A hunter finds his target standing on top of a frozen lake. He shoots with his rifle but in doing so, he too was thrown back over the lake and injured himself. Why did this happen?

Question

I'm thinking Newton's 3rd law but aside from that don't really know how to explain.

Answer 1

FIRST law is in the absence of an EXTERNAL force the momentum of bullet/hunter system remains constant
momentum before firing = 0
so
0 =mass of hunter * velocity of hunter + mass of bullet * velocity of bullet
so
v hunter = - ( mass bullet/mass hunter ) vbullet