# Basic Relative Motion Problem

1. Sep 29, 2016

### physicsparent

1. The problem statement, all variables and given/known data
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.

2. Relevant equations
subtract the speeds, since they are in opposite directions
3. 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!

2. Sep 29, 2016

### Bystander

Is this given? Or, inferred?

3. Sep 29, 2016

### physicsparent

inferred

4. Sep 29, 2016

### Bystander

"Speeds?" Or, "velocities?"

5. Sep 29, 2016

### physicsparent

the problem only mentions 'speed'

6. Sep 29, 2016

### Bystander

7. Sep 29, 2016

### 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 ...?

8. Sep 29, 2016

### Bystander

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

9. Sep 29, 2016

### 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?

10. Sep 29, 2016

### Bystander

You are asking me? What do you get?

11. Sep 29, 2016

### 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

12. Sep 29, 2016

### Bystander

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

13. Sep 30, 2016

### Orodruin

Staff Emeritus
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 begining you would have had an answer a long time ago.

14. Sep 30, 2016

### 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.

15. Sep 30, 2016

### Orodruin

Staff Emeritus
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.

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).

16. Sep 30, 2016

### physicsparent

My reasoning is this: since the bales were thrown at 3 m/s east relative to the tractor, as the tractor is still travelling 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.

17. Sep 30, 2016

### 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?

18. Sep 30, 2016

### 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?

19. Sep 30, 2016

### Orodruin

Staff Emeritus
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 ...

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.

Last edited: Sep 30, 2016
20. Sep 30, 2016

### Staff: Mentor

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.)

Last edited: Sep 30, 2016