• Question: when a bullet is fired from a gun, its fired at a very fast speed but after a while will lose the momentum or speed it started with but if you drop say something like a phone or a watch it picks up momentum why is this???

    Asked by Fintan boyle to Michel on 8 Nov 2016.
    • Photo: Michel Destrade

      Michel Destrade answered on 8 Nov 2016:

      Yes excellent! It’s all to do with the fact that there is horizontal momentum and vertical momentum. When you fire the bullet, the horizontal momentum momentum is huge at the beginning and then it decreases because of the friction with the air. But the vertical momentum is zero at the beginning and then it increases as the bullet is pulled towards the ground by gravity.

      When you drop an object it has no horizontal momentum, but what happens to the vertical momentum is the same as for the bullet: it increases at the object is pulled towards the ground by gravity.

      To put maths onto this, you need new objects called vectors, making it easier to solve the equations. We teach that in first year of a science or an engineering degree here at NUI Galway. Hope you’ll study it one day! In fact studying projectile motion in secondary school (at the time, vectors were taught in secondary school) is exactly what got me interested in physics in the first place!