1 mark question on kinematics, or a stone fallin through air.

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Homework Help Overview

The problem involves a stone dropped from a height of 40 meters, focusing on the time taken for the stone to fall the last 10 meters. The context is kinematics, specifically dealing with free fall and the effects of gravity, while neglecting air resistance.

Discussion Character

  • Exploratory, Assumption checking, Problem interpretation

Approaches and Questions Raised

  • Participants discuss calculating the time taken to fall the last 10 meters by considering the time taken to fall the first 30 meters and the total 40 meters. Questions arise about the initial and final velocities used in these calculations.

Discussion Status

There is an ongoing exploration of different approaches to the problem. Some participants have provided guidance on using kinematic equations, while others are questioning the assumptions made regarding initial and final velocities. The discussion reflects a mix of interpretations and attempts to clarify the setup.

Contextual Notes

Participants note the importance of correctly identifying the initial and final velocities for the different segments of the fall, as well as the need to apply appropriate kinematic equations. There is an acknowledgment of potential confusion regarding the definitions of these velocities in the context of the problem.

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Homework Statement


A stone is dropped from the top of a tower of height 40 m. The stone falls from rest and air
resistance is negligible.
What time is taken for the stone to fall the last 10m to the ground?

A 0.38s B 1.4s C 2.5s D 2.9s


Homework Equations





The Attempt at a Solution



I first found the final velocity for the first 30m. V^2= 2x9.8x30 (ignore directions)

then i used that final velocity as my initial velocity in V=u+at where V=0. My answer is C. But its wrong.
 
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Why not find the time to fall 30 m, then the time to fall 40 m, and take the difference?
 
gneill said:
Why not find the time to fall 30 m, then the time to fall 40 m, and take the difference?

How do i find the time t0 fall 40m? Both the final and initial velocity will be zero then won' it.
 
mutineer123 said:
How do i find the time t0 fall 40m? Both the final and initial velocity will be zero then won' it.

Find a kinematic equation that expresses the distance as a function of acceleration and time.
 
Applying your method V is not = 0.The final speed is just before it hits the ground and you have taken the 'final' speed to be when it hits the ground and stops.
The velocity you calculated at 30m is u for the last 10m
 
Emilyjoint said:
Applying your method V is not = 0.The final speed is just before it hits the ground and you have taken the 'final' speed to be when it hits the ground and stops.
The velocity you calculated at 30m is u for the last 10m


Thanks, yeah forgot about that. Plugged it in and got .383, close enough. Thank you
 

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