Acceleration: chapter 3

What is accelerating?  Car braking?  Car with its cruise control set?  Merry-Go-Round with constant motion?  Leaf falling from a tree? Yes No Yes

Changing Velocity  Anytime you change velocity = acceleration  Acceleration can be positive or negative  Pg. 58: Which Motion Diagrams represent acceleration?

Acceleration  Acceleration – the rate at which an object’s velocity changes  Average acceleration – the change in velocity during some measureable time interval divided by that time interval.  Instantaneous acceleration: can be found by drawing a tangent line on the velocity-time graph at the point of time in which you are interested. The slope of this line is equal to the instantaneous acceleration.  Note: Will often think of average and instantaneous acceleration as being equal in this class unless specified otherwise

Velocity vs. time graph  What does a horizontal line mean on a velocity vs. time graph?  What does a straight line (up) represent on a velocity vs. time graph?  What does a straight line (down) represent on a velocity vs. time graph?

Copy figure 3-4 from page 60

Displaying Acceleration on a Motion Diagram  a avg = Δv/Δt  Δv = v f – v i  We usually measure acceleration in m/s 2 for physics class

Practice Problems pg. 61; 3-5

Answers to Practice Problems 3a. 5.0 – s 3b. 0.0 – 5.0 s 3c – 20.0 s 4a. 2.0 m/s 2 4b m/s 2 4c. 0.0 m/s 2

Acceleration can be positive  Draw figure 3-7a; when the velocity vectors of the motion diagram and acceleration vectors point in the same direction, an object’s speed _____________

Acceleration can be negative  Draw figure 3-7b; when the velocity vectors of the motion diagram and acceleration vectors point in opposite directions, an object’s speed _____________

Note: Negative acceleration  The term DECELERATION; to some implies “slowing down” or reducing speed, to others it implies “negative acceleration” for this class we will just the term NEGATIVE ACCELERATION  A negative acceleration could be EITHER speeding up OR slowing down; it just depends on how the corrdinate system is assigned.

Figure 3-8 pg. 62  Draw the graph; explain what is happening at each point.  A & E =  B=  C=  D =

Discussion Question  A classmate is skateboarding along a level surface into a very strong wind, which slows her and eventually moves her backward. As her motion changes direction, what is her velocity? Is her acceleration as her motion changes positive, negative, or zero Answer: the instant she changes direction, her instantaneous velocity is zero. If her initial direction of motion is positive, then her acceleration is negative, and vice versa

Example Problem 2 on page 63

HW: practice problems 6-11

HW: practice problems 6-11 on pg. 64