
How to compute the force?  
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Posted by: sjob ^{®} 02/24/2008, 23:35:37 Author Profile eMail author Edit 
Dear group, Please see the attached pic. I've a mass m, held against a body A by a spring, which is initially in a state of compression. The mass m, suddenly sees a pressure P, which is much higher than the spring opposing force for the distance a. How can i compute the force imparted to the body B. Is it simply (pressure force  spring force)? I'm just confused if there is any dynamic angle to the force in this problem. Thanks

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: How to compute the force?  
: How to compute the force?  sjob  Post Reply  Top of thread  Forum 
Posted by: eagle ^{®} 02/27/2008, 01:36:06 Author Profile eMail author Edit 
well dear sjob i would suggest that u first define the dimensions and the material of each part so that the deflections and contact forces can be calculated on each part.
regards 
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Posted by: jboggs ^{®} 02/25/2008, 16:55:20 Author Profile eMail author Edit 
Can you enlighten us as to the application and some of the dimensions involved? The "perfect" answer will have some algebraic gymnastics involved. The realistic answer will probably allow you to ignore some theoretical effects. 
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Posted by: sjob ^{®} 02/25/2008, 23:02:09 Author Profile eMail author Edit 
Its a representative model for a pressure switch. The plunger is the mass m and as it sees a high pressure it would be activating the switch near to the ground(not shown in the pic). The position of body B limits the travel of the the plunger to the distance 'a'. The dimension 'a' would be something like .04" , initial spring compression 0.06" and mass m is around .013 lbs , K is aorund 170 lbs/inch. Pressure is 500psi, the exposed dia of mass would be around .037". Thanks

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Posted by: jboggs ^{®} 02/26/2008, 08:21:17 Author Profile eMail author Edit 
If I understood your explanation correctly and did my math right, the answer is ZERO. The mass won't move at all. 
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Posted by: sjob ^{®} 02/26/2008, 22:36:11 Author Profile eMail author Edit 
Sorry, jboggs. There was a typo in my earlier mail. Yday I tried to post it, but net connection was lost. The dia is 0.37" and not 0.037" as in the earlier post. 
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Posted by: jboggs ^{®} 02/27/2008, 08:40:36 Author Profile eMail author Edit 
The total force is exactly that  the vector sum of all forces. The acceleration of the body at any moment is a product of that total force. As the spring compresses its contribution to that total changes. That means the total force changes continuously as the mass travels. That means that its acceleration also continuously changes. (Your description ignored the effects of friction.) All of that is easily calculated. That being said, realistically its all a nonissue because of the dimensions involved. If the structure can withstand the force of the pressure, you should be ok. 
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Posted by: sjob ^{®} 02/27/2008, 03:32:57 Author Profile eMail author Edit 
eagle, all the parts are SS 303. 
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