[单选题]
A ball is released from rest at a height h above the ground. At each bounce At each bounce $75^{\circ}$ of its kinetic energy is lost. Which graph represents the variation of the ball's velocity v with time t from the beginning of motion until the moment just before the third bounce? Assume the acceleration of the ball is in in the negative direction and that air resistance is negligible.
A.
B.
C.
D.
参考答案: B
本题详细解析: Expianation:
Kinetic energy $E_k$ is directly proportional to the square of the magnitude of velocity$v$.
$E_k$=$\frac{1}{2}mv^2$
The velocity will decrease to half of its value at each bounce.
The value of the acceleration is constant, free fall acceleration $g$ . The velocity time graph should be linear and all sections should have a negative gradient to represent downwards acceleration.
The direction of velocity is positive on the way up, and negative on the way down.
