Rocks and Bullets


I need an answer to a question. If you shoot a gun and dropped a rock at the same time which one would hit the ground first the rock or the bullet???


Let's set up the conditions on this thought experiment like this. We have a gun with its barrel parallel to the ground at some height. A rock sits on a shelf at the same height. We shoot the gun and at the same instant that the bullet clears the barrel, the shelf is pulled from under the rock.

Now if we neglect the effect of air resistance on both the rock and the bullet we can think about your question this way. Clearly the rock starts to fall as soon as the shelf is pulled from under it. The Earth attracts it so that it accelerates downward at 9.8 meters per second per second. The same is true of the bullet once it is clear of the gun's barrel. Since the Earth attracts all objects in the same way, the bullet also accelerates downward at 9.8 meters per second per second. This means that they will both hit the ground at the same time.

The fact that the bullet is also traveling horizontally at the same time does not affect its motion in the downward direction. Remember that the bullet does not wait to start falling until it gets where it is going. It starts at the same time as the rock. This ability to separate the motion of an object into independent dimensions is a very useful idea. For as long as you do science it will be used again and again.

This information is brought to you by M. Casco Associates, a company dedicated to helping humankind reach the stars through understanding how the universe works. My name is James D. Jones. If I can be of more help, please let me know.