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Truck vs Tree (first week physics woes)

  • Thread starter youagain
  • Start date
  • #1
5
0
1. The driver of a truck slams on the brakes when he sees a tree blocking the road. The truck slows down uniformly with an acceleration of -5.60 m/s2 for 4.20s making skid marks 62.4m long that end at the tree. With what speed does the truck strike the tree?



Homework Equations



I'm guessing that I have to use Vf2-Vo2=2as?

I'm really not sure... I've been staring at this problem for hours, is It a trick question? Does it even hit the tree?
 

Answers and Replies

  • #2
1,953
248
You need

[tex]
x = x_0 + v_0 t + (1/2) a t^2
[/tex]

you know everyting except for the initial speed.
 
  • #3
5
0
Man i am defenitly going to need to work on knowing which formulas are for which... ummm using that...


x=vo2+1/2at2



vo2= x-1/2at2



So vo = square root x-1/2at2


square root of 62.4-(-5.6x4.22)/2


= 10.573

So he hits the tree at 10.58 m/s?


EDIT: Wait did I miss out a t? let me try that again...
 
  • #4
5
0
x-(1/2)at2/t=vo


62.4-(1/2)-5.6x4.22
/4.2

=

26.617 m/s



So thats his initial speed right?

I still dont understand how to get his final speed?
 
  • #5
5
0
Wait V=Vo+at

so 26.617+(-5.6x4.2) = 3.097

does he hit the tree at 3.097 m/s?
 
  • #6
cristo
Staff Emeritus
Science Advisor
8,107
73
Be careful with your rearranging. If

[tex]x=v_0t+\frac{1}{2}at^2[/tex]

then

[tex]v_0=\frac{1}{t}\Big(x-\frac{1}{2}at^2\Big)=\frac{x}{t}-\frac{1}{2}at[/tex]
 
  • #7
5
0
I think its just my formating making it look wrong? is 26.617 m/s the right answer?
 

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