# Why is the force not included?

I'm reading my physics book trying to understand how to create force equations. (This next test is going to be a flop.) In this example, it states that if we were trying to find the tension in the rope we would only consider the the weight of the piano and the tension in the rope and exclude the force the piano exerts on the rope in our free body diagram. I do not understand how this excluded force is from the outside world and not directly contributing to the tension in the rope.

• Isn't the force that the piano exerts on the rope the same as the tension in that rope? Oct 1, 2019 at 5:09
• The force the rope exerts on the crane is external Oct 1, 2019 at 6:13

Imagine a person applying a force $$\vec F$$ on a box (of mass m) kept on a frictionless floor, now its acceleration, $$\vec a$$ is equal to $$\vec F$$/m. In this case, we don't consider the force that the box applies on the person (pushing the box). Similarly in the piano case, we don't consider the force exerted by the piano on the rope.