1. Physical Science Coach Kelsoe Pages 342–348 S ECTION 11–3: A CCELERATION

2. O BJECTIVES  Identify changes in motion that produce acceleration.  Describe examples of constant acceleration.  Calculate the acceleration of an object.  Interpret speed-time and distance-time graphs.  Classify acceleration as positive or negative.  Describe instantaneous acceleration.

3. A CCELERATION  The rate at which velocity changes is called acceleration.  Acceleration can be described as changes in speed, changes in direction, or changes in both. Acceleration is a vector.

4. C HANGES IN S PEED  We often use the word acceleration to describe situations in which the speed of an object is increasing. This can be slightly misleading  ANY change in speed (speeding up or slowing down) is what scientists would call “acceleration.”  We can call a negative acceleration, or a slowing down effect, a “deceleration.”  Can an object have zero acceleration and still be moving?

5. C HANGES IN S PEED  If you are in a car that is stopped at a traffic light, you will feel a positive acceleration (speeding up) when the light turns green. When you reach a cruising speed, you will have zero acceleration. As you approach the next red light, you will experience a negative acceleration, or deceleration (slowing down).  An example of acceleration due to change in speed is free fall, the movement of an object toward Earth solely because of gravity.  The unit for acceleration is meters per second per second, or meters per second squared (m/s 2 ).

6. C HANGES IN S PEED  Objects falling near Earth’s surface accelerate downward at a rate of 9.8 m/s 2. Each second an object is in free fall, is velocity increases downward by 9.8 m/s.  After one second, an object falling would be moving at 9.8 m/s. At two seconds, its speed would be 19.6 m/s. At three seconds, its speed would be 29.4 m/s.  The change in the object’s speed is due directly to gravity.

7. C HANGES IN D IRECTION  Acceleration isn’t always the result of changes in speed. You can accelerate even if your speed is constant.  You experience this type of acceleration as you ride around a curve at the same speed.  You feel acceleration/deceleration, not speed or velocity.

8. C HANGES IN S PEED AND D IRECTION  You experience this type of acceleration when you ride a roller coaster. Typically your speed and direction are always changing.  You will also experience this type of acceleration when you slow down to enter a curve in your vehicle. In this case, both the speed and direction change.

9. C ONSTANT A CCELERATION  The velocity of an object moving in a straight line changes at a constant rate when the object is experiencing constant acceleration.  Constant acceleration is a steady change in velocity. That is, the velocity of the object changes by the same amount each second.  An example of constant acceleration is illustrated by a jet airplane during takeoff.

10. C ALCULATING A CCELERATION  Acceleration is the rate at which velocity changes.  You calculate acceleration for straight-line motion by dividing the change in velocity by the total time. If a is the acceleration, v i is the initial velocity, v f is the final velocity, and t is total time, this equation can be written as follows: a = (v f – v i )/t  Think about how a change in either your final velocity or initial velocity would have an effect on the acceleration. Acceleration is a vector quantity.

11. C ALCULATING A CCELERATION  A ball rolls down a ramp, starting from rest. After 2 seconds, its velocity is 6 m/s. What is the acceleration of the ball?  Initial velocity (v i ) = 0 m/s (starting from rest)  Final velocity (v f ) = 6 m/s  Total time (t) = 2 s  Acceleration (a) = (6 m/s – 0 m/s)/2 s = 6 m/s/2s = 3 m/s 2

12. G RAPHS OF A CCELERATED M OTION  You can use a graph to calculate acceleration.  When using a speed-time graph, the slope of the line is acceleration.  This slope is change in speed divided by change in time.

13. S PEED -T IME G RAPHS  Constant acceleration is represented on a speed-time graph by a straight line.  A linear graph is a graph in which the displayed data forms straight-line parts.  When the slope of a line is positive, the acceleration is positive. When the slope of a line is negative, the acceleration is negative.

14. D ISTANCE -T IME G RAPHS  Accelerated motion is represented by a curved line on a distance-time graph.  In a nonlinear graph, a curve connects the data points that are plotted.

15. I NSTANTANEOUS A CCELERATION  Acceleration is rarely constant, and motion is rarely in a straight line.  Instantaneous acceleration is how fast a velocity is changing at a specific instant.  An object has an instantaneous acceleration at all times, but sometimes the instantaneous acceleration is zero.

16. V OCABULARY  Acceleration  Free fall  Constant acceleration  Linear graph  Nonlinear graph