Given:
- Time period of the simple pendulum (T) = 4 seconds (when the car is moving uniformly)
- Time period of the simple pendulum (T') = 3.99 seconds (when the accelerator is pressed)

To Find:
- Acceleration of the car (a)

Formula:
- The time period of a simple pendulum is given by the formula: T = 2π√(l/g), where l is the length of the pendulum and g is the acceleration due to gravity.

Assumptions:
- The length of the pendulum remains constant throughout the motion.
- The acceleration due to gravity remains constant.

Explanation:
1. The time period of a simple pendulum depends on the length of the pendulum and the acceleration due to gravity.
2. When the car is moving uniformly on a horizontal road, the acceleration due to gravity is effectively canceled out by the acceleration of the car, resulting in a time period of 4 seconds.
3. When the accelerator is pressed, the car experiences an additional acceleration that affects the time period of the pendulum.
4. By making an approximation, we can assume that the change in the time period is directly proportional to the change in acceleration of the car.
5. Therefore, we can write the equation as: ΔT = kΔa, where ΔT is the change in time period, Δa is the change in acceleration, and k is a constant of proportionality.
6. Rearranging the equation, we get: Δa = ΔT / k.
7. Since the time period changes from 4 seconds to 3.99 seconds, ΔT = 3.99 - 4 = -0.01 seconds.
8. To find the constant of proportionality (k), we can substitute the original time period (T = 4 seconds) into the formula: T = 2π√(l/g).
9. Rearranging the formula, we get: g = (4π²l) / T².
10. Now, we can substitute the values of T and l (length of the pendulum) into the formula and calculate the value of g.
11. Once we have the value of g, we can calculate the value of k using the equation: k = ΔT / Δa.
12. Finally, substituting the values of ΔT and k into the equation Δa = ΔT / k, we can find the acceleration of the car.

Conclusion:
By using the above steps, you can find the acceleration of the car when the accelerator is pressed.