Vertical Projectile Motion Problem

TheRedDevil18

1. The problem statement, all variables and given/known data

Please check my work.

Two students are on a balcony 19.6m above the ground. One guy throws a ball vertically downwards at 14.7m/s; at the same time the other guy throws a second ball vertically upwards at the same speed. The second ball just misses the balcony on the way down.

2.3.1) What is the difference in the balls flight time?
2.3.2) What is the velocity of each ball as they strike the ground?
2.3.3) How far apart are the balls 0.8s after they are thrown?

2. Relevant equations

vf^2 = vi^2 + 2a*y
delta y = vi*t + 1/2*a*t^2
delta y = vf+vi/2 * t
vf = vi + a*t

3. The attempt at a solution

2.3.1) For the ball thrown upwards I decided to first find the maximum height it reached. I used the first equation from the list and got an answer of 11.03m. I then used the third equation to find the time and got 1.5s (another 1.5s for coming down=3s). I then worked out the time for the ball thrown downwards to be 1s. My final answer for the difference in flight time is 4-1=3s.

2.3.2) Each ball would strike the ground at the same speed.
vf = vi+a*t
= 14.7+9.8*1
= 144.06m/s

2.3.3) Using the heights of the two, I worked the distance for the first ball to be 8.624m and the second ball to be 14.896m. Subtracting the two would give a distance of 6.272m as my final answer.

All I am asking is for someone to please check my work because I am not sure if I have approached this problem in the correct way.

rude man

Check the number in red above! Otherwise all is good.

TheRedDevil18

I think the first ball should be 14.896 and the second ball 8.624. Is that my mistake?

rude man

Check the items in red above!

TheRedDevil18

Ok im a bit confused, but this is how I got 14.896m:
delta y = 14.7*0.8 + 1/2(9.8)*0.8^2
= 14.896m

Is there something I did wrong?

rude man

Should that number be + or -?

TheRedDevil18

If the answer should be negative, Why?

rude man

Because when you came up with y1 = +8.624m you defined y = 0 to be the balcony height, so anything below the balcony is negative. It should also be obvious that the second ball will be lower in height than the first, not higher as you had it.

So y1 = vi*t - gt^2/2 = 8.624m (ball tossed up but g is down)
y2 = -vi*t - gt^2/2 = -14.896m (ball tossed down & g is down)
y1 - y2 = 2vi*t = you fill this one in!

Just keep track of signs & you'll be fine.

TheRedDevil18

So the final answer is 23.52m apart?

rude man

Yep.

TheRedDevil18

Ok, thanks for your help