Helicopter Problem: Find Time for Mailbag to Reach Ground

[jaytm2291]

Homework Statement

The height of a helicopter above the ground is given by h = 2.60t3, where h is in meters and t is in seconds. At t = 2.10 s, the helicopter releases a small mailbag. How long after its release does the mailbag reach the ground?

Homework Equations

Y=ViT - (1/2)gt^2

The Attempt at a Solution

h=(2.60)(2.10)^3
24.0786=0t-(1/2)(9.8)t^2

Thank you.

 

[rock.freak667]

The Attempt at a Solution

h=(2.60)(2.10)^3
24.0786=0t-(1/2)(9.8)t^2

Thank you.

That is correct, but your '-' should be a '+' since you will want to take down as the positive direction. Otherwise your displacement is negative.

 

[jaytm2291]

That is correct, but your '-' should be a '+' since you will want to take down as the positive direction. Otherwise your displacement is negative.

Then I would have:

24.0786 = 0t + 4.9t^2
t^2 = 4.914
t=2.2s

This is wrong. Any ideas?

 

[rock.freak667]

Then I would have:

24.0786 = 0t + 4.9t^2
t^2 = 4.914
t=2.2s

This is wrong. Any ideas?

Uhm if that is not correct then it means that the helicopter was rising when the bag was dropped, in which you need to get the initial vertical velocity by differentiating 'h' with respect to 't'.

 

[jaytm2291]

Uhm if that is not correct then it means that the helicopter was rising when the bag was dropped, in which you need to get the initial vertical velocity by differentiating 'h' with respect to 't'.

Vi = h'(t) = 7.8t^2 = 7.8(2.10)^2=34.398 m/s ?

 

[rock.freak667]

Vi = h'(t) = 7.8t^2 = 7.8(2.10)^2=34.398 m/s ?

So now try solving 24.0786=34.398t-(1/2)(9.8)t^2 and see if you get the correct answer.

 

[jaytm2291]

So now try solving 24.0786=34.398t-(1/2)(9.8)t^2 and see if you get the correct answer.

It is a quadratic Equation which gives me..x = 0 when y = .785

 

[rock.freak667]

It is a quadratic Equation which gives me..x = 0 when y = .785

where did you get x and y from? You only had 't' to solve for.

 

[jaytm2291]

Listen I have been trying to figure this problem out for 3 hours and 14 minutes. I'm freaking out over this. Can you just point me in the right direction and tell me which formula to use so I can can get on with this? I still need to do so many other problems and this is really annoying if I'm doing this in the wrong direction.

 

[rock.freak667]

Listen I have been trying to figure this problem out for 3 hours and 14 minutes. I'm freaking out over this. Can you just point me in the right direction and tell me which formula to use so I can can get on with this? I still need to do so many other problems and this is really annoying if I'm doing this in the wrong direction.

What I meant was that you had to solve:

24.0786=34.398t-(1/2)(9.8)t²

to get t, which is why I am confused as to where you got 'x'and 'y' from. I don't mean to cause you anymore frustration but solving for 't' should give the correct answer.

 

[jaytm2291]

I got when x is = 0, y is = to .785 is all I know. This is the most frustrating thing I just want to be done with. Please help me finish this problem. Thank you. Can you solve it from the beginning and not take my word for it because I'm really confused. Thank you.

 

[rock.freak667]

I got when x is = 0, y is = to .785 is all I know. This is the most frustrating thing I just want to be done with. Please help me finish this problem. Thank you. Can you solve it from the beginning and not take my word for it because I'm really confused. Thank you.

24.0786=34.398t-(1/2)(9.8)t²

Do you know how to solve a quadratic equation? That is all you have to do for now.

 

[jaytm2291]

Yes. But I don't even know if these numbers or direction I am going in is correct and you are just telling me to go with what I have been doing. Please tell me I am doing the right thing.

 

[rock.freak667]

I believe that you were using the correct method, you just did not include the initial speed of the helicopter initially.