please help with ap physics

Pre-College Issues High School Life
1

<li>A cow moving with constant acceleration covers the distance between two points 67.0 m apart in 7.10 s. Its speed as it passes the second point is 15.0 m/s.</li>
</ol>

<p>(a) What is its speed at the first point?</p>

<p>(b) What is the acceleration?</p>

<li>An airplane travels 320 m down the runway before taking off. If it starts from rest, moves with constant acceleration, and becomes airborne in 9.00 s, what is its speed, in m/s, when it takes off?</li>
</ol>

<p>thanks a lot and please explain how u got the answer</p>

2

<p>1a. Use a kinematics equation
I think this should work: .5(v<em>i + v</em>f) * t = distance
So... .5 (v<em>i + 15 m/s) * 7.10 s = 67.0 m
7.5 m/s + .5 v</em>i = 67.0 m/(7.10 s) = 9.44 m/s
9.44 m/s - 7.5 m/s = 1.94 m/s</p>

<p>From this use another kinematics equation to find acceleration... (This list might help --> <a href="http://www.glenbrook.k12.il.us/GBSSCI/PHYS/Class/1DKin/U1L6a.html%5B/url%5D"&gt;http://www.glenbrook.k12.il.us/GBSSCI/PHYS/Class/1DKin/U1L6a.html&lt;/a&gt;) </p>

<p>Try doing the rest and if you're still stuck, explain what you did, and I'll try to help...</p>

3

<ol>
<li>1/2(a^2)*t = x (kinematic equation, half of acceleration squared times time equals distance)</li>
</ol>

<p>So: 320 = .5(a^2)*9</p>

<p>320 = 4.5(a^2)</p>

<p>71.1 = a^2</p>

<p>a = 8.43 m/s^2</p>

<p>v = at (velocity equals acceleration times time)</p>

<p>v = 8.43*9 = 75.9 ~ 76 m/s (due to significant digits)</p>

4

<p>thats what i did;</p>

<p>1/2(a^2)*t = x (kinematic equation, half of acceleration squared times time equals distance)</p>

<p>So, 320 = 1/2(a^2)*9</p>

<p>320 = 4.5(a^2)</p>

<p>71.1 = a^2</p>

<p>a = 8.43 m/s^2</p>

<p>v = at (velocity equals acceleration times time)</p>

<p>v = 8.43(9) = 75.87</p>

<p>and do u go to apopka high school?</p>

5

<p>It's 1/2a(t^2)=x...</p>

<p>But the really useful equation here is:
2ax+vi^2=vf^2</p>

<p>vi=initial velocity
vf=final velocity
a=acceleration
x=distance</p>

<p>vf-acceleration*time=vi</p>

<p>Sub for initial velocity, and substitute numbers in, and you should be fine for problem 1...</p>

<p>I cannot believe I forgot that (vi+vf)/2=x (after doing the above).</p>