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Review. A window washer pulls a rubber squeegee down a very tall vertical window. The squeegee has mass 160 g and is mounted on the end of a light rod. The coefficient of kinetic friction between the squeegee and the dry glass is 0.900. The window washer presses it against the window with a force having a horizontal component of 4.00 N. (a) If she pulls the squeegee down the window at constant velocity, what vertical force component must she exert? (b) The window washer increases the downward force component by 25.0%, while all other forces remain the same. Find the squeegee’s acceleration in this situation. (c) The squeegee is moved into a wet portion of the window, where its motion is resisted by a fluid drag force R proportional to its velocity according to R = −20.0v, where R is in newtons and v is in meters per second. Find the terminal velocity that the squeegee approaches, assuming the window washer exerts the same force described in part (b).
(a)
The vertical force with which the washer pulls the squeegee down the window at constant velocity.
Answer to Problem 6.34P
The vertical component of the force that washer must exert when she pulls the squeegee down the window at constant velocity is
Explanation of Solution
The mass of the squeegee is
Write the expression for the net vertical force for the given system is given as,
Here,
Write the expression for the frictional force
Here,
At constant velocity the net vertical force is zero.
Substitute
Rearrange the above expression for
Substitute
Conclusion:
Therefore, the vertical component of the force that washer must exert when she pulls the squeegee down the window at constant velocity is
(b)
The acceleration when downward pulling force increases by
Answer to Problem 6.34P
The acceleration when downward pulling force increases by
Explanation of Solution
The new vertical component of the force exerted by washer against the window is given as,
Write the expression for the new net vertical force for the given system
Substitute
Substitute
Write the formula to calculate the force
Here,
Rearrange the above expression for
Conclusion:
Substitute
Therefore, the acceleration when downward pulling force increases by
(c)
The terminal velocity that the squeegee approaches.
Answer to Problem 6.34P
The terminal velocity that squeegee approaches is
Explanation of Solution
After reaching the terminal velocity there is no acceleration.
Write the new force equation taking into account the drag force
Substitute
Rearrange the above expression for
Substitute
Conclusion:
Therefore, the terminal velocity that squeegee approaches is
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Chapter 6 Solutions
EBK PHYSICS FOR SCIENTISTS AND ENGINEER
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