# 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