Question

A mass of 1000 kg drops from a height of 10 m on a platform of negligible mass. It is desired to design a spring and dashpot on which to mount the platform so that the platform will settle to a new equilibrium position 0.2 m below its original position as quickly as possible after the impact without overshooting. Find the spring constant k and the damping constant b of the dashpot. Be sure to examine your proposed solution x(f) to make sure that it satisfies the correct initial conditions and docs not overshoot. Find, to two significant figures, the time required for the platform to settle within 1 mm of its final position.

Answer 1

k =
`5* 10^(4)\ N/m`

b =
`0.707* 10^(3)`

t =
`7.1* 10^(- 5)\ s`

Solution:

As per the question:

Mass of the block, m = 1000 kg

Height, h = 10 m

Equilibrium position, x = 0.2 m

Now,

The velocity when the mass falls from a height of 10 m is given by the third eqn of motion:

`v^(2) = u^(2) + 2gh`

where

u = initial velocity = 0

g = 10
`m/s^(2)`

Thus

`v = √(2* 10* 10) = 10√(2)\ m/s`

Force on the mass is given by:

F = mg =
`1000* 10 = 10000 N = 10\ kN`

Also, we know that the spring force is given by:

F = - kx

Thus

`k = (F)/(x) = (10000)/(0.2) = 5* 10^(4)\ N/m`

Now, to find the damping constant b, we know that:

F = - bv

`b = (F)/(v) = (10000)/(10√(2)) = 0.707* 10^(3)`

Now,

Time required for the platform to get settled to 1 mm or 0.001 m is given by:

`t = (0.001)/(v) = (0.001)/(10√(2)) = 7.1* 10^(- 5)\ s`