

ftable=85+30 = net force=ma

but acceleration is zero, so ..Now on ftable, it has to be positive, that is , it is pushing up, so when solving with a being zero, ftable becomes negative (it cant happen unless the 85N weight is glued), then acceleration is not zero. I think that happens in c.
Physics - Physics, Tuesday, August 25, 2009 at 9:14pm
wait so if downards was negative
the force of gravity on the first block would be -85 N

the force of gravity on the seocnd block is - 30 N for problem (a)

and the force of tension is equal and opposite to the force of gravity on the second block and is 30 N

net force in y direction = Fg1 + Fg2 + Ft = -80 N - 30 N + 30

net force acting on table is -50 N
Fn exerted by table is equal and opposite all forces acting on it so 50 N

is that right???
Physics - Physics, Tuesday, August 25, 2009 at 9:15pm
oh wow so I\'m not sure how to do this if the net force in the y direciton is zero...
Physics - bobpursley, Tuesday, August 25, 2009 at 9:22pm
I have a type on mine..

Ftable-80+30=ma

where a is the acceleration of the system, clockwise, and of course in a) it is zero.
Physics - Jordan, Wednesday, August 26, 2009 at 8:49am
we know that the force the table is exerting on the box must be equal to the force that the box is exerting on the table.

in reality the 85N of the box is not the force that is being exerted on the table because the 30N on the other end of the rope is acting against the 85N therefor only 55N of force is being exerted on the table. for 90N the force is not -5N because the box is not exerting any force on the table it is 0N

that is my understanding of how these questions work, hope it helps :)

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