Dynamics (Newtons Laws of Motion) and Projectile motions

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Homework Help Overview

The discussion revolves around a dynamics problem involving an 18kg rock released underwater, with a focus on the forces acting on it, including buoyancy and friction. The main question is about determining the time it takes for the rock to fall 1.0m.

Discussion Character

  • Exploratory, Assumption checking, Mathematical reasoning

Approaches and Questions Raised

  • Participants discuss the calculation of acceleration based on net forces and question the units involved. There is an exploration of kinematic relationships to relate distance, acceleration, and time.

Discussion Status

Participants are actively engaging with the problem, with some providing calculations and others questioning the reasoning and units used. There is a focus on understanding the relationships between forces and motion, but no consensus has been reached on the next steps.

Contextual Notes

There are indications of confusion regarding the signs and units in the calculations, as well as the need for a free body diagram (FBD) to clarify the forces acting on the rock.

jelzap
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Homework Statement


An 18kg rock is released under water.
The fricitional Buyoyance add up to 129N
How long will it take to fall 1.0m


Homework Equations





The Attempt at a Solution

 
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jelzap said:

Homework Statement


An 18kg rock is released under water.
The fricitional Buyoyance add up to 129N
How long will it take to fall 1.0m


Homework Equations



You should have something here!

The Attempt at a Solution



... and here.
Hi jelzap.

What have you tried? Where are you stuck?
 
well, i found the acceleration of the rock, i need to know what to do from there

M(A)=Fnety
M(A)= Fa+ Ff
M(a)=-(18*9.8) + 129N
18a=(-176.4)+129
a=47.4 (what are the units here?)

What do i do from here?
 
jelzap said:
well, i found the acceleration of the rock, i need to know what to do from there

M(A)=Fnety
M(A)= Fa+ Ff
M(a)=-(18*9.8) + 129N
18a=(-176.4)+129
a=47.4 (what are the units here?)

What do i do from here?

You have: 18a=(-176.4)+129, which gives →  (18kg)a = (-47.4N).

How do you get a = 47.4?   What happened to the 18?   What happened to the "−" sign?

1 Newton = 1 kg·m/s2.   What units for Newton/kg ??

Look for a kinematic relationship involving: distance, (constant) acceleration, and time.
 
Draw the FBD and find net force acting on the rock
equate it to manet (you can predict the direction pretty easily)

Find anet

now use second eqn of motion
 
Last edited:

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