Can someone explain force couples to me?

Factual Questions
1

Another Statics question. I am confused about force couples. Suppose I have a 14" lever, like a slot machine lever. I apply 400N force straight down at the top of the lever. According to my book I can create a force couple at the bottom of the lever and move the 400N down the lever, yet keep the same moment of force?

I don’t understand how to replace a force like the 400N downward force with a comparable force couple at the joint. Any ideas?

2

Okay, let’s think about what the pivot of the lever has to react.

If you pushed downward right at the pivot of the lever, the pivot would have to react that 400N, right? But you’re not pushing down right at the pivot, you’re pushing downward a meter away (really big slot machine…).

The pivot has to react a 400N downward force. And, because that 400N was not applied at the pivot, there’s a moment (400N times 1 meter = 400N-m) that it has to react, too.

So you can replace the “400N a meter away from the pivot” with a simpler setup: a 400N force applied right at the pivot, plus a 400N-m couple applied at the pivot.

3

Oh, also–

I wanted to say that Statics was the first of my Engineering classes that I didn’t find intuitively obvious, and had to stretch my mind to emcompass. I think that a lot of would-be engineers find the same thing, and that it’s one of the classes that weeds out the future Chosen Ones from the Accursed Rabble. (That, and Thermodynamics…)

So good luck; stay calm, it isn’t that hard if you don’t panic.

4

Well, a force couple is two forces (“couple”) acting in parallel opposite directions a specified distance apart. Think of turning a big wheel like a valve or opening a bank vault.
Now as far as your example goes, is the slot machine handle vertical or horizontal? If it’s vertical and your 400N is vertical, then the moment is zero and no matter where the force is applied, the moment is always zero. In this case, the couple you create at the joint is two forces equal in magnitude and opposite direction. Or am I way off target here? Maybe if you provide a reference (book, page number) I could look it up?