A body strikes the floor vertically with a velocity u and rebounds at the same speed. The change inspeed would be

Options

A. uniform acceleration
B. 3u
C. 2u
D. zero

Answer

To solve the problem, we need to analyze the motion of the body before and after it strikes the floor.

### Step-by-Step Solution:

1. **Identify Initial Conditions**:
- The body strikes the floor with an initial velocity \( u \) (downward).
- We consider downward direction as negative, so we can represent this velocity as \( -u \).

2. **Identify Final Conditions**:
- After rebounding, the body moves upward with the same speed \( u \).
- Since upward is opposite to the downward direction, we represent this final velocity as \( +u \).

3. **Calculate Change in Speed**:
- Speed is defined as the magnitude of velocity, which is always positive.
- The initial speed (magnitude of initial velocity) is \( |u| = u \).
- The final speed (magnitude of final velocity) is \( |u| = u \).
- Change in speed is calculated as:
\[ \text{Change in speed} = \text{Final speed} - \text{Initial speed} = u - u = 0 \]

4. **Conclusion**:
- The change in speed of the body is \( 0 \).

### Summary:
The change in speed when a body strikes the floor with a velocity \( u \) and rebounds at the same speed is \( 0 \).

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