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Unformatted text preview: buoyant force goes down at some point the buoyant force gets too small to suppor t the weight of the balloon, h
and the shrinking air volume increases the mass density of the balloon 11. An electric saw uses a circular spinning blade to slice thro ugh wood. When you start the saw, the motor needs 5 seconds o f con‐
stant angular acceleration to bring the blade to its full angula r blade
velocity. If you change the blade so that the rotating portion of the saw now has 2 times its original rotational mass, how long wi ll the motor need to bring the blade to its full angular velocity? a.
d. 1¼ s 2½ s 5 s 10 s A: Torque of saw is = (rot. mass) x (ang. accel.) = (rot. mass) x (change in ang. speed) / (time needed) = (rot. mass) x (final ang. speed) / (time need ed) So if the rotational mass is 2x bigger, the time will also b e 2x bigger, so 2 x 5 = 10 s , t I , but and are fixed, so t I In symbols: I I t 12. As you swing back and forth on a playground swing, your apparent weight changes. At what point do you feel the heaviest? a.
d. at the top of your swing going either forward or back ward at the top of your swing going backward in the middle of your swing going forward, but not ye t at the highest point somewhere else A: The centripetal force = mass x speed2 / radius = m ass x speed2 / (length of swing chain). This is maximum at the lowest point, because that’s whe re your speed is greatest. So your ‘feeling of acceleration’ is greatest there, and it points do wn. Add your weight, and you see you feel heaviest there. So the answer is not (a), (b), or (c), an d we’re left with answer (d). PHYS115 – Spring 2012 6 Final Test - 1 May 2012 Problems 13 – 15: Water is supplied by a pump to a tank where it is stored at a pressure of 400,000 pascals (4 atmospheres). The tank has an opening at the top (the fountainhead), from which a fountain shoots up. 13. The water leaves the fountainhead into a surrounding pressure of … a. 0 Pa b. 25,000 Pa c. 100,000 Pa d. 40...
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