Homework Help: 1D Motion

1. Sep 23, 2010

dramallama

1. The problem statement, all variables and given/known data
Just when a Frisbee passes over the head of a student at a speed of 4m/s and decelerating at a rate of 1.5 m/s2, she starts to run accelerating at a rate of 1 m/s2. [neglect vertical motion of Frisbee]

a)How long will it take the student to catch the Frisbee?

b) If instead of accelerating the student were able to run at a constant speed of 2 m/s as soon as the Frisbee passes her head how far would she have to run to catch it?

2. Relevant equations
x=x0+V0t+1/2at2

3. The attempt at a solution

So for part a, I figured if I made both xfrisbee and xstudent equal to each other I could solve for t
xfrisbee=0+4t+(0.5)(-1.5)t2
xstudent=0+0t+(0.5)(1)t2

0.5t2=4t+0.75t2
t=0s t=3.2s

And something similar for part b:
xfrisbee=0+4t+(0.5)(-1.5)t2
xstudent=0+2t+(0.5)(0)t2

4t+(0.5)(-1.5)t2=2t
t=0s t=2.7s
x=(2m/s)(2.7s)=5.4m

I just wanted to know if I got these right. Thanks!

2. Sep 24, 2010

Staff: Mentor

Looks perfect to me.

(Nitpick: Careful not to round off until the last step--especially if you're submitting your work to an online system. They can be picky.)

3. Sep 24, 2010

jhae2.718

Looks like you did everything correctly.

One caution: be careful with your signs; had you gone off of this equation, you would have gotten the result that the student will never reach the frisbee:
What Doc Al says about rounding is good advice. The distance, if you carry figures through to the end, ends up at about 5.3333 m.

4. Sep 24, 2010