2.3 Newton’s Third Law

Describe the interaction of two objects or systems using Newton’s third law and a representation of paired forces exerted on each object or system.

• Newton’s third law describes the interaction of two objects or systems in terms of the paired forces that each exerts on the other.
• Interactions between objects within a system (internal forces) do not influence the motion of a system’s center of mass.
• T ension is the macroscopic net result of forces that infinitesimal segments of a string, cable, chain, or similar system exert on each other in response to an external force.
  • i. An ideal string has negligible mass and does not stretch when under tension.
  • ii. The tension in an ideal string is the same at all points within the string.
  • iii. In a string with nonnegligible mass, tension may not be the same at all points within the string.
  • iv. An ideal pulley is a pulley that has negligible mass and rotates about an axle through its center of mass with negligible friction.