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# Homework Help: Physics

Posted by Emily on Thursday, September 30, 2010 at 6:39pm.

If 100 m sprinters accelerate from rest for 3.5 s at 2.8 m/s^2, how far have they run to this point? How long will it take them to complete the 100 m sprint, assuming they maintain their speed the rest of the way?

I found the distance, which is 17 m, and the distance they still have to run to complete the race, which is 83m. I also calculated their second velocity, which is 9.8 m/s. But when I divide 83m by 9.8 m/s,(since that's the velocity for the rest of the race) I don't get the correct answer, which is 12 s.
• Physics - Henry, Thursday, September 30, 2010 at 10:20pm

V = at = 2.8 m/s^2 * 3.5 s = 9.8 m/s,
d1 = 0.5 * 2.8 m/s^2 * (3.5 s)^2 = 17.15 m,

t(total) = 3.5 s + 82.85 m / 9.8 m/s =
3.5 + 8.5 = 12 s.

• Physics - Emily, Thursday, September 30, 2010 at 11:00pm

Thanks!

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