STRAIGHT-LINE MOTION

The term position means the location of a specified object. To defined a position of an object, we must give a reference point or location. For example, the entrance to your house is 10 meters away from your mailbox. Or your cat is under the table. And so on.

If an object changes its position, then the motion has occurred. If an object is undergoing a continuous change in position, then we would say the object is in motion.

Since the position is relative, the motion is also relative. For example, "The car is driving 1 kilometers per minute," indicates that the car is changing its position at 1 kilometer for each minute of time, relative to the road, or ground. Since the earth itself is moving. Therefore when we are describing speed, it's very much like comparing the moving object to another object. The road for example, is the frame of reference for the car's motion. Similarly, if we said, " The car is traveling at the rate of 90 miles per hour," we are using the road as the reference for the car's motion.

An example of straight-line motion is an car driving east on a level highway. The motion of the car may or may not be at a constant rate, or at a constant speed. In either case, the automobile is moving at a straight line. It's like a ray in geometry, it has the same slope, and will not change.

Combining length and time will give the time rate of change of position. It is the basis of describing motion in terms of speed and velocity.