In an isolated system with no external impulses, which quantity is conserved?

• A. Total momentum.
• B. Total kinetic energy.
• C. Total angular momentum.
• D. Total momentum is conserved in an isolated system with no external impulses.

Answer: (D)

Conservation of momentum: in an isolated system with no external impulses, the total momentum remains constant. Momentum is the product of mass and velocity for all parts of the system, and the net external impulse is zero, so the total momentum cannot change. Internal forces can shuffle momentum between objects, but their effects cancel out in the sum.

Kinetic energy, on the other hand, need not be conserved—internal interactions can convert kinetic energy into other forms like heat or deformation, even though the total momentum stays fixed. Angular momentum is conserved only if there are no external torques; simply having no external impulses doesn’t automatically guarantee zero external torque in every situation, so you can’t rely on angular momentum being conserved from that condition alone.

So, the quantity that is conserved under these conditions is the total momentum.