1. Chapter 3 Describing Motion In this chapter, you will begin to use tools to analyze motion in terms of displacement, velocity, and acceleration. These concepts will help you to determine how fast and how far an object will move, whether the object is speeding up or slowing down, and whether it is standing still or moving at a constant speed.

2. Picture Motion When an object is in motion, its position changes, and that its position can changed along the path of a straight line, a circle, a graceful arc, or a back and forth vibration. Motion Diagrams – A series of images of a moving object that records its position after equal time intervals. Examples are Fig. 3-5 pg. 45. 4 parts of motion: At rest Speeding up Slowing Down Constant speed

3. Particle Motion Replacing an object by a single point is called the particle model. Look at Figure 3-6. page 46. Use the particle model to draw a motion diagram for a car that starts from rest, speeds up to a constant speed, and then slows to a stop.

4. Scalar Quantity – A quantity that only has magnitude. Vector Quantity – A quantity that has both magnitude and direction. Scalar quantities are represented by simple letters such as m, t, and T. Vector quantities tells you not only the magnitude of the quantity, but also the direction.

5. Displacement – defines the distance and direction between two positions. The difference between two times is called the time interval.

6. If it takes a sprinter to run the 100 meter- dash in 9.68 seconds, what was his time interval? Distance – a scalar quantity that is the length, or size, of the displacement vector.

7. Remember* Distance is a scalar quantity. Draw a dot diagram of a car speeding up. Add displacement vectors to the diagram. Draw a dot diagram for a runner slowing down to a stop. Add displacement vectors between each dots. Draw a diagram of a bus that speeds up, then moves at constant speed, then brakes to a halt. Add displacement vectors.

8. Velocity – Speed and direction. (sound familiar?) Suppose you record a speedy jogger and a slow jogger, you will be able to tell that the position of the jogger changes more than that of the walker. In other words, for a fixed time interval, the displacement is larger for the jogger because she is moving faster.

9. Average Velocity To get average velocity, you need both distance and time.

10. Average Speed Average speed is the ratio of the total distance traveled to the time interval. Example: Automobile speeds are measured in miles per hour, but in physics, things will be measured in meters per second. (m/s)

11. Instantaneous Velocity Instantaneous Velocity – The speed and direction of an object at a particular unit of time.

12. Displacement Average Velocity and Time

13. Acceleration An object in motion whose velocity is changing is said to be accelerating. Recall that an object’s velocity changes when either the magnitude or direction of motion changes. Average Acceleration: The change in average velocity divided by the time.

14. Average Acceleration