Matter in Motion

Section 1 Measuring Motion What You Will Learn •" Describe the motion of an object by the position of the object in relation to a reference point. • Identify the two factors that determine speed. • Explain the difference between speed and velocity. • Analyze the relationship between velocity and acceleration. • Demonstrate that changes in motion can be measured and represented on a graph." "Look around you—you are likely to see something in motion. Your teacher may be walking across the room, or perhaps your friend is writing with a pencil." "Even if you don’t see anything moving, motion is still occurring all around you. Air particles are moving, the Earth is circling the sun, and blood is traveling through your blood vessels!" Observing Motion by Using a Reference Point "You might think that the motion of an object is easy to detect—you just watch the object. But you are actually watching the object in relation to another object that appears to stay in place. The object that appears to stay in place is a reference point. When an object changes position over time relative to a reference point, the object is in motion. You can describe the direction of the object’s motion with a reference direction, such as north, south, east, west, up, or down". What is a reference point? A reference point in physics is a point in space, regardless of the type of geometry that you are using that stays still and does not move.  Speed Depends on Distance and Time "Speed is the distance traveled by an object divided by the time taken to travel that distance. Look again at Figure 1. Suppose the time interval between the pictures was 10 s and that the balloon traveled 50 m in that time. The speed of the balloon is (50 m)/(10 s), or 5 m/s." "The SI unit for speed is meters per second (m/s). Kilometers per hour (km/h), feet per second (ft/s), and miles per hour (mi/h) are other units commonly used to express speed". Determining Average Speed "Most of the time, objects do not travel at a constant speed. For example, you probably do not walk at a constant speed from one class to the next. So, it is very useful to calculate average speed using the following equation:" Recognizing Speed on a Graph "Suppose a person drives from one city to another. The blue line in the graph in Figure 2 shows the total distance traveled during a 4 h period. Notice that the distance traveled during each hour is different. The distance varies because the speed is not constant. The driver may change speed because of weather, traffic, or varying speed limits. The average speed for the entire trip can be calculated as follows:" Bibliography: http://my.hrw.com/tabnav/controller.jsp?isbn=0030462495