Drawing a Free-Body Diagram

The whole trick of a free-body diagram is subtraction. A sled, a rope, snow, friction, gravity, the angle of the pull — that's a lot to hold in your head at once. Shrink the sled to a dot. Now draw an arrow for every force that has a real source touching or pulling on it: gravity down, the normal force from the snow up, friction opposing slide, and tension along the rope at whatever angle it's pulled. Each arrow starts at the dot, points in the force's direction, and roughly scales with its size. Once the mess becomes four or five labeled arrows, you can split them into x and y components and run F = ma on each axis separately.

Inclines are where axis choice matters most. Instead of keeping x horizontal and y vertical, rotate both axes so x runs along the slope and y runs perpendicular to it. Gravity now has to be split into two components — one along the slope pulling the object down the incline, one into the surface — while the normal force and friction already line up neatly with your new axes. That rotation turns a two-force-mixing headache into simple addition along each axis.

A free-body diagram never includes velocity, acceleration, or momentum as an arrow, because those describe motion, not force. It connects directly to Newton's second law, the next step: once every force arrow is drawn and broken into components, you add them up to get net force, divide by mass, and that gives you acceleration. The diagram is the setup; the second law is the payoff.