* Static friction: This is the force that opposes the motion of an object that is in contact with a surface. When the net force acting on an object is zero, the object will remain at rest.
* Kinetic friction: This is the force that opposes the motion of an object that is moving across a surface. Kinetic friction is always less than static friction, so an object will always eventually stop moving if it is not acted on by another force.
* Air resistance: This is the force that opposes the motion of an object that is moving through the air. Air resistance is proportional to the speed of the object, so an object will eventually stop moving if it is not acted on by another force.
* Gravity: This is the force that pulls objects towards the center of the Earth. Gravity is always acting on objects, but it is only significant when the net force acting on an object is small.
