A man squatting on the ground gets straight up and stand. The force of reaction of ground on the man during the process is.

Options

A. constant and equal to mg in magnitude
B. constant and greater than mg in magnitude
C. variable but always greater than mg
D. at first greater than mg and later becomes eual to mg

Explanation

To solve the problem regarding the force of reaction of the ground on a man as he transitions from a squatting position to standing up, we can break down the steps as follows:

### Step-by-Step Solution:

1. **Understanding the Situation**:
- When the man is squatting, he is in a position where his center of mass is lower, and he needs to exert a force to maintain his balance against gravity and any frictional forces.

2. **Identifying Forces**:
- The forces acting on the man include:
- His weight (W = mg, where m is mass and g is acceleration due to gravity).
- The normal reaction force (R) exerted by the ground on the man.
- Any frictional force (F) that helps him maintain balance while squatting.

3. **Analyzing the Squatting Position**:
- While squatting, the man exerts a force on the ground, and according to Newton's third law, the ground exerts an equal and opposite reaction force on him.
- In this position, the reaction force (R) must counteract both his weight and provide additional force to balance the frictional force. Thus, R > mg when he is squatting.

4. **Transition to Standing**:
- As the man begins to stand up, he straightens his body. During this transition, the frictional force becomes negligible as he is no longer needing to maintain balance in the same way.
- At this point, the reaction force (R) starts to decrease and approaches his weight (mg).

5. **Final Position**:
- When the man is fully standing, the only force acting on him vertically is his weight (mg), and the reaction force (R) from the ground equals his weight (R = mg).

6. **Conclusion**:
- Therefore, during the process of standing up, the reaction force is initially greater than mg and then becomes equal to mg once he is standing.
- The correct answer is that the reaction force is at first greater than mg and later becomes equal to mg.

### Final Answer:
The correct option is: **At first greater than mg and later becomes equal to mg.**

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