What is the Correct Direction of Hay Bales in a Basic Relative Motion Problem?

[physicsparent]

Homework Statement

A farmer is throwing bales of hay off the back of his tractor with a speed of 3 m/s relative to the tractor, which is moving west with a speed of 7 m/s. Determine the speed and direction of hay bales relative to the ground.

Homework Equations

subtract the speeds, since they are in opposite directions

The Attempt at a Solution

4 m/s -- this is where the issue arises: the direction of the hay bales. my son's teacher says one thing, and i insist it's the opposite. without revealing which of us believes what here, can you please tell me what the answer SHOULD be? Thank you!

 

[Bystander]

Homework Equations

subtract the speeds, since they are in opposite directions

Is this given? Or, inferred?

 

[physicsparent]

inferred

 

[Bystander]

"Speeds?" Or, "velocities?"

 

[physicsparent]

the problem only mentions 'speed'

 

[Bystander]

There has been no specification of direction; i.e., it's indeterminate. See
https://www.google.com/search?noj=1......0j0i7i30k1j0i7i10i30k1j0i131k1._fEeCa1SFC8

 

[physicsparent]

the direction of the tractor is specified (west) and since the question asks for a direction, i would think that the teacher wants one. i know speed is scalar and velocity is vector, but here direction is required as part of the answer ...?

 

[Bystander]

off the back

Assume the tractor is moving "forward," whatever "forward" means, then "off the back" implies the opposite direction.

 

[physicsparent]

yes. so if we make the problem 'the tractor is moving 7m/s west, and the hay bale is thrown east at 3m/s,relative to the tractor' then what would be the answer?

 

[Bystander]

what would be the answer?

You are asking me? What do you get?

 

[physicsparent]

Trying not to reveal that, since I don't want to influence responses. I got one direction as the answer; the teacher got the other

 

[Bystander]

to influence responses.

You won't get an answer from anyone until you commit yourself. We do not play that game on this forum.

 

[Orodruin]

As Bystander mentions, the rules for the homework forums require you to not only commit to an answer, but to tell us how your reasoning for arriving at that answer.

This problem is trivial for most people here so the idea of "influencing" people's answers is very far fetched. If you had provided your thought process and result from the beginning you would have had an answer a long time ago.

 

[physicsparent]

Wow. I'm a parent of a physics student, trying to get clarification on a test response from a week ago - not playing some game or trying to trick people into doing homework. The insinuation is both inaccurate and insulting.

Answer = 4 m/s WEST

 

[Orodruin]

Wow. I'm a parent of a physics student, trying to get clarification on a test response from a week ago - not playing some game or trying to trick people into doing homework. The insinuation is both inaccurate and insulting.

There is no insinuation or implication that you are not telling the truth. That students occasionally attempt to get others to do their homework for them is only one of the reasons that this rule is in place. The rule is also there because we do not strive to be a Q&A site where you go simply to get an answer. We expect that people who ask questions here are genuinely interested in learning the subject and willing to do the minimal amount of work required. It is also a nightmare to start guessing how people have argued when they are wrong if they do not supply the thought process.

Apart from the above, this is all explained in the rules that you agreed to when registering. Spending the time to do this properly, you might actually learn something more than the answer as well.

Answer = 4 m/s WEST

And what is your reasoning? What is the reasoning of your son's teacher? And since you have a different response from the teacher, you should also be able to answer the question: Where is the flaw in the teacher's reasoning? It is not a shouting contest or contest of authority, it is a question about making a coherent argument for why the other is wrong (or accepting a coherent argument for why you are wrong).

 

[physicsparent]

My reasoning is this: since the bales were thrown at 3 m/s east relative to the tractor, as the tractor is still traveling at 7 m/s west relative to the ground, then the bales still have a velocity of 4 m/s to the west, relative to the ground. The bales would have to be thrown at a velocity greater than that of the tractor to move east in relation to the ground.

The above is my reasoning. The teacher gave no reasoning, only that the answer is 4 m/s east. I'm looking for either 1) someone of authority (ie someone who knows physics -- I took it as a junior in high school, 20+ years ago) to confirm my reasoning or 2) someone of authority to explain why it should be east, hopefully in a way i can understand :) I actually do want to understand this, and here's why: I know this is a basic physics question and matters will only get more complicated from here.

Thanks in advance for any assistance.

 

[kuruman]

Consider this: If the tractor were not moving and the farmer tossed the bale out with a speed of 3 m/s east relative to the tractor, the speed of the bale relative to the ground would also be 3 m/s east. How reasonable is it for the bale to be moving faster than that in an easterly direction if the tractor is moving at 7 m/s west?

 

[physicsparent]

EXACTLY! The alternative, as you stated, (4 m/s east) makes no common sense, even if I know nothing of physics (which I don't). The concern is, the EAST response came from an AP Physics teacher, and when questioned very nicely as to why it wouldn't be WEST, no attempt was made at all to reread the question, give some explanation, or help us understand. Just a digging-in-of-the-heels, if you will, that their answer was correct. Worries me because my son has tests coming up, and he's conflicted as to how to answer these types of questions now. He doesn't want to question his teacher (understandably), but we don't want him building on a false understanding. Am I being crazy?

 

[Orodruin]

You are correct and the teacher really should know better and be able to provide ample reasoning. If your son's teacher argues against this, send him here or back to taking AP physics. By the same reasoning as the teacher has, the bale should travel at 7 m/s east when just dropped at 0 relative velocity relative to the tractor...

It is particularly disturbing to not offer any reasoning and rely on authority.

Edit: The only way to get east is to mix up the speeds of the obects ...

The alternative, as you stated, (4 m/s east) makes no common sense

Although it leads you right in this case, "common sense" will not always do this. In particular when you move out of realms where we have everyday experience, eg, at the quantum or relativistic level.

 

[Doc Al]

Here's how velocities combine: $V_{a/c} = V_{a/b} + V_{b/c}$

Let a = Hay; b = Truck, c = Ground

Using this (and a suitable sign convention, such as East = +) you can calculate the velocity of the hay with respect to the ground.

(Edit: This is offered if you wanted a more "formal" method of showing the instructor's error. Certainly not needed.)

 

[physicsparent]

Thank you very much, Orodruin and Doc Al. I literally lost sleep trying to figure out why this answer was wrong. Now I have the uneviable task of trying to broach this with the teacher, without coming off as undermining and insulting. So far, polite suggestions like "oh, maybe you misread the question/directions" have been met with defensiveness :(

I appreciate the help -- validates that at least some small part of my brain is still working!

 

[Orodruin]

To be honest, I would not be so polite. I would just tell him to check this thread. Unless he (well, or you) misread the question and you are discussing different problems, he should not be teaching physics.

 

[physicsparent]

the problem as stated at the top of the thread is verbatim, except for the farmer's name. So ... yeah :(

 

[Bystander]

verbatim

Cross your fingers (and mine) you don't run into brick wall.

 

[physicsparent]

unfortunately, today's test came home, and I think the teacher is in error again on a completely different question :(

 

[Doc Al]

unfortunately, today's test came home, and I think the teacher is in error again on a completely different question :(

Uh oh. Well, let's have a look.

 

[physicsparent]

How do I show the scenario without having to explain my 'I'm just a parent, not a student looking for free answers' situation again? Please know I would never wish for someone to lose their job or get in 'trouble' because of me, but I am worried that this will progress as we go on and snowball. Can't imagine I'm the only parent who's catching this

 

[Greg Bernhardt]

How do I show the scenario without having to explain my 'I'm just a parent, not a student looking for free answers' situation again?

You can link to this thread and it doesn't really matter if you try to work through the problem like this thread :)

 

[Doc Al]

How do I show the scenario without having to explain my 'I'm just a parent, not a student looking for free answers' situation again?

What Greg said.

Please know I would never wish for someone to lose their job or get in 'trouble' because of me, but I am worried that this will progress as we go on and snowball. Can't imagine I'm the only parent who's catching this

If the next issue is anything like the first, you owe it to everyone to wake this guy up.

 

[hmmm27]

Suggestion: write the teacher a printed note. Include a copy of the question, just in case the teacher's edition of the book has a typo or something.

(i noticed where you didn't explicitly say that you have contacted him/her personally)