Chapter 2 Motion and Speed

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.

Speed  Rate—any change over time  Speed—the distance an object travels per unit of time. –Speed = distance s = d time t 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. 

Distance vs. Time

VelocityVelocityVelocityVelocity  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 = v f – v i = v f – v i  We can rewrite the acceleration equation as a = (v f – v i ) a = (v f – v i ) 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 = (v f – v i ) = (80 m/s – 0 m/s) t20 s t20 s = 4 m/s 2  The airplane is speeding up, so the acceleration is positive.

Velocity vs Time Graph of Positive Acceleration

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

Velocity vs Time Graph of Negative Acceleration

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

An Object Changing Acceleration