# Homework Help: Please help- Arrow shot in sky question

1. Jan 17, 2005

### mrd59

Ok, I'm trying to help my son with this test question he got wrong. He is in a basic high school physics class and I have virtually no experience with physics. The book and classroom handouts have nothing that completely explains this question. Any help and explaination would be most wonderful!

An arrow is shot straight up into the air and then falls back down to the ground. On the way up , the arrow could be described as......

A) Positive inertia, positive velocity, positive acceleration and positive momentum. (he marked this and got it wrong)

B)Negative inertia, positive velocity, positive acceleration and negative momentum.

C) positive inertia, negative velocity, negative acceleration and positive momentum

D) Positive inertia, positive velocity, negative acceleration and positive momentum. (we think this might be the correct answer but are not sure why)
:surprised

2. Jan 17, 2005

### ek

Negative acceleration.

Gravity is slowing the arrow down, if it was positive acceleration the arrow would go flying off into space.

3. Jan 17, 2005

### chroot

Staff Emeritus
Is this the entire question?

This question actually has no answer as stated, because it fails to make clear its assumptions. Velocity cannot be said to be positive or negative without first defining which direction should be considered positive! Acceleration suffers the same problem; you must first define the directions. People often consider gravity to be a negative acceleration, but this is just a convention. You could just as well consider it a positive acceleration.

Besides, the terms "positive inertia" and "negative inertia" mean nothing at all. Perhaps the teacher meant kinetic energy?

If this is the entire problem as given, the teacher should really be ripped to shreds. If I were you, I'd be making a phone call.

- Warren

4. Jan 17, 2005

### ek

It's just grade 10 or 11 physics. I don't think there's a need to over complicate things for students. Assumptions are made for the ease of the students I'm guessing. That inertia thing though, I was thinking wtf too.

5. Jan 17, 2005

### chroot

Staff Emeritus
A problem that is not completely specified does not have a completely specified answer, no matter what "easing" is done. If the teacher told the class that "upwards velocities are always positive in my class," then so be it, but we here on physicsforums.com do not have access to that information. We cannot answer it.

- Warren

6. Jan 17, 2005

### mrd59

Yes this is the entire question. He gets some extra points for correcting his test questions and explaining why the wrong answers were wrong and the correct answer is correct. Of course the problem with that is the teacher will not help by telling you which one is the correct answer. All we have are some very sketchy handouts from class and a Conceptual physics book. Most of test questions have taken us a long time to help him figure out (post test) even with these resources. It seems to be a vicious circle. Material is not explained, test questions do not come from the book or handouts, student fails, teacher says find the right answer, but that is hard if you don't know it (which is why you failed), and resources are minimal. A parents' (and students') nightmare!

Last edited: Jan 17, 2005
7. Jan 17, 2005

### Curious3141

As chroot said, the statement of this question is flawed. Although, one can infer the convention from the fact that 3 of the quantities should be signed one way and one the other way meaning there's only one choice that fits. Nevertheless, feedback should be given to the teacher.

I've had bitter experience with teachers who misinform, insist they're right when they're wrong, and then sneakily correct themselves later without fanfare or even acknowledgement.

For the sake of your son and the rest in his class make sure the teacher knows why this question is incomplete. You can always ask the teacher to come here to get chastised. :tongue2:

8. Jan 17, 2005

### chroot

Staff Emeritus
I fondly remember a ninth-grade chemistry teacher spending a half an hour of class time trying to convince me that a milliliter of any substance weighs one gram.

- Warren

9. Jan 17, 2005

### dextercioby

We wouldn't be that lucky... :tongue2:

To the OP:My advice is to start l'exposeĆ© with the words:
"I chose the Oy axis pointing upwards".

Then the answer will be correct and the idiot would have nothing to say...

Daniel.

10. Jan 17, 2005

### mrd59

Thanks to all of you! My daughter is a physics major in college but she's back at school now so not around to help. I might be posting quite often if this class continues to go in this direction!

-m

11. Jan 17, 2005

### Integral

Staff Emeritus
I would bet that in class the teacher had a standard set of definitions for the coordinate system. Clearly he expected his default definitions to be known and understood by the students. We are not privy to such definitions. Your son needs to examine, notes etc. to gain an understanding of these unstated but implied definitions.