Newton S Second Law Is Usually Written As F = Ma

Force Notes: Mr. Davis

Newton’s Second law is usually written as F = ma.

The F in this case is the sum of all the forces acting on an object, and the sum of the forces, if non- zero, produces an acceleration.

Since F  ma, a non-zero Force means a non-zero acceleration (and vice versa).

Fnet means the sum of all of the forces.

So Fnet = Fappl – Ff means that the sum of all of the forces (Fnet) = the force pushing (or pulling) on an object minus Ff.

If Fnet = 0 (sum of the forces equals zero), then any Force being applied to the object equals the force of friction Ff).

The normal force (Fn) is the force pushing on an object by a surface that keeps the object from falling though the surface. There is no normal force for an object in free fall. There is no normal force for an object hanging from a rope.

The force of friction is the force opposing the motion of an object due to interaction with its environment. Usually this is at a solid-solid interface, although air resistance is described as a frictional force. For a frictional force at a solid-solid interface, the coefficient of friction is defined as

 = Ff/Fn, or Ff =  Fn.

On a flat horizontal surface Fn = Fg = mg = the weight of the object. On an inclined plane Fn = mg cos (Note that for a horizontal surface, = 0 and mg cos(= mg. This means that on an inclined plane the Ff will be lower than for the same object sitting flat on a surface. This makes sense, since as you tip the plane the Ff diminishes until the object slides. Of course, the Fg parallel to the plane is also increasing and is given by

Fg = mg sin(

Force is a vector quantity, and can be broken into components.

Newton’s third law (For every action there is an equal and opposite reaction) means that every pair of objects produce equal forces on each other. Mostly this comes down to stopping baseballs or hockey pucks, or accelerating blocks of ice or baseballs or hockey pucks, or rocket ships.