AP Physics C: Mechanics Flashcards: Newton's Third Law
Written by AP Content Team, Verified for 2026 AP Exams, Last updated: May 2026
Review key ideas with interactive flashcards. This set includes 10 cards to help you master important concepts.
A hammer hits a nail. The hammer exerts a force on the nail. What is the paired force described by Newton's third law?
The paired force is the force the nail exerts on the hammer, which is equal in magnitude and opposite in direction to the force the hammer exerts on the nail.
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A hammer hits a nail. The hammer exerts a force on the nail. What is the paired force described by Newton's third law?
The paired force is the force the nail exerts on the hammer, which is equal in magnitude and opposite in direction to the force the hammer exerts on the nail.
Define tension in the context of physics.
Tension is the macroscopic net result of forces that infinitesimal segments of a string, cable, or similar system exert on each other in response to an external force.
What effect do internal forces have on the motion of a system's center of mass?
Interactions between objects within a system, known as internal forces, do not influence the motion of the system's center of mass.
Why don't the equal and opposite forces of a Newton's third law pair cancel each other out?
They do not cancel out because they act on two different objects. Net force is calculated for a single object, so these forces are never added together.
Do the two forces in a Newton's third law pair act on the same object?
No, the paired forces are exerted on the two different interacting objects or systems; one force acts on the first object, and the reaction force acts on the second object.
What is the mathematical representation of Newton's third law for two objects, A and B?
The equation is $\vec{F}_{A \text{ on } B}=-\vec{F}_{B \text{ on } A}$, indicating the forces are equal in magnitude and opposite in direction.
When you jump, you push down on the Earth. What is the Newton's third law reaction to this force?
The reaction force is the Earth pushing up on you with an equal and opposite force, which causes you to accelerate upwards.
What is meant by a 'paired force' in the context of Newton's third law?
A paired force refers to the set of two forces that arise from an interaction between two objects, where each object exerts a force on the other.
How is tension an example of an internal force?
Tension represents the paired forces that segments of a string or cable exert on each other, which are interactions between objects within that system.
State Newton's third law of motion.
Newton's third law describes the interaction of two objects or systems in terms of the paired forces that each exerts on the other, which are equal in magnitude and opposite in direction.