A rider on horse back falls when horse starts running all of a sudden because

Options

A. Rider is taken back
B. Rider is suddenly afraid of falling
C. Inertia of rest keeps the upper part of body at rest whereas lower part of the body moves forward with the horse
D. None of the above

Answer

To understand why a rider on horseback falls backward when the horse suddenly starts running, we can analyze the situation using Newton's laws of motion, particularly the concept of inertia.

### Step-by-Step Solution:

1. **Understanding Inertia**:
- Inertia is the tendency of an object to resist changes in its state of motion. An object at rest will remain at rest unless acted upon by a net external force.

2. **Initial State of the Rider**:
- When the horse is stationary, both the horse and the rider are at rest. The rider's body is in a state of rest due to inertia.

3. **Horse Starts Running**:
- When the horse suddenly starts running, it accelerates forward. The lower part of the rider's body (the legs) is in contact with the horse and begins to move forward with the horse.

4. **Upper Body's Inertia**:
- The upper part of the rider's body (the torso and head) has inertia and tends to remain at rest. Since it is not immediately pushed forward with the horse, it appears to fall backward relative to the horse.

5. **Resulting Motion**:
- As the horse accelerates forward, the rider's lower body moves with the horse, but the upper body lags behind due to inertia. This difference in motion causes the rider to fall backward.

6. **Conclusion**:
- The rider falls backward because the upper part of the body remains at rest while the lower part moves forward with the horse. This phenomenon is a clear demonstration of inertia.

### Final Answer:
A rider on horseback falls backward when the horse starts running suddenly because of the inertia of rest. The upper part of the rider's body remains at rest while the lower part moves forward with the horse, causing the rider to fall backward.