Question

current can open a 2.5 metre wide Door by applying a minimum force of 3 Newton find the moment of force needed to open the door​

Answer 1

Answer:

Yes, the rod will start to rotate; and if the force continues to act in the same direction and the wheel rotates for long enough, then the torque will change. However, we can take the limit where a very large force acts for a very short time, in such a way that their product is constant. (That is, instead of 120 N on the CM for 1/10 sec, apply 1200 N on the CM for 1/100 sec, or 12000 N on the CM for 1/1000 sec, etc.) In this limit, the actual angular rotation of the rod goes to zero, and we can assume that the impulse is all exerted at one spot and at one time. This is the only limit in which the rest of your questions are easily addressable, so I'll assume this is true from now on