Forces
Scientists consider both forces and velocities as vectors. Vectors are shown by arrows: they represent quantities that have both a specific magnitude—size or strength—and direction. Velocity, for example, has both magnitude and direction. Although the words speed and velocity are used interchangeably, speed is properly only the magnitude of the velocity vector. A complete description of an object's velocity requires both a knowledge of the object's speed and the direction in which it is traveling. For example, a stone whirled in a circle at the end of a string has a changing velocity even if it moves at a fixed number of revolutions per minute. The stone's speed is constant, but its direction of travel, and therefore its velocity, changes continuously. The force on the stone that causes the change in velocity is another vector, called a centripetal force. Its magnitude is the tension in the string, and its direction is radially inward toward the center of the circle described by the spinning stone.

Two forces applied simultaneously to the same point have the same effect as a single equivalent force. The magnitude and direction of this resultant force can be found by drawing the two original force vectors head to tail and then drawing a new vector—the resultant force vector—from the tail of the first vector to the head of the second. Similarly, vectors can also be added by the use of parallelograms (see diagram).
The same forces can have different effects depending on how they are applied and on the specific body to which they are applied. For example, if applied in a certain way, a force may cause a body to spin, or rotate. The tendency of a force to rotate the body to which it is applied is called torque, or moment of the force. Torque is also a vector. The magnitude of the torque can be calculated by multiplying the perpendicular distance between the line of the force and the axis of rotation.
The force that resists the motion of a body along a path or the torque that opposes rotation is called friction. Both frictional forces and frictional torques are passive and do not exist alone. They appear only when other forces are applied or if a body is already in motion. Friction may be undesirable, as in the case of air resistance that slows down an airplane, or it may be useful, as it is in the case of car brakes, which slow down a car by means of friction