Instantaneous Velocity: drag racing problem

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The figure shows a graph of actual position-versus-time data for a particular type of drag racer known as a "funny car."
Physicsproblem2.png


Estimate the car's velocity at 2.0s

Equation:
v = m/s



I thought I knew how to solve this, and I made a tangent line at the curve and 2s, and when I do the slope at different points on the tangent I keep getting somewhere in the field of 50m/s. However, it keeps telling me I'm wrong. What should I be doing differently?

Thank you for your help!
 

Answers and Replies

  • #2
BruceW
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your answer looks ok to me. what keeps telling you that its wrong?
 
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The program I have to use for class (Mastering Physics). It says express the answer with proper units and 2 significant figures, but whether I put in 50m/s or 50.0 m/s, it's still saying it's incorrect?
 
  • #4
SammyS
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The program I have to use for class (Mastering Physics). It says express the answer with proper units and 2 significant figures, but whether I put in 50m/s or 50.0 m/s, it's still saying it's incorrect?
I get a somewhat steeper slope. ≈ 65 m/s

Have you tried entering scientific notation?
 
  • #5
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Thanks!

67 m/s ended up being the correct answer. I redrew my tangent line, and took another slope and I ended up getting an answer extremely close to that, so my tangent line was just off before. Only problem is now, I have to do this again for t=4s. I have, once again, exhausted most of my attempts trying to get the correct answer. What am I doing wrong?? Here is my tangent line for t=4:
Physicsproblem22.png


I am getting a slope around 80m/s. Is my tangent line really that off?

Thank you so much for your help!
 
  • #6
SammyS
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Thanks!

67 m/s ended up being the correct answer. I redrew my tangent line, and took another slope and I ended up getting an answer extremely close to that, so my tangent line was just off before. Only problem is now, I have to do this again for t=4s. I have, once again, exhausted most of my attempts trying to get the correct answer. What am I doing wrong?? Here is my tangent line for t=4:
Physicsproblem22.png


I am getting a slope around 80m/s. Is my tangent line really that off?

Thank you so much for your help!
That's at t = 4 seconds !
 
  • #7
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I know that. :) This is for the second question, which is for the velocity at t = 4s. The first question was t = 2s. I got that answer for t = 2s correct now, it was 67 m/s. But now I can't get the correct answer for t = 4s.
 
  • #8
SammyS
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I know that. :) This is for the second question, which is for the velocity at t = 4s. The first question was t = 2s. I got that answer for t = 2s correct now, it was 67 m/s. But now I can't get the correct answer for t = 4s.
How do you figure that slope to be 80 ?

It's much more like ≈250/2 = 125
 

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