Engineering Mechanics
The total momentum of a system of masses (i. e. moving bodies) in any one direction remains constant, unless acted upon by an external force in that direction. This statement is called

Newton's first law of motion
Newton's second law of motion
Principle of conservation of energy
Principle of conservation of momentum

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Engineering Mechanics
Non-coplanar concurrent forces are those forces which

Meet at one point, but their lines of action do not lie on the same plane
Do not meet at one point, but their lines of action lie on the same plane
Meet at one point and their lines of action also lie on the same plane
Do not meet at one point and their lines of action do not lie on the same plane

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Engineering Mechanics
According to the law of moments, if a number of coplanar forces acting on a particle are in equilibrium, then

Their algebraic sum is zero
Their lines of action are at equal distances
The algebraic sum of their moments about any point is equal to the moment of their resultant force about the same point
The algebraic sum of their moments about any point in their plane is zero

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