解答第19题---五、计算题 (本大题共2小题, 6+7=13分)
19. 在中考体育力量训练中, 俯卧撑可简化为杠杆模型。如题 19 图甲所示为某同学做俯卧撑时的简化示意图, 该同学质量为 50 kg, 其重心位于 A 点, 如图乙所示。当他每次撑起身体时, 重心竖直上升 0.3m。(取 g=10N/kg) 求:
(1) 该同学所受重力 G 的大小;
(2) 该同学每次将身体撑起克服自身重力所做的功;
(3) 如题图甲和乙所示, 该同学保持静止状态, 此时地面对双手的支持力 F 的大小。
Chart/Diagram Description:
**Diagram 甲 (Figure A):**
* Type: Lever model diagram.
* Elements:
* A horizontal line representing a lever arm, pivoted at point O.
* Point O is the pivot. It is represented by a triangle symbol, indicating a support or fulcrum.
* Point A is on the lever arm to the left of O.
* A force vector labeled "G" is shown acting downwards at point A. This represents the gravitational force (weight). A dashed vertical line goes from A downwards.
* A force vector labeled "F" is shown acting upwards at the leftmost point of the lever arm. A dashed vertical line goes upwards from this point.
* Distances are indicated along the horizontal dashed line below the lever arm: 0.6m from the leftmost point (where F acts) to the vertical line below A, and 0.9m from the vertical line below A to the vertical line below O. This implies the distance from the leftmost point to A is 0.6m and the distance from A to O is 0.9m.
**Diagram 乙 (Figure B):**
* Type: Schematic diagram of a person doing a push-up.
* Elements:
* A person is shown in the push-up position, supported by their hands and feet.
* The hands are on the left, and the feet are on the right.
* Point A is labeled above the person's back, indicating the position of the center of gravity. A dashed vertical line goes from A downwards.
* Distances are indicated below the person: 0.6m from the hands to the vertical line below A, and 0.9m from the vertical line below A to the feet. This corresponds to the distances shown in Figure A.
**Overall:** Figure 甲 is a lever model representing the push-up position shown in Figure 乙. Point O in Figure 甲 represents the pivot point at the feet in Figure 乙. The force F in Figure 甲 represents the support force from the hands in Figure 乙. Point A represents the center of gravity, and G represents the weight acting at the center of gravity. The distances shown are the distances along the horizontal direction.