Mail Time: Weird Fundamentals Quiz Questions and Answers

Georgian Michael Gray asks about a couple of questions I ask on a kinematics fundamentals quiz:

I am looking at your fundamentals quiz 1 and I don't quite understand what answer you are looking for in these two Questions.

1.       An object has a negative acceleration.  Explain briefly how to determine whether the object is speeding up, slowing down, or moving with constant speed.

If an object has a negative acceleration... how could it move with constant speed?

Exactly, Michael.  A correct student response says that a positive velocity means slowing down, a negative velocity means speeding up, and the object can NOT move at constant speed.*

(* Just so everyone understands, in this portion of the course we are explicitly discussing straight-line motion only.)

2.       Given the velocity of an object, how do you tell which direction that object is moving?

I mean, velocity has magnitude and direction... so if v is positive then you see which way you defined to be positive... and that way.

Yup... you're two for two.  I'd make it an even simpler response:  "The object moves in the direction of the velocity vector."

 

These seem like silly questions. I'm trying to get my students to articulate their understanding in a variety of situations. They think they know what acceleration means... then they'll say that something moving at a constant speed of 10 m/s must be falling near earth's surface. Or, they'll get so caught up in the mathematicalness of the words "positive" and "negative" that they think too hard... when the direction of motion IS the direction of velocity. The more different ways they're asked the same kind of question, the more they'll understand.

 

GCJ