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...
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.
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???
oh wow so I\'m not sure how to do this if the net force in the y direciton is zero...
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.
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 :)
To solve this problem, let's break it down step by step.
First, let's consider the forces acting on the box. We have the weight of the box itself (85 N) acting downward, which we can call the gravitational force (Fg). We also have a tension force (T) in the rope, which is transmitted from one end of the rope to the other.
Now, let's look at the forces acting on the weight hanging from the other side of the pulley. We have the weight of the hanging weight (30 N, 60 N, or 90 N), which is also the gravitational force (Fg). We have the tension force (T) in the rope, pulling it upwards.
Considering a weightless and frictionless pulley, the tension in the rope is the same on both sides of the pulley. So, the tension in the rope is equal to the force exerted by the table on the box.
Coming back to the box, we can use Newton's second law to determine the force exerted by the table on the box. The sum of the forces in the vertical direction should be zero, as the box is in equilibrium.
So, for each case:
(a) The gravitational force acting on the hanging weight is 30 N. Since the tension force in the rope is equal to this gravitational force, the force exerted by the table on the box is also 30 N.
(b) The gravitational force acting on the hanging weight is 60 N. Again, since the tension force in the rope is equal to this gravitational force, the force exerted by the table on the box is also 60 N.
(c) The gravitational force acting on the hanging weight is 90 N. Similarly, the tension force in the rope and the force exerted by the table on the box are both 90 N.
Therefore, to find the force exerted by the table on the box in each case, you simply need to consider the gravitational force acting on the hanging weight. The force exerted by the table on the box is equal to that gravitational force.