Which free body diagram is in equilibrium? a b both a and b. neither a nor b.

The block in Diagram-A is in equilibrium.  The horizontal forces acting on it add up to zero, and the vertical forces acting on it also add up to zero.  Since the block is in equilibrium, it will move in a straight line with a constant speed.  It won't speed up, slow down, or move in a curve, until one of the forces on it now changes size or direction, or some new force shows up.

The block in Diagram-B is NOT in equilibrium.  It has a net force acting on it ...  5 Newtons to the left.  That block will accelerate to the left.

Note:  If you want to come right down to it, Diagram-B is actually absurd. The horizontal force pointing to the left is labeled "Force of friction", which is silly.  Friction is not actually a real force, even though it may behave like one in some situations.  For example, friction can never ADD mechanical energy to an object, or INCREASE the object's velocity.  In Diagram-B, there could never be a force of 5N due to friction acting on the block and pointing to the left, without something else going on.  It's only when you try to pull the block to the right with REAL force, that a left-pointing force ... due to friction ... SEEMS to show up.  That's why friction is called a "pseudo force".

explanation: in any substance/object, the particles inside it (atoms/molecules) constantly move in random directions and with random speeds (this motion is called brownian motion). as a result, the particles have some kinetic energy (which is proportional to the temperature of the substance).

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insects!

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