1. Unit 2 Section 2 NotesAcceleration

2. Acceleration  Acceleration: rate of change in velocity  An object accelerates if:  It’s speed changes: if an object is speeding up or slowing down, its acceleration is changing  It’s direction changes: if an object is traveling at the same speed but changes direction  Both speed and direction change  Acceleration is a vector quantity: includes both magnitude and direction.

3. Acceleration  An object is accelerating even it its speed stays the same and only its direction changes. Why?  Acceleration is defined as the rate at which velocity changes with time; velocity includes both speed and direction, so an object accelerates if its speed, direction, or both change.  Therefore, you can constantly accelerate while never speeding up or slowing down!

4. Centripetal Acceleration  Centripetal Acceleration: acceleration that occurs in a circular motion.  If you move at a constant speed in a circle, even though your speed doesn’t change, your direction does; therefore, you are accelerating.  Examples: The moon accelerates around the earth Riding on a Ferris wheel Blades on a windmill Roller coaster

5. Calculating Acceleration  To find the acceleration of an object in a straight line, you need to measure the object’s velocity at different times.  To mathematically find acceleration, take the change in an object’s velocity divided by time.  or 

6. Acceleration Values  What do acceleration values tell you?  If acceleration is small, velocity is increasing very gradually.  If acceleration is large, velocity is increasing more rapidly.  Positive acceleration: an object’s velocity is increasing; object is speeding up  Negative acceleration: an object’s velocity is decreasing; object is slowing down

7. Acceleration Values  In science, acceleration describes any change in velocity; not just speeding up  When you slow down, you have a negative acceleration because it is opposite the direction of motion.

8. Graphing Accelerated Motion  When graphing acceleration:  Independent variable, which is time, is on the x- axis  Dependent variable, which is speed/velocity, is on the y-axis

9. Graphing Accelerated Motion  A straight line on a speed vs. time graph means the speed changes by the same amount over equal time intervals. This is called constant acceleration.  The slope of a straight line on a speed vs. time graph is equal to acceleration.  Line with a positive slope: object is speeding up  Line with a negative slope: object is slowing down  A curved line on a distance vs. time graph means the object is accelerating.

10. Acceleration Sample Problems  Natalie accelerates her skateboard along a straight path from rest to 4.5 m/s in 2.5 seconds. Find her average acceleration.

11. Acceleration Sample Problems  Find the average acceleration of a northbound subway train that slows down from 12 m/s to 9.6 m/s in 8 s.

12. http://www.physicsclassroom.com/Class/1DKi n/U1L1e.cfm http://www.physicsclassroom.com/Class/1DKi n/U1L1e.cfm