Hello, Friction is a force that acts in the opposite direction of an object's motion. Most friction problems involve static friction (F sub s) and kinetic friction (F sub k). Both static and kinetic friction can be calculated from F=mu*N, where mu is a lowercase Greek letter, N is the normal force, and F is the friction force. Mu is referred to as the coefficient of static or kinetic friction, depending on the problem. Let's say you are trying to push a heavy box on the ground; if you are pushing and the box hasn't budged, that is because your pushing force is not bigger than the static friction force. If you push hard enough to move the box, the static friction force has been surpassed in magnitude and now the friction force is caused by kinetic friction. Drag force is also another form of friction. Drag is caused by an object moving in a fluid or gas, like air. If you need more help with this or have questions on some problems, feel free to contact me! Kind regards, Josh V

whenever a body moves or tends to move over a surface of another body a force comes into play acting parallel to the surface of contact which opposes the relative motion of the body, this opposing force is called friction. e.g. when we try to push a box kept on the ground, friction comes into play and it opposes the motion of the box and makes it difficult to move. this frictional force depends on the roughness of the surface of the body in contact, thus on a smooth surface the friction force is less so it is easier to move an object over a smooth surface while on a rough surface the frictional force is more so it is difficult to move object over a rough surface.