# Spring Constant of a block

## Homework Statement

Two 47 blocks are held 30 above a table. As shown in the figure, one of them is just touching a 30-long spring. The blocks are released at the same time. The block on the left hits the table at exactly the same instant as the block on the right first comes to an instantaneous rest.

## Homework Equations

I tried using everything, T=2*pi root m/k

## The Attempt at a Solution

Really have no idea where to go with this..

Thanks for any help!

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SammyS
Staff Emeritus
Homework Helper
Gold Member
Having units of measure and a figure would help us to help you.

Thats the entire question I was given. But I was given a figure and I attached it!

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jambaugh
Gold Member
You can use the period formula but remember one period is the time to complete a full cycle i.e. how long until the spring attached mass bounces back up to its starting point.

You should be able to figure the the time of flight for the free falling mass given the distance and gravity.

Given both traverse the distance in the same time...

I tried using that formula but we dont have T and K. so its 2 unknowns. am i missing something ?

SammyS
Staff Emeritus
Homework Helper
Gold Member
That's the entire question I was given. But I was given a figure and I attached it!
Well, the unit of length in the figure is cm.

The blocks could each have a mass of 47 kg, or 47 grams - or they could have a weight of 47 N. Maybe it doesn't matter unless you want to reproduce this situation in the lab. (I don't want to have to release two 103 pound blocks simultaneously!)

How long does it take the block on the left to hit the table after it's released?

How long will it take the block on the right to rise back to it's initial position?

Im sorry, im really bad at physics hahaa, are you hinting that i should use conservation of energy?

jambaugh
Gold Member
I tried using that formula but we dont have T and K. so its 2 unknowns. am i missing something ?
You're looking for k, right? You certainly know m right? So what is the meaning of T the period?

It is how long it takes for the spring system to complete a cycle. Remember it bounces up and down and up and down and up and down and .....

One period is the time from say up to down to up again.

Also can you figure how long it takes a mass to drop a given distance?

BTW You don't need to mess with energy here. It's a simpler problem than that. But don't just look for values of variables to plug into the formulas, if that were all there is to it you'd be doing a pure math problem. What are the meanings of the variables in the formulas. (as in what does period T mean)

jambaugh
Gold Member
Im sorry, im really bad at physics hahaa, ...
That's a temporary condition, that is if you want it to be. Practice is what makes you not bad at any endeavor. But you can't just watch it done, and too much help doesn't practice the critical skills and test your own understanding. You have to approach the subject with enough audacity and determination to do it on you own.

A hint may speed this process up but if you need too many hints, you need to go back and study the prerequisite concepts and understand the meaning of definitions before attempting the problems. The ability to solve problems you have already (you use it to function on a daily basis) and has nothing to do with physics per se. You simply apply it to physics once you understand the meaning of the components of a physics problems (and of course the math).

This problem has two basic components, a freely falling mass, and a mass and spring. Go back and look at your text and notes on each of these systems if you are still having trouble with this and similar problems. Keep at it until you can solve similar problems without getting hints.

(Pardon my tirade, it isn't really aimed at you. I see too many students despairing at their lack of ability as if it were fixed in concrete and so you struck a nerve.)

BTW You don't need to mess with energy here. It's a simpler problem than that. But don't just look for values of variables to plug into the formulas, if that were all there is to it you'd be doing a pure math problem. What are the meanings of the variables in the formulas. (as in what does period T mean)

the period means how long it takes to get through one repeat of the cycle, so in this case it would be from the pt of dropping to when the box bounces back up the spring...but we are not given a time at all in order to use that...

i looked through my notes as well as the textbook and dont see anything along the lines of this question anywhere in there :(

SammyS
Staff Emeritus
Homework Helper
Gold Member
the period means how long it takes to get through one repeat of the cycle, so in this case it would be from the pt of dropping to when the box bounces back up the spring...but we are not given a time at all in order to use that...

i looked through my notes as well as the textbook and don't see anything along the lines of this question anywhere in there :(
I asked earlier, how long it takes for the mass on the left to reach the table.

How does that time compare to one period of the mass-spring combination?

BTW: You do need to know the units for the mass. 47 -(what units?)

gneill
Mentor
The problem statement says explicit things about time. Reread the problem statement.

jambaugh