Need Help with Project

[TRB]

Hi!

Me and two others are building an engineering project to be completed by next week. The project is a double pendulum impact rig for testing helmets. So far, we have completed everything - including the pendulum rig. One very important thing still remains a problem though - the formula for calculating the actual force hitting the helmet.


Let me explain exactly what the pendulum impact rig does and how we have to use it.
Basically, its like any other impact test rig for measuring the strength of metals, except there are two pendulums, one containing the helmet with the aluminium head mould with a mass of 10kg, and the other pendulum containing the hemisphere surface for helmet impact with mass of 5kg.
During operation, the helmet and mass is raised to a certain angle, while the other pendulum containing the impact surface remains at its rest location. The helmet and mass pendulum is released and impacts the other pendulum (hopefully causing it to break).

Now, what we need is the formula for calculating the impact force using the data taken.

The data we are to use for each operation is :

1) the angle of the raised pendulum initially (helmet pendulum)
2) the max angle of the pendulum impacted (surface pendulum)
3) the angle at which the rebound of the second pendulum hits the first.
4) all the usuals including mass of both pendulums and length of pendulum arms etc.

If any of you guys could please supply us with a formula for calculating the force of impact using the information.

Thank you very much in advance

[C1ay]

Basically it's just the force of the falling body accelerated by gravity. Can you calculate how fast a 10kg body is falling after x seconds? The force is then f=ma.

[TRB]

Can you calculate how fast a 10kg body is falling after x seconds? The force is then f=ma.

Sorry C1ay, we are not allowed measure time. We built the pendulum under the instructions to use three angles during the test, the initial angle of the first pendulum raised, the second pendulums angle after being impacted, and then the rebound angle of the first pendulum once been hit by the second.

We can use the mass of both pendulums and the length of both pendulum arms (if necessary) too.

Any ideas?

Thanks for the reply C1ay

[C1ay]

You do not have enough information to solve the problem them. An analogy might be, if a car leaves New York for California at 60 mph hour, how long will it talk Mary to walk to the store? Bodies falling under the force of gravity do so at a rate of 9.8 m/s^2 where s is seconds. You could attempt to back into this by calculating the vertical distance the first mass will fall before string the second mass and using the gravitational constant to calulate the acceleration so that you can then compute the force.

[cwes99_03]

Originally Posted by C1ay

You do not have enough information to solve the problem then.

Sure you do, you have the length of the pendulum arm.

Assuming that both pendulum arms are of equal length and that the period of each pendulum is equal then you must only calculate the angles and the lengths. Simply put, how much energy is being provided by gravity to the system is only a measure in the change of potential into kinetic energy.

PS. I expect a mention in the paper!

[TRB]

arms are of equal length and that the period of each pendulum is equal then you must only calculate

Thanks for the reply cwes99.

Im not sure what you mean by the period of each pendulum?

PS. I expect a mention in the paper!

No problem Ill dedicate 3 pages of Acknowledgements to you

[cwes99_03]

The only reason I mention the period is to figure out where in the path of the pendulum's motion will the two pendulums collide. If the periods are equal and the two pendulums are released simultaneously then they'll collide at the bottom of their arcs where kinetic energy is the greatest.