# Energy Absorption from Impact

Tags:
1. Mar 13, 2015

### 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?

2. Mar 13, 2015

### Suraj M

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

3. Mar 13, 2015

### ClapClap

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

4. Mar 13, 2015

### Suraj M

no, change in kinetic energy!

5. Mar 13, 2015

### ClapClap

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

6. Mar 13, 2015

### 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 thats 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.

7. Mar 13, 2015

### ClapClap

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

8. Mar 13, 2015

### Suraj M

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

9. Mar 15, 2015

### ClapClap

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

10. Mar 15, 2015

### Suraj M

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.

11. Apr 5, 2015

### 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 travelled , the with shinguard , take same measurement.

12. Apr 6, 2015

### 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.

13. Apr 15, 2015

### 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?

#### Attached Files:

• ###### Screen Shot 2015-04-15 at 23.11.55.png
File size:
14.8 KB
Views:
158
14. Apr 15, 2015

### Staff: Mentor

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

15. Apr 15, 2015

### 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.

File size:
65.3 KB
Views:
177