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

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Engineering Mechanics
The static friction

Is independent of the area of contact, between the two surfaces
Always acts in a direction, opposite to that in which the body tends to move
Bears a constant ratio to the normal reaction between the two surfaces
All of these

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