Distance Is the Length of a Path Between Two Points

Motion (Chapter 9) An object is in motion if its distance from another object is changing. A reference point is a place or object used for comparison to determine if something is in motion.

Distance is the length of a path between two points. Displacement is the length and direction that an object has moved from its starting point. If you run half way around the oval track, you are traveling an actual path you can measure the length of, which is 200 meters. You ran a distance of 200 meters, but you were displaced from one side of the track to the other in a given direction. A vector is an arrow showing size and direction. Your displacement across the track can be shown with a vector.

You also ran a certain speed around the track. Speed can be measured by dividing the distance by the time it took you to walk around the track. If it took you 2 minutes, or 120 seconds, to run half way around the track your speed was 200 meters divided by 120 seconds equaling 1.67 meters per second. S= 200m/120s = 1.67 m/s

If you start and stop running a number of times, you can measure your average speed by dividing the total distance by the total time of running. You could also measure your speed at any given time while running and this would give you your instantaneous speed. Velocity is the speed in a given direction. Velocity can be measured with a vector. Changes in velocity can be due to changes in speed, direction, or both. When you run around the track you are constantly changing directions. To show speed on a distance verses time graph you can measure the slope of the line by finding the vertical difference between two points and dividing it by the horizontal difference between the same two points. See pg. 346-347.

Acceleration

When you run a few laps you are probably not traveling at a constant speed or velocity. Acceleration includes increasing speed, decreasing speed, or changing direction. Acceleration equals final velocity minus starting velocity divided by the time it took to change the velocity. A= final velocity (m/s) – starting velocity (m/s) time (s)

Example: when a car gets on the freeway on-ramp how can we find the acceleration of the car?

25 m/s 32 m/s 39m/s 46 m/s 53m/s 60m/s │ ______│ ______│ ______│ ______│ ______│ 0 s 1 s 2 s 3 s 4s 5s

2 A = Vf – Vs = 60m/s – 25m/s = 35m/s = 7m/s/s or 7m/s t 5s 5s Each second the car increased its velocity at the rate of 7 m/s.