Misconception in activity 2 sports

[Fred Meshna]

On page 21 in the physics talk section of activity two it states that " When speed decreases, the acceleration has a negative value.  Please tell me how I explain to my students how the acceleration due to gravity is negative 9.8m/s/s when an object's speed is increasing as it falls?  Negative acceleration does not indicate that something is slowing down.  Things slow down only when the sign for acceleration is opposite from the sign of velocity.  The negative or positive designation only refers to direction not increase or decrease in speed.  I am surprised that mistatement was printed.

Fred Meshna

[Janie Martin]

I told my students that acceleration is negative when the direction of the velocity is opposite to the acceleration. Does that sound OK?
I also noticed that we are told that the speed of light through air is 3X10* m/s but that is the speed of light in a vacuum, light moves slower throug various "airs".

[Fred Meshna]

Janie,

Acceleration is negative if velocity is positive and opposite in direction to acceleration.

The negative sign only indicates direction.

Fred Meshna

[Matt Anthes-Washburn]

Janie,

Acceleration is negative if velocity is positive and opposite in direction to acceleration.

The negative sign only indicates direction.

Fred Meshna

My students and I discussed a simple number line, where a positive vector means you are moving in the positive direction, and a negative vector is in the negative direction.

A positive acceleration indicates that the velocity is changing toward the positive velocity. That can be making a positive velocity more positive, or a negative velocity less negative. Like Fred said, positive and negative are just directions, and you choose to assign them at your own convenience.

Mateo

[Janie Martin]

Sorry , I had not been able to access the site for some time. But, yes Fred what I did tell the students was that the negative sign on acceleration is only a direction and negative when the velocity is positive and the acceleration is going in the opposite direction.
I use the number line analogy also.
I mistyped what I meant.
Darn keyboards.