Dynamics (Newtons Laws of Motion) and Projectile motions

AI Thread Summary
An 18kg rock released underwater experiences a buoyant force of 129N, leading to a net force calculation. The net force equation is established as M(A) = Fnety, where the gravitational force and buoyancy are considered. The acceleration of the rock is derived to be negative, indicating it is moving upwards against the buoyant force. Participants suggest using kinematic equations to relate distance, acceleration, and time to determine how long it takes to fall 1.0m. The discussion emphasizes the importance of correctly applying Newton's laws and understanding 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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