A man is running with a constant acceleration on a plank which is placed on a horizontal smooth surface. Then choose the correct option(s):

Options

A. Work done by friction on the man is negative
B. Work done by friction on the man is positive
C. Work done by friction on the plank is positive
D. Work done by friction on the plank is negative

Answer

To solve the problem, we need to analyze the situation involving a man running on a plank that is on a smooth horizontal surface. We will evaluate the work done by friction on both the man and the plank.

### Step 1: Understand the scenario
- A man is running with constant acceleration on a plank.
- The plank is on a smooth horizontal surface, meaning there is negligible friction between the plank and the surface beneath it.

### Step 2: Identify the forces acting on the man
- The man experiences a frictional force from the plank as he runs. This frictional force acts in the direction of his acceleration.
- Since he is accelerating forward, the displacement of the man is also in the forward direction.

### Step 3: Determine the work done by friction on the man
- Work done (W) is defined as the product of the force (F) and the displacement (d) in the direction of the force: \( W = F \cdot d \).
- In this case, the frictional force on the man is in the same direction as his displacement.
- Therefore, the work done by friction on the man is positive.

### Step 4: Analyze the forces acting on the plank
- The plank experiences a frictional force from the man that acts in the opposite direction to the plank's displacement.
- As the man accelerates forward, the plank will experience a backward frictional force.

### Step 5: Determine the work done by friction on the plank
- The frictional force acting on the plank is in the opposite direction to its displacement.
- Hence, the work done by friction on the plank is negative, as the force and displacement are in opposite directions.

### Conclusion
- The work done by friction on the man is positive.
- The work done by friction on the plank is negative.

### Final Answer
- Work done by friction on the man is positive.
- Work done by friction on the plank is negative.