Frictional forces come in three kinds, each of which replaces a different interaction with the surroundings.
Grip forces are for getting something going, or stopping two things sliding past each other.
Slip and Drag forces always slow something down, and still depend on moving past the surroundings. Slip from moving past solids, drag from moving through liquids or gases.
Expect a grip force exerted on something when a rough solid is in contact, but the thing does not move past the thing. Re-imagine the thing extracted from its environment by choosing to replace the rough environment by a grip force exerted on the thing.
The re-imagining is simpler, and useful if the grip force is chosen so that it has the same effect as the rough surroundings that it replaced.
The rougher the environment and the more you push on this rough surface, the greater grip force.
Expect a slip force exerted on something when the thing moves over a rough solid in contact. Re-imagine the thing extracted from its environment by choosing to replace the rough environment by a slip force exerted on the thing.
The re-imagining is simpler, and useful if the slip force is chosen so that it has the same effect as the rough surroundings that it replaced.
The rougher the environment and the more you push on this rough surface, the greater the slip force.
Expect a drag force exerted on something when the thing moves through a liquid or a gas. Re-imagine the thing extracted from its environment by choosing to replace the environment by a drag force exerted on the thing.
The re-imagining is simpler, and useful if the drag force is chosen so that it has the same effect as the replaced surroundings.
The faster the thing moves through the liquid or gas, the greater drag force. Changing the liquid or gas, or the shape of the thing, or the size of the thing, may also vary the drag force. Drag forces are complicated.