# Dynamics (Newtons Laws of Motion) and Projectile motions

## 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

SammyS
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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

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?

SammyS
Staff Emeritus
Science Advisor
Homework Helper
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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

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