last minute revision

[Dr. No]

hey guys i just want to check that i know energy correctly.  my book has this question:

A ball of mass 6kg is dropped from a height of 10 metres above the ground.  It then compresses a spring as it gets close to the ground.  By how much will the spring be compressed if the spring has a force constant of 3 Nm-1?

At the bottom of the fall, the only force acting on the ball will be that of gravity and the the hooke force, yeah?

F = kx which is equal to F = mg

3*x = 6 * 10
x = 60/3 = 20 metres.

is that correct?

[Ahmad]

EPIC LUL

[Tea.bag]

umm not quite right...

[Tea.bag]

it is the energy that is conserved not the force...

[/0]

But that's not the point, is it? Weight will continue pushing downwards until the spring can balance it, and that point is at displacement x... right? I would have used an energy approach myself, but that would require the length of the unstretched spring.

[Tea.bag]

mgh=0.5kx^2...

[/0]

mgh=0.5kx^2...

, where L is the length of the unstretched spring

[dcc]

mgh=0.5kx^2...

, where L is the length of the unstretched spring

The Question did say "as it gets close to the ground", so would it be possible for one to assume that

[enwiabe]

Except if you assume Ug = 0, you get x = 14. Now, that's pretty far off the fricken ground. Shit question, IMHO.

I seriously would write as an answer to that question
"How long is a piece of spring?"

[/0]

I get x = 20 using conservation of energy :O. Even x = 14 is an impossible answer! Did we just break physics?

[cara.mel]

To new people viewing: (because there is a lot of you after mid years. )

This is an in-joke. There is a person on this site: http://www.artofproblemsolving.com/ with that username who seemingly answers every single question wrong

To old people: you've had like 2 days to have fun

[Ahmad]

He even gives multiple approaches to prove his incorrect statements!

Profile: http://www.artofproblemsolving.com/Forum/profile.php?mode=viewprofile&u=8491

[Neobeo]

hey guys i just want to check that i know energy correctly.  my book has this question:

A ball of mass 6kg is dropped from a height of 10 metres above the ground.  It then compresses a spring as it gets close to the ground.  By how much will the spring be compressed if the spring has a force constant of 3 Nm-1?

At the bottom of the fall, the only force acting on the ball will be that of gravity and the the hooke force, yeah?

F = kx which is equal to F = mg

3*x = 6 * 10
x = 60/3 = 20 metres.

is that correct?

As much as I like you Dr. No, I'll have to politely disagree with your claim. The main problem is that the ball has an upward acceleration at its lowest point. It trivially has zero velocity since it is at the lowest point, but if it had no acceleration as well then it would stay at that lowest point forever. Which we all know isn't how springs work!

But since I like the force approach myself, I have tried to modify it slightly to make it work. Instead of having a zero net force at the lowest point, we have a zero net force when the spring reaches equilibrium instead, then adding the amplitude of the oscillation to that. To that end I have created the following visual aid:



So it turns out you were right after all, Dr. No. I should never have doubted you from the start.

Regards,
Your Dr. No 1 Fan,
Neobeo

P.S.: If you're new to this forum I'd love to show you around  .

[Mao]

LOL!!!

[/0]

Omg you're so colourful! Your notes must be awesome