AP Physics C: Mechanics Flashcards: Conservation of Linear Momentum
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.
Under what condition is the total momentum of a system constant?
The total momentum of a system is constant when the net external force on the selected system is zero.
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Under what condition is the total momentum of a system constant?
The total momentum of a system is constant when the net external force on the selected system is zero.
What is an 'isolated system' in the context of linear momentum?
An isolated system is a collection of objects on which the net external force is zero, meaning its total linear momentum is conserved.
What is the equation for the velocity of a system's center of mass?
The velocity of a system's center of mass is calculated using the equation $\vec{v}_{cm}=\frac{\sum\vec{p}_{i}}{\sum m_{i}}$.
If the total momentum of a system is conserved, what must be true about the velocity of its center of mass?
If the total momentum of the system is constant, the velocity of the system's center of mass must also be constant.
Why is the selection of a system crucial for determining if momentum changes?
The selection of a system determines which forces are considered external versus internal; only a zero net external force guarantees the conservation of the system's momentum.
In a system with no net external forces, how are internal momentum changes balanced?
Any change to the momentum of an object within the system must be balanced by an equivalent and opposite change of momentum elsewhere within the system.
A cannon fires a cannonball. If the system is defined as *only* the cannonball, is its momentum conserved after firing? Why?
No, its momentum is not conserved because the cannon exerts a net external force on the cannonball, causing its momentum to change.
A cannon fires a cannonball. If the system is defined as the cannon and cannonball *together*, is the system's total momentum conserved (ignoring friction)?
Yes, the total momentum of the system is conserved because the forces between the cannon and cannonball are internal, and the net external force on the system is zero.
What is the direct consequence of a non-zero net external force on a system's momentum?
A non-zero net external force will cause the total momentum of the system to change; it will not be conserved.
What is the principle of conservation of linear momentum?
If the net external force on a selected system is zero, the total momentum of the system is constant.