Motion dropping a stone: Time question

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Homework Help Overview

The discussion revolves around a problem involving the motion of a stone being dropped, with a specific focus on the time it takes for the stone to reach a leaf on the water's surface. Participants are exploring the relationship between distance, initial velocity, and time in the context of this motion problem.

Discussion Character

  • Exploratory, Assumption checking, Problem interpretation

Approaches and Questions Raised

  • Participants are questioning the relevance of the distance provided and whether it should be used in the current context. There is discussion about the timing of the stone's drop in relation to the leaf's position and the implications of the water's velocity.

Discussion Status

Some participants have suggested potential calculations involving distance and time, while others express uncertainty about the connections between different parts of the problem. There is an ongoing exploration of the necessary parameters and assumptions, particularly regarding the height from which the stone is dropped and the timing of the drop.

Contextual Notes

There is mention of a bridge in later posts, which raises questions about the initial setup described in the first post. Participants are also considering the implications of the stone's initial velocity and the acceleration due to gravity, but there is no consensus on how these factors integrate into the problem.

Jeff97
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Homework Statement
A boy watches a leaf move down the stream at 0.30ms^1. He wants to drop a stone onto the leaf. Determine the position of the leaf at the instant when he must drop the stone.
Relevant Equations
I'll put one here once I know what one to use.
what I know d=35.75 t=? what am I meant to do with this problem? I'll add more to this area once I know some more. thanks. Is the leafs velocity considered inital? if so I know Vi
 
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Jeff97 said:
I know d=35.75
What distance is this and how do you know its value?
Suppose he drops the stone when the leaf is underneath. What will happen?
 
@haruspex. I got the distance from the other problem, should I not be using it? If he drops the stone when the stones underneath him it will be past him once the stone reaches the leaf?
 
Jeff97 said:
I got the distance from the other problem, should I not be using it?
They sounded like completely different problems to me, but since I cannot see both in context I may be missing something. You do need to know the height from which the stone is being dropped.
Jeff97 said:
If he drops the stone when the stones underneath him it will be past him once the stone reaches the leaf?
Right, but assuming you know the height (call it d) by how much will he miss the leaf?
 
I suppose I need a formula? that contains d, Vi? t?
 
Jeff97 said:
I suppose I need a formula? that contains d, Vi? t?
Yes, and one more (known).
 
Jeff97 said:
Vf? If so could you use d=Vi+Vf/2 t?
No, I wrote "known" (as an edit). You do not know Vf.
 
acceleration?
 
Jeff97 said:
I suppose I need a formula? that contains d, Vi? t?

Perhaps you could forget formulas for a moment. Suppose this was a real task. You really have to hit something moving along a stream by dropping a stone on it. Imagine that you get lots of chances. Each time you miss, someone releases another leaf and gives you another stone. Let's assume you have a watch. How do you achieve the task?
 
  • #10
Jeff97 said:
acceleration?
Yes.
 
  • #11
Ok, so what we know is that he must drop the stone so that it will land on the leaf 2.7 seconds later. If the water is running at 0.3 m/s, then we can find the distance from the bridge to the leaf position (horizontal):

d=vt =0.3 x 2.7 = 0.81 m, so he must drop the stone when the leaf is 0.81 metres before the bridge.

The speed of the stone when it hits the water is not needed in the question. And I use the time from the first question.
 
  • #12
Jeff97 said:
Ok, so what we know is that he must drop the stone so that it will land on the leaf 2.7 seconds later. If the water is running at 0.3 m/s, then we can find the distance from the bridge to the leaf position (horizontal):

d=vt =0.3 x 2.7 = 0.81 m, so he must drop the stone when the leaf is 0.81 metres before the bridge.

The speed of the stone when it hits the water is not needed in the question. And I use the time from the first question.
That's all good, except that I am still not convinced the two questions are connected.
You refer to a bridge. There was no mention of such in post #1. Are you sure there's nothing else you've left out?
 

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