You try to pound a nail into a piece of wood by holding the piece of wood in your hand. It is hard, so then you get a heavy piece of iron and put the wood on top of that. You hold the iron and start pounding. Why is it easier now to get the nail into the wood?
Your friend is not wearing a seat belt and slams into the windshield when you abruptly stop. Why?
A 1 kg water bottle is thrown up. Ignoring friction what is the net force on the water bottle in Newtons?