Confusing Problem 4

[04jchatter]

Hello Everyone,

I am a product designer so my mechanical engineering is a bit rusty. I have been tasked with putting together a test program for a traffic sign. One of the tests is a simulated impact. The way this is done is by hitting the product with an (undetermined) mass using a pendulum. The arm has to be 1.25 meters long and the impact energy has to be 150 Nm.

My problem is that, that is the only information the standard gives to work with. I have estimated (as best I can) the period of the pendulum:

T=2π√L/G For which I have calculated T=2π√1.25/9.81 = 2.25

However After that I am not sure were to go, I need to prove that the energy that will impact the sign is 150 Nm.

If possible a break down of how the Pendulum period works and how to calculate pendulum energy would be excellent.

Many Thanks,

04jchatter

 

[04jchatter]

Hello everyone,

Thank you all for your responses, maybe I am being a bit dense and maybey as IRstuff says my calculator may be broken, but I cant seem to see where you have calculated my Velocity? I seem to always get a value of 5.942m/s ? If there is anyway someone could explain where they are getting (4.95m/s) I would be very grateful.

Racookpe1978 - The impact will be 250mm of the ground, but the bumper is quite big (500mm/250mm plywood with weights on the back) Its a test to ensure there is no determinant deflection on the base of the product after it rebounds.

Once again thank you to everyone for your help,

04jchatter

 

[04jchatter]

GregLocock - The sign is designed to rebound so hopefully 12.2Kg should be sufficient, I just need to prove its the right mass

 

[desertfox]

Hi 04jchatter

A diagram of your setup would help however you need to find the center of percussion of your pendulum because what you will design is a compound pendulum not a simple one.
A simple pendulum is where the mass is connected to a piece of string and the string is effectively massless.
In your case the rod compared to the pendulum bob will not be negligible so search for the mechanics of a compound pendulum.

 

[LittleInch]

04,

As zdas04 says, the equation where you get velocity is v=(2*g*L*(1-cos(α))^0.5

or V squared = 2*g*L*(1-cos(α)

thus V^2 = 2 x 9.81 x 1.25 x (1-0)

= 24.525

Thus V = Sqrt 24.525 = 4.952 m/sec

I've tried a few ways of incorrectly calculating this and can't get to your figure.

Anyway, checking it against the simple energy equation shows it is incorrect.


Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[IRstuff]

Also, bear in mind that the calculations performed provide only the MINIMUM required mass. Any losses, like friction, will lower the KE, and to regain the desired KE, additional mass or velocity is required.

btw, you stated 5.96 in your first result, and now, you're stating 5.942, so something changed. The calculation is pretty much bulletproof, so it's unclear where your math is coming from:


TTFN
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[IRstuff]

Note also that MIL-S-901D, which is the US Navy specification for shipboard shock testing uses a similar kind of pendulum, and there is a graduated scale that's used to determine the angle of the pendulum to achieve a desired impact. In the case of 901D, the UUT is mounted on an "anvil" which is struck by the pendulum hammer.

TTFN
faq731-376

Need help writing a question or understanding a reply? forum1529


Of course I can. I can do anything. I can do absolutely anything. I'm an expert!
There is a homework forum hosted by engineering.com:

http://www.engineering.com/AskForum/aff/32.aspx

 

[Dougt115]

Change your calculator from radians to degrees or use pi/2 radians.

Also was the 150Nm the amount of energy absorbed by the sign or how much energy the pendulum has to have when it hits? The equations we have been using are to calculate the energy of the pendulum at impact but if "bounce" is required then some of that energy will be reflected back to the pendulum decreasing the energy absorbed by the sign. There is also the mechanics of the impact, sharp point, blade edge, rubber bumper.... Each of these will result in different test results.

Z and LI you keep dropping a ")" from your equations.

 

[LittleInch]

That's because I copy and pasted his!

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[racookpe1978]

What is the shape of the pendulum where it hits the plywood?

"Weighted plywood" => but NOT anchored sign post!! => you've suddenly changed the impact problem to that of a moving (pointed ?) object hitting a stationary object and forcing it sideways (some), up (a little bit because the plywood will rotate up and away from its pivot point on the BACK side of the weighted bottom flat plywood, and in deformation (crushing) of the plywood at point of impact.