# Physics question

#### f1_champ

Banned
Ok , so there was a physics test today and there was this 5 mark question that I got wrong. In the velocity-time graph I've atttached( I know the graph isnt perfect but you get the idea); I 'm supposed to find out the accelration of the object between 0secs and 8 secs, what I did was used the formula
a=(v[2]-v[1])/t

Where 'a'=accelration, 'v[1]'= initial velocity, 'v[2]'=final velocity & 't'=time
So since the speed of the object was 32 m/s at the end of 8 seconds and obviously the object is at rest at 0secs the answer should be 4 m/s^2, shouldn't it? But my teacher crossed out my answer and the only explanation she gives is that "since this is a velocity-time graph and the resulting line is a straight one and a straight line on a velocity time graph means no accelration"

Isnt the speed of the object at 0 secs supposed to be 0 m/s (at least thats what the graph indicates) and since the velocity is 0m/s, the displacement at that paticular instance of time would obviously be 0. Can someone help? Tell me that she's wrong and I'm right

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Banned
anyone?

#### valis

Moderator
f1_champ said:
I got wrong.
f1_champ said:
1: f1champ is never wrong
2: When in doubt refer to rule 1
to be honest, I have no idea about the answer to your physics question. I just saw the moment to make some levity. Good luck, though.

#### Kenny94

Banned
Where is the Tip or Trick in this thread....

#### ekim68

Mike
Is this a test? Give us a minute...

#### f1_champ

Banned
Can I ever get a straight answer?

#### brendandonhu

Well, acceleration is a change in velocity (speed or direction), and the graph shows a change in velocity. Are you sure the question was "What is the acceleration?" and not "What is the change in acceleration?"

#### f1_champ

Banned
brendandonhu said:
Well, acceleration is a change in velocity (speed or direction), and the graph shows a change in velocity. Are you sure the question was "What is the acceleration?" and not "What is the change in acceleration?"
Yes, I'm pretty sure that the quesstion was "What is the acceleration?" even though I dont have the test with me right now.

But for the sake of argument let's say that the question was "What is the change acceleration?" even now what my teacher says

"since this is a velocity-time graph and the resulting line is a straight one and a straight line on a velocity time graph means no accelration"
is false since there is definate change in accelration because the object obviously cannot have an accelraton at 0 seconds (unless their is a concept of "negetive time").

Either ways what my teacher says is wrong

#### brendandonhu

There is no change in acceleration in this graph-- the acceleration is constant.
the object obviously cannot have an accelraton at 0 seconds
Acceleration isn't at a certain point in time. Its the difference between two velocities over a certain period of time. Perhaps the question was "What is the change in acceleration?" and your teacher meant to say "a straight line on a velocity time graph means no change in accelration".

#### iltos

that straight line graph shows a constant rate of acceleration (if you start at 0km/sec and your acceleration is 2km/sec, at the end of 1 sec, your velocity is 2km/sec.....but you're still accelerating at 2km/sec, so at the end of 2 sec, you're velocity is 4km/sec, but your rate of acceleration is unchanged....that is hopefully what you're teacher was referring to, or that person is in the wrong job

#### brendandonhu

iltos said:
But that's a velocity...your acceleration would be 2km/sec^2

#### f1_champ

Banned
brendandonhu said:
There is no change in acceleration in this graph-- the acceleration is constant.

Acceleration isn't at a certain point in time. Its the difference between two velocities over a certain period of time. Perhaps the question was "What is the change in acceleration?" and your teacher meant to say "a straight line on a velocity time graph means no change in accelration".
if the ques was "What is the change in acceleration?" so that would mean the original ques is "find out the change in accelration of the object between 0secs and 8 secs" There is no way to find the accelration at a paticular instant tof time like you said above that would mean the ques is invalid

#### iltos

brendandonhu said:
But that's a velocity...your acceleration would be 2km/sec^2
details

#### f1_champ

Banned
iltos said:
that straight line graph shows a constant rate of acceleration (if you start at 0km/sec and your acceleration is 2km/sec, at the end of 1 sec, your velocity is 2km/sec.....but you're still accelerating at 2km/sec, so at the end of 2 sec, you're velocity is 4km/sec, but your rate of acceleration is unchanged....that is hopefully what you're teacher was referring to, or that person is in the wrong job
have a look at my graph

#### brendandonhu

f1_champ said:
if the ques was "What is the change in acceleration?" so that would mean the original ques is "find out the change in accelration of the object between 0secs and 8 secs" There is no way to find the accelration at a paticular instant tof time like you said above that would mean the ques is invalid
That makes sense, but the acceleration in this case is constant. The change is 0 for any 2 points you pick on the line. Its really hard to say without knowing the original question.

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