所算答案有偏差 求解?

Consider an observer inside the car (which is a non-inertia reference system). The pendulum bob, when in equilibrium inside the car, is subject to the following real and inertia forces:

(i) the resolved component of the weight of the bob that is pointing downward parallel to the inclined plane. This equlas to mg.sin(alpha), where m is the mass of the bob, and g is the acceleration due to gravity;

(ii) the resolved component of the weight of the bob that is normal to the inclined plane, which is mg.cos(alpha);

(iii) tension in the suspended string of the bob;

(iv) the inertia force due to the downward acceleration of the car. This equals to mg.sin(alpha) in direction pointing upward paralllel to the inclined plane.

In the above four forces, forces in (i) and (iv) balance one another. The same happens for forces in (ii) and (iii).

When the pendulum bob is set into oscillation from its equilibrium position, its period of oscillation T is given by,

T = 2.(pi).square-root[L/g']
where g' is the apparent acceleration due to gravity under the situation where the bob is placed. In this case, g' = g.cos(alpha)

Hence, T = 2.(pi).square-root[L/(g.cos(alpha))]