Chapter 2 Motion and Speed http://www.nsf.gov/news/special_reports/olympics/bobsled.jsp

Sec. 1 Describing Motion Motion occurs when an object changes position. You do not need to see something move to know that motion has taken place. To know if position has changed, you need a reference point

Distance and Displacement Distance—how far an object moved The SI unit for distance is the meter (m) Displacement—the distance and direction of an object’s change in position from the starting point.

https://www.youtube.com/watch?v=Qhm7-LEBznk https://www.youtube.com/watch?v=6OVvF4xoWzU

Speed Rate—any change over time Speed—the distance an object travels per unit of time. Speed = distance s = d time t Suppose you ran 2 km in 10 min, what is your speed? s = 2 km / 10 min s = 0.2 km / min

Average & Instantaneous Speed Average speed—the total distance traveled divided by the total time of travel. Instantaneous speed—the speed at a given point in time.

Bubble Gum Lab Speed Challenge Lab/ wkst

Graphing Motion On a distance vs. time graph, time goes on the x-axis and distance goes on the y-axis. Time is the independent variable Distance is the dependent variable The slope (steepness) of the line represents the speed of the object. http://www.nsf.gov/news/special_reports/olympics/mathletes.jsp

Distance vs. Time

Velocity Speed describes only how fast something is moving. Velocity—includes the speed of an object AND the direction of its motion Example: a race car has a constant speed of 100km/hr around an oval track. The speed remains constant, but the velocity changes.

Acceleration Acceleration—the rate of change of velocity. The change in velocity over time acceleration = change in velocity time When something speeds up, it has a positive acceleration When something slows down, it has a negative acceleration.

Changing Direction A change in velocity can be either a change in speed or a change in direction. Any time an object changes direction, its velocity changes and its acceleration changes.

Calculating Acceleration As you know, acceleration = change in velocity time How do you find the change in velocity? change in velocity = final velocity – initial velocity = vf – vi We can rewrite the acceleration equation as a = (vf – vi) t

Calculating Positive Acceleration An airplane starts at rest and travels down a straight runway and reaches a top speed of 80 m/s in 20 s. What is its acceleration? a = (vf – vi) = (80 m/s – 0 m/s) t 20 s = 4 m/s2 The airplane is speeding up, so the acceleration is positive.

Calculating Negative Acceleration A skateboarder is moving at 3 m/s and comes to a stop in 2 s. What is the acceleration? a = (vf – vi) = (0 m/s – 3 m/s) = -1.5 m/s2 t 2 s The skateboarder slowed down, so the acceleration is negative.

Velocity vs Time Graph of Positive Acceleration

Velocity vs Time Graph of Negative Acceleration

Zero Acceleration When an object has a constant velocity, the acceleration is zero.

An Object Changing Acceleration