A spring is actually a very convenient way of measuring the maximum force that occurs during an event. Here's how it would work:

1) A stiff spring is placed in a loose-fitting white cylinder (like a short PVC pipe) so that half of the spring is exposed and half is inside the cylinder.

2) A marker could be placed on the inside of the cylinder, attached to the spring near the top of the cylinder.

3) Secure this apparatus inside a helmet so that hitting the top of the helmet will depress the spring (that is, support the bottom of the white cylinder so it doesn't move when the helmet is hit. It can rest on the head of someone wearing the helmet, for example...but it's best practice not to use real people for these experiments!)

4) When the helmet is impacted, the spring will compress and the marker will draw a line inside of the cylinder, marking the maximum amount the spring was compressed during the collision.

5) Given the amount of compression in the spring, you can calculate the force by knowing the spring's Hook's Constant (Force = -k*x, where x is the (compressed length - original length).

Yeah, good question! The instrument you want is something that measures the change of shape of the helmet. Some options to consider:

1. Place a space-filling but fragile object inside of the helmet, then whack it. My suggestion would be a plastic bag filled up with water and tied off (and remember to put it into the helmet upside down and fill it while inside the helmet). Water is incompressible, so the bag will be made very fragile by this.

2. Use a high-speed camera (or movie camera, if you already have one - many modern digital cameras and cell phones do, so it's not unlikely that you already do). Then take a movie of the helmet being whacked, and analyze the shape of the helmet frame-by-frame, and measure how much it changes relative to, say, a fragile but semi-rigid object (a melon of some kind or other large squash like a pumpkin would be perfect).

Likely yes, although it is controversial. The NFL is not unbiased in its internal studies, see many articles here about the current state of understanding:

head-injuries

This website has instructions on how to make various items like pressure sensors that might work:

analog-pressure

You will need some access to items like a voltmeter though.

Alternatively you might be able to rig up a bike inner tube or balloon with a pressure gauge then convert the change in pressure to force (pressure = force/area). It depends on how much force you want to measure really.

If I were measuring force, I would visit a local hardware or home improvement store to see if they had a spring that could be compressed (the opposite of stretched). This sort of spring would have coils that are far apart when it is relaxed.

If I could find a spring of the right strength, I would get PVC pipe and make two tubes, one that would fit inside the other. Both would be a little over half the length of the spring. The spring would fit inside the smaller tube and the bigger tube would go face the other way and cover the other end of the spring. Then when you pushed on the ends of the whole device, it would get shorter.

You could measure the amount of force it took to compress the spring a certain distance by stacking items of known weight on top of it and measuring how much it compresses. If you attach a marking pen to the side, and have that touch a paper, you can see how much it compressed when something hit it.

If all of this is a bit too much, you can use a big ball of modeling clay, mount it on a wide dowel, and measure how much it squishes.

You then need a way to hit things with a consistent force. I suggest using pendulum--a weight on a rope that you swing from a consistent height. Heres a picture of what I mean:

This is maybe safer than dropping something straight down, but keep clear because it will bounce back. For a weight, I would probably pick up a kettle bell

Kettlebell

if it were cheap enough or if I could borrow one. Otherwise, I would pick up a strong duffle bag at a thrift store and fill it with sand. I would be afraid that a milk jug would break.

Have an adult help you out to keep things safe. Back when I played rugby, I heard that there were a lot fewer head injuries in rugby than in football (per person hour) because wearing a helmet makes a person do things like use their head as a weapon. Back when I played rugby, no one wore any protective head gear other than a mouth guard. I was curious whether injury rates are different when people wear soft head protectors, as many do now. I looked it up and found that there was no difference

You might want to read this article about whether helmets help or hurt in American football:

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