tension direction?

Question

how come sometimes tension is said to be "in both directions", but if you stick a cable horizontally from a wall and hang a mass from the end, the tension is towards the wall? shouldn't there never be any net tension, and if so, why does the value factor in during calculations of other forces??

Answer 1

In a tug o' war, neither side is going to say, "I don't feel nothing!". In the example you gave, a cable is anchored to a vertical wall, goes out horizontally, goes around a wheel, goes down to where it's holding up a mass. The mass is trying to pull the cable out of the wall, and the wall is trying to pull the cable out of the mass, so tension works both ways. The reason why "direction of tension" went 90 degress down is because of the pulley wheel at the corner, and the resultant force force diagram does in fact balance out. Something is holding that pulley wheel in place, and it experiences a net force at 45 degrees towards the ground. To help visualize tension, think of a tightly-wound spring, long like a cable, but capable of being stretched. Whenever it is stretched, tension exists, and it's the same in either direction along the spring.