Forces, find net with velocity and distance

  • Thread starter yogoes123
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  • #1
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Homework Statement



Driver brakes 1420kg car moving at 64.8km/h [W]. Car slows down and comes to stop after 729m [w]

find net force


Homework Equations



a=v/t f=ma

The Attempt at a Solution



t=40.5 a= 0.44

f=1420(0.44) f= 624.8

My answer is 624.8N

TB Answer: 316N


what am i doing wrong?
 
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Answers and Replies

  • #2
NascentOxygen
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Hi yogoes123. http://img96.imageshack.us/img96/5725/red5e5etimes5e5e45e5e25.gif [Broken]

Your first mistake is probably in how you worked out that 40.5 secs, as you haven't shown the equation you used.
 
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  • #3
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I used v=d/t swapped it around and solved for t=d/v and that gave me 40.5s, is that wrong?
 
  • #4
NascentOxygen
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I used v=d/t swapped it around and solved for t=d/v and that gave me 40.5s, is that wrong?
That equation relates time to the distance travelled at a constant speed.
 
  • #5
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So it would be correct, no? I cant figure out what i did wrong.
 
  • #6
NascentOxygen
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f=1420(0.44) f= 624.8

ans is: 316N


what am i doing wrong?
You [somehow] worked out a figure of 624.8N
then you decided to halve it, and call that your answer?

How to explain that halving?
 
  • #7
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The TB answer is 316, my answer is 624.8, sorry I didnt make that clear.
 
  • #8
NascentOxygen
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So it would be correct, no? I cant figure out what i did wrong.
As I emphasised "at a constant speed". At no time here is the vehicle travelling at a fixed speed, the vehicle is constantly slowing.

To begin over....

you need equations relating distance, initial velocity, final velocity (= 0 here) etc.

So, write down some suitable candidates.
 
  • #9
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I thought it starts to slow down after the 729m. Are you saying that is it slowing down within 729m?
 
  • #10
NascentOxygen
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I thought it starts to slow down after the 729m. Are you saying that is it slowing down within 729m?
At the 729m mark, it is motionless. :smile:

Where speed is constant it has acceleration = 0 (for straight line travel).
 
  • #11
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Ok I think I got this, I used Vf^2=Vi^2+2ad

Then I did a= 18^2/2/729 and i got 0.2222222 then I used F=ma and got 316, thanks so much for clearing that up.
 
  • #12
NascentOxygen
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Good. :smile:
 

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