Engineering Mechanics
Which is the correct statement about law of polygon of forces?

If a polygon representing forces acting at a point is closed then forces are in equilibrium
If any number of forces acting at a point can be represented in direction and magnitude by the sides of a polygon, then the forces are in equilibrium
If any number of forces acting at a point can be represented in direction and magnitude by the sides of a polygon taken in order, then the forces are in equilibrium
If any number of forces acting at a point can be represented by the sides of a polygon taken in order, then the forces are in equilibrium

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Engineering Mechanics
The Cartesian equation of trajectory is (where u = Velocity of projection, α = Angle of projection, and x, y = Co-ordinates of any point on the trajectory after t seconds.)

y = x. tanα + (gx²/2u² cos²α)
y = (gx²/2u² cos²α) + x. tanα
y = (gx²/2u² cos²α) - x. tanα
y = x. tanα - (gx²/2u² cos²α)

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Engineering Mechanics
According to Newton's first law of motion,

To every action, there is always an equal and opposite reaction
None of these
The rate of change of momentum is directly proportional to the impressed force, and takes place in the same direction, in which the force acts
Everybody continues in its state of rest or of uniform motion, in a straight line, unless it is acted upon by some external force

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