If a particle and a system have zero momentum

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A particle with zero momentum must have zero velocity, leading to zero kinetic energy since mass cannot be zero. However, a system of particles can have zero momentum while individual particles may still possess momentum, such as those vibrating within a lattice. This distinction highlights that a system's overall momentum does not necessarily imply that all components are at rest. Therefore, while a single particle with zero momentum has zero kinetic energy, a system can maintain zero momentum with active particles. Understanding this difference is crucial in analyzing momentum and kinetic energy in physics.
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Is it true that they both must also have zero kinetic energy?
 
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If we are talking of one particle, and it has zero momentum, it must have zero velocity (mass cannot be zero), and so its kinetic energy is zero.
If a system of particles has zero momentum, anyone particle in the system may still have momentum, say ,if it is vibrating in a lattice, but the system as a whole does not.
 

Thread 'Magnitude of buoyant force in fluids of different densities'

The book claims the answer is that all the magnitudes are the same because "the gravitational force on the penguin is the same". I'm having trouble understanding this. I thought the buoyant force was equal to the weight of the fluid displaced. Weight depends on mass which depends on density. Therefore, due to the differing densities the buoyant force will be different in each case? Is this incorrect?
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