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Velocity to acceleration then conversion from km/h to m/s2 confusion

  • #1
A car travels in a straight line for 5.3 h at a constant speed of 60 km/h.
What is its acceleration? Answer in units of m/s2

So far, to my understanding so far acceleration is change in velocity over time. So 60 over 5.3, which is 11.32 km/h2

Then I converted to m first, which is 1132 m/h2, then from h2 to sec, which is .00524 m/sec2.

But that isn't the answer, what did I do wrong?
 

Answers and Replies

  • #2
arildno
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"a constant speed of 60 km/h.
What is its acceleration?"

What does CONSTANT speed mean, when you are travelling along a straight line? In particular, what is the acceleration then?
 
  • #3
I'm not understanding what your asking. The acceleration is (60-0)/time right?
 
  • #4
arildno
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What was the INITIAL speed?
What is the FINAL speed?

In particular, in those 5.3 hours, does the speed change?
 
  • #5
tiny-tim
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hi survivorboiii! welcome to pf! :smile:
… acceleration is change in velocity over time. …
hint: what does "change" mean? :wink:
 
  • #6
The car probably started at 0km/h, then speeds up to 60km/h where it stayed for 5.3hr
 
  • #7
arildno
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"0km/h, then speeds up to 60km/h "
Really?
What does the book say about the speed during the time interval we're looking at?
 
  • #8
Enlighten me, I don't have my book with me. I guess I'm still confused about speed, vel, and acc
 
  • #9
arildno
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What does the word CONSTANT mean?
 
  • #10
constant velocity means acceleration is 0 doesn't it because the velocity isn't changing
 
  • #11
arildno
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constant velocity means acceleration is 0 doesn't it because the velocity isn't changing
Correct!

So what was wrong with your two statements:

1. " The car probably started at 0km/h, then speeds up to 60km/h where it stayed for 5.3hr"

2. "The acceleration is (60-0)/time right? "

In particular:
How should you reframe the equation in 2. ?
 
  • #12
Would it be 0-60 over time?
 
  • #13
arildno
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Why?
You said yourself:
"constant velocity means acceleration is 0 doesn't it because the velocity isn't changing "

How can the speed change from 0 to 60 if it isn't changing?
 
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  • #14
tiny-tim
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(just got up :zzz:)

average acceleration = final velocity minus initial velocity divided by time

instantaneous acceleration (usually just called "acceleration") = final velocity after an extremely short time minus initial velocity divided by that extremely short time = the slope of the velocity-time graph :smile:

this question is asking for acceleration​

so draw a graph … what is the slope? :wink:
 
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  • #15
IT just hit me...the acceleration is 0 because the vel is constant! :) Thanks both of you!
 
  • #16
arildno
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IT just hit me...the acceleration is 0 because the vel is constant! :) Thanks both of you!
:smile:



PS:
Remember to give your answer as 0m/s^2, because the exercise specifically asked you to include units
 

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