You wear this heavy helmet. The main stiff part has to weigh at least as much as the ball or whatever that hits the helmet, so the ball will stop or bounce off. There is enough padding so that the main part doesn't hit your head while it's absorbing the momentum from the ball. Once it's moved back the small distance and the ball has bounced off it, it hits an equal mass weight on the back of the helmet, which absorbs all the momentum and flies off the back. To protect against side impacts a you can also have weights on each side, though of course each weight makes the total heavier. The combination of back and side weights would also protect against diagonal impacts, if I've got it right.

And you can have a weight at the front, protecting you from all directions except vertical.

You don't have the weights attached with string to a point above your head like a proper Newton's cradle, because that would be silly, and also because if they were hit hard enough they might swing all the way around and hit you.
-- caspian, Sep 26 2016

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