Hi Daidai – you can work this one out for yourself. The mass of a penny is about 3 grams. The terminal velocity (maximum speed in air resistance) is about 100 mph. The kinetic energy is half the mass times the velocity squared.
If you work this out, you can then convert it to how fast a cricket ball would have to be going to have the same energy. It’s not exact, because the penny will keep a fair bit of its energy after it hits you and bounces off, but even if you assume all the energy from the penny goes into your head, it will only sting a bit.
Hi daidai! I personally don’t really feel like I want to test this one out in person! Drew’s calculation should give you an idea though. That’s my favorite way. But, if your more of an experimentalist, you could always do a “model” experiment and start dropping 1p pieces into a sand box (or maybe cornstarch and water – that’s a little harder) from higher and higher up and see what you observe. Could be interesting!!