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Grade 11 Kinematics Question

  • Thread starter bcaie16
  • Start date
  • #1
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Homework Statement


In a long distance race, Michael is running at 3.8 m/s and is 75 m behind Robert, who is running at a constant velocity of 4.2 m/s. If Michael accelerates at 0.15 m/s^2, how long will it take him to catch Robert


Homework Equations


D=vi(delta)t + 1/2a(delta)t^2
etc


The Attempt at a Solution


I know how to do this type of problem when the accelerating runner starts from a rest, but I have no idea where to go from here. I've tried framing the problem a bunch of different ways but nothing.
 

Answers and Replies

  • #2
tiny-tim
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welcome to pf!

hi bcaie16! welcome to pf! :smile:

(have a delta: ∆ and try using the X2 and X2 icons just above the Reply box :wink:)

you need to write two separate equations, one for each runner …

what do you get? :smile:
 
  • #3
Delphi51
Homework Helper
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Welcome to PF, bcaie16! Nice to see some high school work here.
The thing that determines the answer is that the two people meet - their distances become equal. So you should begin with
Michael's distance = Robert's distance

Robert's distance is easy - just use d = vt and add 75 because he is starting out that far ahead.

Michael's distance is complicated because of the initial velocity and acceleration. Use the standard distance formula for accelerated motion: d = Vi*t + .5*a*t².

It looks like you will have a quadratic equation to solve. If you are just starting grade 11 math you may need to look up how to solve a quadratic on a calculator or using the general formula:
http://en.wikipedia.org/wiki/Quadratic_equation

You can, of course, solve it by trial and error. A spreadsheet works really well for that.
 
  • #4
5
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I figured I would have to use the quadratic formula. I wasn't sure how to frame it kinematics wise. What are the a, b, and c values respectively in terms of velocity, acceleration, etc.
 
  • #5
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Ok i figured that the respective equations would be ∆t= 4.2 ∆D +75, and
75= -∆t +/- square root ∆t -2.28/2
 
  • #6
tiny-tim
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Ok i figured that the respective equations would be ∆t= 4.2 ∆D +75, and
75= -∆t +/- square root ∆t -2.28/2
nooo :redface:

(btw, why are you using all these ∆s? it's not normal, and it really doesn't help)

∆t= 4.2 ∆D +75 is completely wrong, isn't it?

and i can't work out what your second equation is :confused:

just write two general equations, one for each runner (ignoring the other) …

when you have them, then you can decide how to use them​
 
  • #7
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Ok being completely new to physics, I am kind of lost. I am not really sure to how to get a general equation. Should they be in relation to time or distance
 
  • #8
tiny-tim
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both …

each should be x = a function of t

(ie it tells you where it is at each time t :wink:)
 
  • #9
Delphi51
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Michael's distance = Robert's distance
vt + .5*a*t² = vt + 75
3.8*t + .5*(.15)*t² = 4.25 + 75
collect like terms, all on one side, so you have an equation of the form
at² + bt + c = 0
Then use one of the methods of solving the quadratic for t.
 
  • #10
Delphi51
Homework Helper
3,407
10
Oops, forgot a t in the previous post. Too late to edit. Should be like this:
Michael's distance = Robert's distance
vt + .5*a*t² = vt + 75
3.8*t + .5*(.15)*t² = 4.2t + 75
collect like terms, all on one side, so you have an equation of the form
at² + bt + c = 0
Then use one of the methods of solving the quadratic for t.
 

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