Is my working correct?

  • Thread starter Stroodle
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  • #1
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A car is advertised as taking [tex]11.3\ s[/tex] to reach a speed of [tex]100\ kmh^{-1}[/tex]. About how far does it travel before reaching this speed?

Firstly I have tried to work out the acceleration: [tex]a=\frac{\frac{100000}{3600}}{11.3}\approx 0.2175\ ms^{-2}[/tex]

I've then used this to find the displacement: [tex]x=0.2175\times 11.3^2\approx 21.7\ m[/tex]

Am I doing something wrong? It seems like too short a displacement...

Thanks for your help
 

Answers and Replies

  • #2
671
2
Check your conversion from [tex]\tfrac{kilometers}{hour}[/tex] to [tex]\tfrac{meters}{second}[/tex].
Remember that for motion with constant acceleration, starting at rest, [tex]S=\tfrac{1}{2} a t^2[/tex]
Don't round your results so much. Keep your answer parametric until the very end. This is a skill you'll NEED once you get to the more complicated stuff.
You were off by a factor VERY different from [tex]10^n[/tex] in your calculation of the average acceleration.

Another thing I feel I should mention is that you need to state your assumption that the car accelerates with a constant acceleration, as that is not always the case, and your results will vary greatly should you consider such a case.
 
Last edited:
  • #3
26
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Awesome. Thanks for your help.
I'm now getting the answer as [tex]\approx 314\ m[/tex] with
[tex]a=\frac{2500}{1017}\ ms^{-2}[/tex]
Is that correct?
 
Last edited:
  • #4
671
2
Awesome. Thanks for your help.
I'm now getting the answer as [tex]\approx 314\ m[/tex] with
[tex]a=\frac{2500}{1017}\ ms^{-2}[/tex]
Is that correct?
That is very close, you forgot the factor of [tex]\tfrac{1}{2}[/tex] in the displacement as a function of time formula.
 
  • #5
26
0
Oh yeah. Oops.

Thanks again for your help. I'm sure these will become really easy after I do a few more of them.
 
  • #6
671
2
Oh yeah. Oops.

Thanks again for your help. I'm sure these will become really easy after I do a few more of them.
Oh yeah, completely. By the time you get to the subject of work and energy, you'll be reciting these in your sleep. :)

And no problem, you're very welcome.
 

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