Drag is a resistive force. Two familiar examples:

• Friction occurs when two objects are in contact and sliding with respect to each other. In this case, the friction force equals F_f = μ N, and it opposite to the direction of motion.
• Air resistance. This arises, for example, when running into the wind. In this course, we will typically ignore air resistance.
  
  
  

Thrust is a force that is typically caused by gas molecules being ejects from an engine at high speeds.