 Now we look at torque. Now torque is related to linear force, so let's just review a little bit more about that to start with. When I had forces by Newton's law, forces cause an acceleration. And I had my general equation that the net force was the mass times the acceleration. Now this assumed that the object was at a single point. And so that point had a specific position, velocity, and acceleration. If we're talking about the center of mass of the object, then a force applied to the center of mass will cause the entire object to move with that acceleration. Now if I've got a tangential force, then I can have a chance of getting some rotational motion. So if I have a tangential force, and if it's offset from the axis, then I'm going to get an angular acceleration. Here's where we start defining our torque. So torque is a combination of the tangential force and the distance from the axis. So tau is the symbol we use for torque. And it's got a unit of Newton meters. Now be careful of this. When we did energy, we said that a Newton meter was equal to a unit of energy. Torque has the same units, but it's not equivalent to energy. So we never combine these Newton meters to joules because that's an energy unit. In this case, we want to keep it as Newton meters as we're describing our torque. Now there's some alternate equations we can look at. The first one is the one we've already seen with our tangential force. And I'll give you a picture of what that looks like. Imagine I've got a solid object here and I've got some sort of point here that's going to act as a rotation pivot point. And I've got some force that's being exerted on that. Well, the tangential component to the force is tangential to the circular motion. See, when I pull up on here, it's going to cause this bar to rotate around the pivot point. And it's the tangential part of the force that causes that rotation. And the distance from the axis that I need to use here is not necessarily the entire bar's length, but just the distance from the pivot point up to where the force is being applied. Now this tangential force, if I were to look at my triangles, if this is my angle phi that I'm defining, then the tangential part uses the sine of phi. Now another way to look at it is to look at the tangential distance. And what do I mean by that? Well, if I took this force line down and took my pivot point straight out, then I've got some distance between the pivot point and this line of force. Well, that line of force I'm going to define as being my lever arm. That's what my L stands for here. So if I multiply the force by the lever arm, that is also going to give me the torque. Now if I just look at the angle in between, what I'll see is that if I plugged in my lever arm as being r sine theta, or if I plugged in my tangential force as being f sine theta, then both of these equations reduce to that the torque is equal to the force times the distance between the pivot point where the force is applied and the sine of the angle in between those two lines. So that introduces you to what torque is. We still have to look at some other equations that involve torque.