1. The position of an object can be described by locating it relative to another object or the background.

The location of an object can be described by stating its distance and direction from another object or reference point. Distance may be measured using a ruler, measuring stick, measuring tape or other appropriate instrument. Terms such as left, right, above, or below may be used to describe the direction of an object from a reference point. Direction may also be measured by using a compass positioned at the reference point, and reading the number of degrees the object is from a specified reference direction. A reference background or system, such as the lines of latitude and longitude that appear on globes and maps, provide another way of describing the location of an object.

2. An object’s motion can be described by tracing and measuring its position over time.

The word “motion” implies that there is a change in position over time. An object in motion moves a certain distance in a given time. Speed measures how fast something is moving. Average speed is the distance an object travels divided by the time of travel.
Instantaneous speed is the speed that an object has at any given instant. Instantaneous speed is determined by finding the average speed over an infinitesimally small time interval. (Example: The speedometer of a car indicates the instantaneous speed but is actually determined by finding the average speed during a single revolution of the tires.) Velocity describes not only how fast an object is moving but also specifies in what direction. Sometimes people loosely use the term “velocity” interchangeably with “speed,” but there is a distinct difference. Velocity is speed and direction. (Example: A car travels at 60 km/hr to the north)

3. Motion can be changed by increasing or decreasing the speed of an object and/or changing the direction of its motion.

Because velocity is both speed and direction, the velocity of an object changes whenever it speeds up, slows down, or changes direction (such as going around a curve). Whenever the velocity of an object changes in any of these ways, we say that an object accelerates. Acceleration describes how quickly the velocity changes:

NOTE: Many people erroneously define acceleration as simply “change in velocity.” It is actually the “rate at which velocity changes.” EXAMPLE : It takes 4 seconds for a remote-controlled model airplane traveling north at 10 meters/second to speed up to 18 meters/second in the same direction. In this example, the change in velocity of the airplane is 8 meters/second to the north, and the acceleration is 2 meters/second/second or 2 m/s².

4. In the absence of forces such as gravity or friction, an object will keep its speed and direction of motion.

If an object moves at a constant velocity, then there are either no forces acting on it, which is unlikely in the real world, or any forces acting on it are balanced (net force = 0). Similarly, if an object is at rest, then any forces acting on it are balanced (net force = 0). An object at rest can be thought of as having a constant velocity of zero.

5. The motion of an object can be changed by an unbalanced (or net) force.

If the velocity of an object is changing, there is an unbalanced force acting on the object (net force is not equal to zero). A net force acting on an object in the direction of its motion will increase the speed. A net force acting opposite to the direction of motion of the object will decrease the speed. A net force acting at a right angle to the motion of the object will change the direction of motion. A net force acting at any other angle will change both the speed and the direction of motion.

6. The quickness with which an object changes its motion is related to the mass of the object and the size of the force acting on it.