# Homework Help: Which graph(s) illustrates a car that is accelerating?

1. Sep 30, 2016

### physicsparent

[mod note: OP is ok, ref: https://www.physicsforums.com/threads/basic-relative-motion-problem.887296/] [Broken]

I can't figure out how to snap a picture and post here. So I'll try to explain the given scenario as best I can. Four distance versus time graphs are shown:

Graph 1 is a constant/straight diagonal line from 0 upwards
Graph 2 is a curved line from 0, trending increasingly upwards
Graph 3 is a straight upwards diagonal line from 0-2, then a horizontal line from 2-5, then a straight downwards line from 5-7 (all connected)
Graph 4 is a flat horizontal line midway up the graph

The question is: which graph(s) illustrates a car that is accelerating?

Last edited by a moderator: May 8, 2017
2. Sep 30, 2016

### Staff: Mentor

Looks to me like there are two choices that illustrate acceleration. (I wonder if the instructor realized that?)

3. Sep 30, 2016

### physicsparent

Graphs 2 and 3? I thought on distance vs time graphs, a curved line shows acceleration, and a diagonal line showed constant speed?

4. Sep 30, 2016

### Staff: Mentor

Right.

Right. But graph 3 shows three different segments, each with a different velocity. Thus it shows acceleration, since the velocity changes. (Rather abruptly, but whatever.)

5. Sep 30, 2016

### kuruman

Forgive me for butting in, but I am wondering why is physicsparent seeking help for his/her son and not the son himself? How will the help we offer benefit the son better, us offering it directly to him or through physicsparent who by his/her own admission has limited understanding of physics? Also, why is this new thread not placed under a separate topic? Just wondering...

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

### Orodruin

Staff Emeritus
Well, there is the (s) in the question "which graph(s)?"

It is worth clarifying that there is no acceleration in the straight segments, only when the slope of the line changes, but that these changes in the slope represent points where there is acceleration.

7. Sep 30, 2016

### physicsparent

Happy to explain, kuruman. I try really hard not to question teachers in front of my son - after all, I want him to respect them. So I am seeking clarification on things outside of his knowledge and presence so that, if I'm right,we can approach the teacher respectfully, and if I'm wrong, I can get help here to understand why. My son takes what his teacher says as Bible, so he's not questioning a thing. He in fact believes that I'm wrong on the relative motion matter, since his teacher dismissed the question so abruptly. (By the way, it's kind of oxymoronic to ask about helping me: if I didn't have limited understanding of physics, why would I need to seek help here? ;)

8. Sep 30, 2016

### physicsparent

In other words, if I had unlimited knowledge of physics, why would I be here? ;)

I included the question here, as opposed to a new thread, to avoid having to rehash history, as stated above.

9. Sep 30, 2016

### kuruman

Even if they teach your son something wrong?
That's a pity. It would be good for your son to understand that no matter what his teacher says, no matter what I say, no matter what a person with physics "authority" says, nature wants it her way and her way only. It's not a matter of opinion and it's not a matter of say so. The teacher's job, my job, the authority's job is to clarify to your son (in a way that he can understand) the way nature wants it. If he doesn't understand the offered explanation, he is entitled to a better one. Respect is earned by sensible and intelligible physical explanations, not by obfuscation and evasion.
Perhaps I was not clear. Helping you is not what I questioned, I questioned the effectiveness of helping your son through you.

10. Sep 30, 2016

### FactChecker

A parent may just need very infrequent reminder of principles whereas the child may need a lot more.

11. Oct 1, 2016

### hmmm27

There is an assumption missing, that all graphs are labelled as time(x) vs distance(y).[edit: confirmed as such in post #3]

Within the context of the specific course language standards, Graph 3 might be considered to portray accelerations or not, ie: "negative acceleration" vs "deceleration" (and possibly even "well it isn't curving so it isn't accelerating; where'd I put my beer ?").

Last edited: Oct 1, 2016
12. Oct 2, 2016

### sophiecentaur

You have to hope that a helpful parent will be disciplined enough to read the labels on the graph axes carefully - then make sure the child is also doing it! An adult would (should) have an advantage in that respect.
It is so tempting to interpret a graph that 'goes up and down' as representing a bouncing object.