Why do things collide?

When two things collide, the energy of the objects’ motions is released — sometimes with destructive outcomes. Think of what happens when you drop a watermelon off the roof of a ten-story building, or when two cars collide! The energy of motion goes into rearranging the matter of the objects. And sometimes the outcome is a mess! But as Cosmic Collisions illustrates, the forces of nature can also transform that mess into new and beautiful forms: galaxies re-shape themselves, new stars are born from old ones, and even life on Earth has been made possible by collisions. Cosmic collisions drive change on cosmic scales, and thus the evolution of the universe. The universe is constantly in motion, and things in motion will maintain that same motion…unless acted upon by an outside force. This is Isaac Newton’s 1st law of motion, the law of inertia. Newton’s 2nd law of motion, F=ma, conversely states that a force is something that changes an object’s motion. When a driver steps on the brakes, the car slows; when a comet passes by the Sun, its orbit is defl ected by the Sun’s gravitational pull. If this sounds rather circular, it is! Scientists use the concept of force to describe how things behave, not what a force is. Investigating what a force is remains a fundamental goal of science.


Gravity - the force of attraction between any two bodies with mass — is by far the most important force in setting the objects in the universe in motion. The more mass an object has, the greater the gravitational pull it exerts. And the closer the two objects are, the stronger the pull of gravity they exert on each other. That’s why we feel more gravity on Earth than on the Moon. Gravity is responsible for the fall of that proverbial apple onto Newton’s head, as well as the inexorable attraction between the Milky Way and our nearest neighbor, the Andromeda Galaxy. Although it is much weaker than the other forces of nature, gravity acts over enormous distances. Things are also in motion because they are attracted or repelled by electromagnetism. Like gravity, this is a fundamental force at work in the universe — and it’s much stronger than gravity. It’s driven by the interaction of electrical charges (such as protons or electrons), causing attraction or repulsion between them. It is the repulsion between the electrons in a baseball and the electrons in a bat (helped by the momentum of the swing) that sends the ball fl ying when the two collide. Another manifestation of electromagnetism is the magnetic fi eld generated by the movement of electrical charges in the outer core of the Earth as it rotates, which protects us from collisions with solar particles. Since everything is in motion in our dynamic universe, and the forces of gravity and electromagnetism operate throughout, collisions are inevitable.


Now Try This!

Experience Gravity Free Water


What you'll need:

  • A glass filled right to the top with water
  • A piece of cardboard



  1. Put the cardboard over the mouth of the glass, making sure that no air bubbles enter the glass as you hold onto the cardboard.
  2. Turn the glass upside down (over a sink or outside until you get good).
  3. Take away your hand holding the cardboard.


What's happening?

If all goes to plan then the cardboard and water should stay put. Even though the cup of water is upside down the water stays in place, defying gravity! So why is this happening? With no air inside the glass, the air pressure from outside the glass is greater than the pressure of the water inside the glass. The extra air pressure manages to hold the cardboard in place, keeping you dry and your water where it should be, inside the glass.