How Can I Calculate Energy Absorbed by a Shin Guard During Impact?

[ClapClap]

Hi,

I am doing a drop impact testing on a football shin guard. I know how to find the impact force but how do I determine the energy absorbed by shin guard during impact?

 

[Suraj M]

welcome to PF...
could it be loss in kinetic energy?

 

[ClapClap]

welcome to PF...
could it be loss in kinetic energy?

Well that is what I thought. I'm not sure if it's equal to the kinetic energy of the falling body

 

[Suraj M]

no, change in kinetic energy!

 

[ClapClap]

no, change in kinetic energy!

This is going to sound really stupid and I'm sure that as soon as I see the answer I'll realized how obvious it was, but how would I find that?

 

[Suraj M]

It might be a bit hard to do, but you should measure the velocity of the ball before and after the collision, if that's difficult, do it using the height to which the ball rises, with no horizontal component and then find the velocity, but with air friction, it's a bit difficult.

 

[ClapClap]

It might be a bit hard to do, but you should measure the velocity of the ball before and after the collision, if that's difficult, do it using the height to which the ball rises, with no horizontal component and then find the velocity, but with air friction, it's a bit difficult.

I'm not sure how i can measure the height after collision as i am dropping a meta stud on it :/ as for the air friction that won't matter as i won't be considering it

 

[Suraj M]

Do you have means of finding the velocity of the ball after the impact?

 

[ClapClap]

Yea. Can I assume that the impact velocity is the same as the rebound velocity or is that wrong?

 

[Suraj M]

t the impact velocity is the same as the rebound velocity or is that wrong?

I think you mean SPEED, because velocity cannot be the same after the REBOUND.
Well theoretically, you can , assuming that the shin guard is as rigid as a wall. But still in reality you can't, however rigid it is, the coefficient of restitution cannot be =1, there will be some energy loss. Also if you assume that the magnitude of velocity remains same, you mean to say that no energy had been absorbed by the shin guard.

 

[Helmet]

you can test the shinguard either with a drop test, like at a helmet lab, with accelerometers in the headform, or on a simpler , self constructed pendulum apparatus. on pendulum put object on bottom of swingarm, hit without shinguard, take measurement of distance traveled , the with shinguard , take same measurement.

 

[Randy Beikmann]

You say you know how to find "the force." Are you measuring it, vs. time? If so, you should be able to calculate energy loss from that directly: the work done on the shin guard during compression of the impact, minus the work done by the shin guard during rebound, is the energy loss. To calculate this you'd either need to have displacement of the impacting object vs. time during the impact also, whether measured directly or from measuring the acceleration vs. time.

 

[ClapClap]

I have actually been struggling reading my sensors measurements. I have attached an acceleration vs time graph i have developed with the measurements taken. Can you help me understand it?

 

[mfb]

That graph looks messy. And I cannot read the axis labels.
Do you have a sketch of the setup? Where is the sensor?

 

[ClapClap]

Sensor was attached to a falling mass of 4.3 kg. The mass was dropped at different heights (this graphs shows acceleration measurements for impact drops of 20 cm). The striking mass was dropped onto a football shin guard which was placed on a metal flask. I assumed that the graph is messy because the weight bounced a few times on the guard.
The y-axis shows the acceleration measurements recorded by the accelerometer, the x-axis shows the time.