Question:
A particle of mass m is attached between two horizontal springs of stiffness 2k and 5k, each of un-stretched length a (see figure below). The mass is held stationary at a displacement from its resting position of x = 1.03m.
![](https://static.wixstatic.com/media/20c892_63151b8503ed450b889c6ff43c324541~mv2.png/v1/fill/w_49,h_16,al_c,q_85,usm_0.66_1.00_0.01,blur_2,enc_auto/20c892_63151b8503ed450b889c6ff43c324541~mv2.png)
Show that the system satisfies the differential equation given by:
![](https://static.wixstatic.com/media/20c892_bd44ad72a4894130a6431ba2b695d9ea~mv2.png/v1/fill/w_60,h_24,al_c,q_85,usm_0.66_1.00_0.01,blur_2,enc_auto/20c892_bd44ad72a4894130a6431ba2b695d9ea~mv2.png)
and use Laplace transforms of derivatives to find L{x}.
Answer:
Resolving horizontally (+ve direction from left to right):
![](https://static.wixstatic.com/media/20c892_a117dfd8f11844fab4b7c526081c45c0~mv2.png/v1/fill/w_49,h_24,al_c,q_85,usm_0.66_1.00_0.01,blur_2,enc_auto/20c892_a117dfd8f11844fab4b7c526081c45c0~mv2.png)
Take Laplace transforms of both sides:
![](https://static.wixstatic.com/media/20c892_b30c50c7f6e94f8f942878b029cdb1be~mv2.png/v1/fill/w_83,h_5,al_c,q_85,usm_0.66_1.00_0.01,blur_2,enc_auto/20c892_b30c50c7f6e94f8f942878b029cdb1be~mv2.png)
Sub in Laplace transform of derivatives:
![](https://static.wixstatic.com/media/20c892_9b7887a73976420eb66dd6cf1c4d8076~mv2.png/v1/fill/w_81,h_14,al_c,q_85,usm_0.66_1.00_0.01,blur_2,enc_auto/20c892_9b7887a73976420eb66dd6cf1c4d8076~mv2.png)
Sub in initial conditions x0 and x'0:
![](https://static.wixstatic.com/media/20c892_77ae8dcd28f8423eb52cec1befcf1d1b~mv2.png/v1/fill/w_83,h_29,al_c,q_85,usm_0.66_1.00_0.01,blur_2,enc_auto/20c892_77ae8dcd28f8423eb52cec1befcf1d1b~mv2.png)
Question:
Determine the poles and zeros of the system and plot them on a pole-zero diagram. Use the initial value theorem to check your Laplace Transform.
Answer:
Find the Poles:
![](https://static.wixstatic.com/media/20c892_f0c1fe5de97346eca3d7c28360cc5d00~mv2.png/v1/fill/w_82,h_23,al_c,q_85,usm_0.66_1.00_0.01,blur_2,enc_auto/20c892_f0c1fe5de97346eca3d7c28360cc5d00~mv2.png)
Find the Zeros:
![](https://static.wixstatic.com/media/20c892_4949363ec4834c39a76f8451782f69f0~mv2.png/v1/fill/w_82,h_10,al_c,q_85,usm_0.66_1.00_0.01,blur_2,enc_auto/20c892_4949363ec4834c39a76f8451782f69f0~mv2.png)
Draw the Pole-Zero diagram:
![](https://static.wixstatic.com/media/20c892_5da266ad498640f39db4c5062cfc0648~mv2.png/v1/fill/w_79,h_43,al_c,q_85,usm_0.66_1.00_0.01,blur_2,enc_auto/20c892_5da266ad498640f39db4c5062cfc0648~mv2.png)
Determine initial values:
![](https://static.wixstatic.com/media/20c892_a4c9a4862e8746d3b398e4d29b91eca6~mv2.png/v1/fill/w_85,h_44,al_c,q_85,usm_0.66_1.00_0.01,blur_2,enc_auto/20c892_a4c9a4862e8746d3b398e4d29b91eca6~mv2.png)
We know this is correct as the displacement at time = 0 was known.
Question:
By using tables to find the Inverse Laplace Transform, determine the equation for the displacement x(t)
Inverse Laplace:
![](https://static.wixstatic.com/media/20c892_e70f5129eb2946fe9893940df477b191~mv2.png/v1/fill/w_81,h_28,al_c,q_85,usm_0.66_1.00_0.01,blur_2,enc_auto/20c892_e70f5129eb2946fe9893940df477b191~mv2.png)