Describing Motion

Motion An object is in motion if its distance from another object is changing.

Reference Point A reference point is a place or object used for comparison to determine if something is in motion. An object is in motion if it changes position relative to a reference point

Relative Motion Whether or not an object is in motion depends on the reference point.

Speed and Velocity

Speed If you know the distance an object travels in a specific amount of time, you can calculate the speed of the object. Speed of an object is the distance the object travels per unit of time

Calculating Speed To calculate the speed of an object, divide the distance the object travels by the amount of time it takes to travel that distance.

Average Speed Speed of most moving objects not constant Average Speed is total distance (d) traveled divided by the total time (t).

Average Speed Example Total distance = 36 miles + 24 miles = 60 miles Total time = 3 hours + 1 hour = 4 hours Average speed (v) = d = 60mi t 4 = 15 mph

Instantaneous Speed Instantaneous speed is the rate at which an object is moving at a given instant in time.

Velocity Velocity is speed in a given direction. Like displacement, velocity is a vector – it has both magnitude (speed) and direction. Changes in velocity may be due to changes in speed, changes in direction or both.

Graphing Speed Slope – steepness of a line on a graph Slope formula:

Acceleration

Changing Velocity Acceleration is the rate at which velocity changes with time. In science, acceleration refers to increasing speed, decreasing speed or changing direction.

Increasing Vs. Decreasing Speed Whenever an object’s speed increases, the object accelerates. Just as objects can speed up, they also slow down, this is called negative acceleration.

Changing Direction An object can accelerate even when it’s speed is constant – when it’s changing direction. Circular motion

Calculating Acceleration To determine the acceleration of an object, you must calculate its change in velocity per unit of time. Acceleration = Final velocity – Initial velocity Time

Graphing Acceleration You can use both a speed – versus - time graph and a distance – versus - time graph to analyze the motion of an accelerating object.

Speed – Versus – Time Graph The slope of a line on a speed – versus – time graph represents acceleration.

Distance – Versus – Time Graph You can also show the motion of an accelerating object with a distance – versus – time graph.