| Conceptual Example� |
16� |
The Motion of a Water Skier |
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Figure 4.33 shows a water skier at four different moments:
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(a)�� |
The skier is floating motionless in the water. |
(b)�� |
The skier is being pulled out of the water and up onto the skis. |
(c)�� |
The skier is moving at a constant speed along a straight line. |
(d)�� |
The skier has let go of the tow rope and is slowing down. |
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For each moment, explain whether the net force acting on the skier is zero.
Reasoning and Solution
According to Newton's second law, if an object has zero acceleration, the net force acting on it is zero. In such a case, the object is in equilibrium. In contrast, if the object has an acceleration, the net force acting on it is not zero. Such an object is not in equilibrium. We will consider the acceleration in each of the four phases of the motion to decide whether the net force is zero.
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(a)�� |
The skier is floating motionless in the water, so her velocity and acceleration are both zero. Therefore, the net force acting on her is zero, and she is in equilibrium. |
(b)�� |
As the skier is being pulled up and out of the water, her velocity is increasing. Thus, she is accelerating, and the net force acting on her is not zero. The skier is not in equilibrium. The direction of the net force is shown in Figure 4.33b. |
(c)�� |
The skier is now moving at a constant speed along a straight line (Figure 4.33c), so her velocity is constant. Since her velocity is constant, her acceleration is zero. Thus, the net force acting on her is zero, and she is again in equilibrium, even though she is moving. |
(d)�� |
After the skier lets go of the tow rope, her speed decreases, so she is decelerating. Thus, the net force acting on her is not zero, and she is not in equilibrium. The direction of the net force is shown in Figure 4.33d. |
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