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Old Oct 26th 2009, 03:04 PM   #1
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Newton's 2nd Law help.

5. Consider the situation where a person that has a mass of 68 kg is descending in an elevator at a constant velocity of 4.0 m/s. After 3 s, the elevator starts to slow to a stop at a rate of 2.0 m/s2.

a. Using Newton’s 2nd Law, find the net force acting on the person in the elevator when it is descending at constant speed.













b. Using Newton’s 2nd Law, find the net force acting on the person in the elevator when it is accelerating.













c. Construct quantitative force diagrams (include magnitudes) for the person in the elevator as it descends at constant speed and during its period of acceleration.














d. If the person in the elevator were standing on a bathroom scale calibrated in Newtons, what would the scale read while the elevator was descending at constant speed and while slowing to a stop? Please explain your answers.


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Old Oct 26th 2009, 03:16 PM   #2
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sorry and a. my first attempt was to use the f=ma formula but would the 2.0 m/s^2 be the acceleration when it is descending at a constant speed?

b. once again i am in the same dilemna and i cant figure out if you would use the f=ma and once again what would be the a?

c. what would the magnitudes on the two end because i can easily draw my force diagrams but i tempted to multiply 68 kg by 9.8 ...

d. i honestly have no idea how to do this ...
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Old Oct 26th 2009, 05:07 PM   #3
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For part a) draw a free body diagram and find each force acting on the person. The net force will be the sum of them.
By the way note that descending at a constant speed means no change in acceleration.
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