Calculate Velocity of 100 lb Block Dropped onto Stiff Spring

[the_d]

How do i find the velocity of a block of mass 100 lb being dropped a distance 5 ft onto a spring whose stifness is 100 lbs/ft after the spring has deformed 4 in?

im sure it has somthing to do with work of a spring force but i don't know exactly how. any ideas?

 

[jamesrc]

Can you write equations for the total energy of the mass and spring (kinetic and potential) before being dropped and at the moment the spring is deformed 4"? If you can, then you are well on your way to solving the problem.

 

[the_d]

ok ill see what i can do, thanx

 

[the_d]

do i use T1 + U = T2?

 

[Dorothy Weglend]

How does the work done by gravity relate to the work done by the spring force?

 

[the_d]

its in the opposite direction

 

[Dorothy Weglend]

What do they sum to?

 

[the_d]

What do they sum to?

zero.....

 

[Dorothy Weglend]

Well, there you go then. Problem solved.

 

[Doc Al]

No reason to think that the work done by the spring force will equal the work done by gravity. Note that it doesn't say that 4 in is the maximum compression of the spring... the block keeps going!

Instead, consider conservation of energy (assume no energy is lost when the falling block hits the "massless" spring). What's the initial energy of the block just as its being dropped? What's the final energy of the block + spring system when the spring has been depressed 4 in?

 

[the_d]

good explanation, i think i got it now thanks 4 help