Final answer:
Performing sit-ups on a slant board is more difficult than on a flat surface because the inclined plane increases the component of gravitational force that acts against the motion, requiring the abdominal muscles to work harder to overcome this resistance.
Step-by-step explanation:
The difficulty of doing sit-ups on a slant board compared to a horizontal surface can be explained through the principles of physics.
When you perform sit-ups on a slant board, your body has to work against the force of gravity to a larger extent than when you are on a flat surface. On a slant board, you are not only lifting your upper body against gravity but also working to maintain your position on the inclined plane, which requires additional effort to overcome both the gravitational force and maintain stability on the slope.
In a more technical sense, the angle of the slant board creates a component of gravitational force that acts along the plane on which your body is situated. This force increases the resistance that your abdominal muscles must overcome during the sit-up.
Therefore, compared to a flat surface where the gravitational force is perpendicular to your body and does not significantly affect the movement of a sit-up, an inclined plane creates a situation where gravity is opposing your motion more directly, making the exercise more challenging.