Hard Motion Question- Help?

1. Feb 1, 2008

fraserperry

Hard Motion Question- Help???

I am really stuck on this motion question, can anyone help please????

"A balloon is 30.00m above the ground and is rising vertically with a uniform speed when a coin is dropped from it. If the coin reaches the ground in 4.00 seconds, what is the speed of the balloon?

2. Feb 1, 2008

Ben Niehoff

So, the coin has an initial height of 30 m, and an initial velocity of v. It has a constant acceleration of -9.8 m/s^2. Find the time it takes to fall to a height of 0 m, in terms of your variables v and t. Then plug in 4.00 s for t, and solve for v.

3. Feb 1, 2008

fraserperry

which motion equation do i use?

4. Feb 1, 2008

rsala

solve for the initial velocity of the coin,,since it is equal to the constant velocity of the balloon, try

0 = 30 + Vi * 4 - .5g(4^2)

thing to know is that if the balloon is traveling , say 500 m/s when the coin is dropped....the coin is actually dropped AT 500m's...so its initial velocity when you insert it into an equation is 500m/s,, NOT 0 as you might assume.
though thats irrelevent here, thought i might say. so that you understand the purpose of the question really

5. Feb 1, 2008

fraserperry

are you saying the coin when dropped is would be travelling the same speed of the balloon in an upwards direction?

6. Feb 1, 2008

rsala

yes, ya and its acceleration is changed from 0 to -9.8 when you put it in the equation because of gravity...because of course it will be hitting the ground soon

7. Feb 1, 2008

fraserperry

I got 12.1 from that equation, but what is that number?

8. Feb 1, 2008

rsala

the coins initial velocity once it is released from the balloon. = 12.1

which just so happens to be the balloons constant velocity, since when you initially dropped the coin from the balloon its initial velocity is = to the balloon velocity.
so, Balloon-velocity = 12.1

ithink