http://www.ck12.org Chapter 9. Energy and Force Version 2
9.1 The Big Idea
The law ofconservation of momentumstates that in any closed system (including the universe) the total quantity of
momentum is constant. Momentum can be transferred from one body to another, but none is lost or gained. If a
system has its momentum changed from the outside it is caused by animpulse,which transfers momentum from one
body to another.
When any two bodies in the universe interact, they can exchange energy, momentum, or both. We saw in an earlier
chapter that the law ofconservation of energystates that in any closed system (including the universe) the total
quantity of energy remains fixed. Energy is transferred from one form to another, but not lost or gained. If energy is
put into a system from the outside or vice versa it is often in the form ofwork,which is a transfer of energy between
bodies.
At this point, we have an opportunity to explore the relationship between force, energy and work. These are really
important concepts in physics, so we should take our time to understand them. At this point the situation can be
summarized like this: energy is the capacity to create motion or change, force is what creates the change, and work
is a book keeping tool to keep track of forces.