Which of the following describes an object in static equilibrium? a. a platypus lies motionless on the sand. b. a tuna speeds up from 2 m/s to 3 m/s c. a dolphin jumps out of the water d. a zebra shark swims at a constant speed of 8 m/s

A).  A platypus lies motionless on the sand.

Explanation:

As we know that equilibrium is defined as the system in the state of ZERO net force or zero acceleration condition.

So equilibrium is of two types

1) Static equilibrium : Here object will remain at rest and do not have any type of motion.

2) Dynamic equilibrium : Here object may move with constant speed in same direction so it will not have any acceleration.

So here

B.A tuna speeds up from 2 m/s to 3 m/s

C.A dolphin jumps out of the water

Not in equilibrium as speed is changing so acceleration is not zero

D.A zebra shark swims at a constant speed of 8 m/s

It is in dynamic equilibrium case where speed is constant

so correct answer will be

A.A platypus lies motionless on the sand.

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