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ciciccz · Oct 5, 2011, 06:12 PM

A sprinter can accelerate with constant acceleration for 3.70 before reaching top speed. He can run the 100-meter dash in 10 s.
What's he's velocity when crossing the finish line?

FadedMaster · Oct 5, 2011, 06:17 PM

Please note that we do not give answers on here, but we do help you with your homework. Have you made any attempts on your problem? If so, please post them. If not, please let us know where you are stuck.

3.70 what? What are your units?

ciciccz · Oct 5, 2011, 06:29 PM

since he travels 100m in 10s, which means the average velocity is 10m/s but I don't find that useful at all.
since acceleration is not given, the first part covered distance is 0.5a*3.7^2, then the distance remaining will be 100-0.5a*3.7^2. therefore the average velocity for the second half will be (100-0.5a*3.7^2)/6.3. To be honest, I don't even know if I am doing this right, but I couldn't find anyway to solve this problem... please helppppp

ciciccz · Oct 5, 2011, 06:33 PM

Sorry, it is 3.7 seconds.

FadedMaster · Oct 5, 2011, 08:17 PM

Okay, for me it always helps to just chart out what my knowns and unknowns are.

We can also look at this in two parts, the first part where the runner accelerates, and the second part where the runner is no longer accelerating and has reached his maximum velocity.

For first part:
$x_i = 0 \text meters$

$x_f = ? \text meters$

$t_i = 0 \text seconds$

$t_f = 3.7 \text seconds$

$a = ? \text {m \over s^2}$

$v_i = 0 \text {m \over s}$

$v_f = ? \text {m \over s}$

For the second part:
$x_{2i} = x_f \text meters$

$x_{2f} = 100 \text meters$

$t_{2i} = 3.7 \text seconds$

$t_{2f} = 10 \text seconds$

$a_2 = 0 \text {m \over s^2}$

$v_{2i} = v_f \text {m \over s}$

$v_{2f} = v_f \text {m \over s}$

Now that it is mapped out a little bit, are you able to see what you need to do next? If you have formulas written out in front of you it should be a little bit easier to see what to do.

Also, is this calculus based physics you are taking, or just an intro course?

ciciccz · Oct 5, 2011, 08:46 PM

... I think I just made this question harder for myself. I thought he will decelerate after accelerating. Sigh... took me more than an hour figuring this out

Unknown008 · Oct 6, 2011, 02:23 AM

Could you post the answer that you got?

FadedMaster · Oct 6, 2011, 05:30 AM

Originally Posted by ciciccz

.... i think i just made this question harder for myself. I thought he will decelerate after accelerating. sigh... took me more than an hour figuring this out

Believe me, it happens to the best of us. I always over think problems. Please post what you found so we can take a look.