When a body is in dynamic equilibrium, then work done is

Options

A. positive
B. negative
C. zero
D. infinity

Answer

To solve the question "When a body is in dynamic equilibrium, then work done is:", we can follow these steps:

### Step 1: Understand Dynamic Equilibrium
Dynamic equilibrium occurs when a body is moving at a constant velocity. This means that while the body is in motion, the net external force acting on it is zero.

**Hint:** Remember that dynamic equilibrium implies constant velocity, which means no change in speed or direction.

### Step 2: Analyze Forces Acting on the Body
In dynamic equilibrium, the forces acting on the body must balance each other out. Therefore, the sum of all forces (net force) acting on the body is zero.

**Hint:** Think about Newton's first law of motion, which states that an object in motion will remain in motion with the same speed and in the same direction unless acted upon by a net external force.

### Step 3: Relate Work Done to Forces
The work done (W) on an object can be calculated using the formula:
\[ W = F \cdot d \cdot \cos(\theta) \]
where:
- \( F \) is the net force acting on the body,
- \( d \) is the displacement,
- \( \theta \) is the angle between the force and the direction of displacement.

**Hint:** Consider what happens to the work done when the net force \( F \) is zero.

### Step 4: Apply the Concept of Work Done in Dynamic Equilibrium
Since the net force \( F \) is zero in dynamic equilibrium, substituting this into the work done formula gives:
\[ W = 0 \cdot d \cdot \cos(\theta) = 0 \]

**Hint:** Even if the body is moving (displacement is occurring), if there is no net force acting on it, the work done is still zero.

### Step 5: Conclusion
Thus, when a body is in dynamic equilibrium, the work done on the body is zero.

**Final Answer:** The work done is zero when a body is in dynamic equilibrium.