# Projectile motion helicopter

1. Nov 21, 2014

### gracy

A helicopter on a flood relief mission flying horizontally with a speed vo at an altitude H, has to drop a food packet for victims standing on the ground. At what 'distance from the victims should the food packet be dropped'?the victims are standing in the vertical plane of the helicopter.
In my textbook diagram for this problem is given
(a)

but what i am confused about is why victims are standing there, can't they stand just beneath the helicopter like this (b)
after all ,victims are standing in the vertical plane of the helicopter in this picture also .So 'distance from the victims should the food packet be dropped 'is equal to height H .That's what i thought and chose the option H as answer for this MCQ.But in my textbook 'distance from the victims should the food packet be dropped' i.e D is taken as in Picture (a) and hence solved this problem in totally different way.So what is wrong in picture b ,victims are still in the vertical plane of the helicopter as mentioned in the question.please do reply.
Thanks.

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Last edited: Nov 21, 2014
2. Nov 21, 2014

### Staff: Mentor

When dropped from a moving helicopter (looks like a plane to me), do the packets fall straight down?

3. Nov 21, 2014

### gracy

assume it as helicopter.

4. Nov 21, 2014

### gracy

I didn't understand what you are trying to ask.please clear your question

5. Nov 21, 2014

### Staff: Mentor

I'll ask in a different way: When the packets first leave the helicopter, what velocity do they have?

6. Nov 21, 2014

### gracy

it is not given.i am just concerned and confused about position of victims.please review my full question.

7. Nov 21, 2014

### DocZaius

I think you are confused by the fact that a helicopter is mentioned. It seems like since the problem statement mentions a helicopter and you don't see a helicopter in the figure, you are ignoring other parts of the figure (such as the fact that the vehicle in question is not hovering over the survivors). I think it'd be reasonable for you to assume that the helicopter/plane is not hovering over the survivors. Then this becomes a non-trivial (in that you have to use at least one equation) projectile motion problem.

I recommend you use figure a. Assume the vehicle is some lateral distance away from the survivors when it drops the package.

8. Nov 21, 2014

### Staff: Mentor

Actually, it is given.

I understand your question. Please answer my questions, as they are meant to clear up yours.

(And read DocZaius's post above.)

9. Nov 21, 2014

### gracy

i think there is some misunderstanding.picture (a)is given in my textbook but not in question but as a solution.

10. Nov 21, 2014

### gracy

how?initial velocity of packet is same as that of helicopter i.e vo,right?

11. Nov 21, 2014

### lep11

yes

12. Nov 21, 2014

### BiGyElLoWhAt

which is a velocity, represented as an arbitrary unknown rather than a specific number.

13. Nov 21, 2014

### DocZaius

Ah ok. In that case I understand your confusion more. However the problem statement does say the helicopter is flying horizontally. This means that the package will be moving both horizontally and vertically upon being dropped. So the helicopter cannot be directly above the survivors at the moment it drops the package. In your (second) diagram, the package would miss the survivors since it would move horizontally at the speed of the vehicle. The vehicle needs to be located laterally "before" the survivors. Find that distance.

14. Nov 21, 2014

### Staff: Mentor

The picture fits the question (except they show a plane, not a helicopter).

Right. So what sort of trajectory would you expect it have as it fell? Would you expect it to fall straight down?

15. Nov 21, 2014

### gracy

why?can you please make me understand this point.

16. Nov 21, 2014

### Staff: Mentor

Drop a ball. How does it fall to the ground?

Throw a ball horizontally. How does it fall to the ground?

17. Nov 21, 2014

### gracy

in first case vertically and in second case it's path would be projectile.

18. Nov 21, 2014

### Staff: Mentor

Right. The second case would take a parabolic trajectory. (Both cases are projectile motion.)

The second case is exactly like the one in your problem. Since the packet has an initial horizontal velocity, it is just as if it were thrown at that speed.

19. Nov 21, 2014

### gracy

how first case is an example of projectile motion?

20. Nov 21, 2014

### BiGyElLoWhAt

better example than throwning a ball. Drop the ball. Have your friend get in a car and drive past your house and then drop the ball. The second case is identical to the situation, except it's a helicoptor (plane?), not a car.