To every action there is an equal and opposite action, i.e. an equal reaction. Internal forces are one part of a system acting on another part of the system, they do not cause motion, they keep a system together. The force which causes the motion of a system is the resultant external force. The effect of force through time is the red area under the graph and written as . A conservative force does no net work in going through a closed path.
Power is the rate at which energy is delivered, or the rate at which work is done.
The greatest challenge facing humanity in the next twenty years is to preserve our fundamental life support systems in the face of increasingly inequitable and unsustainable production and consumption patterns. The microscopic view of matter has particles that can't be seen, heard, touched etc.