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Unformatted text preview: eter block therefore moves up. You could also argue that the block in the accelerometer must also get a downward acceleration, to stay with the elevator car, and this happens when the top spring in the accelerometer is com pressed (so it pushes the block down) and the bottom spring streched (so it pulls the block dow n). 17. In which direction is the driver’s apparent weight? (a) The acceleration to the rear gives a ‘feeling of acceleration’ pointing forward. Together with the driver’s downward weight, the apparent weight is (b). Just in case you wonder, this acceleration to (b)
the rear could occur when the car breaks or (c)
when it starts to back up from rest, for instance. 19. The buoyant force on an object depends on its volume (n ot on its mass). Since the decoration has double the volume of the rock, the buoyant force act ing on it is also twice that on the rock. 20. your
o a. 14 s b. 3.4 s c. 200 s d. 0.29 s force = mass x acceleration = mass x (change in spee d) / time so then time = mass x (change in speed) / force = 81 2 x 4.06 / 2 = 14 s 236 PHYS115 – Spring 2012 9 Final Test - 1 May 2012 21. What is the unit of spring constant k (in SI units)? a. N b. N∙m c. N/m d. 1/m 22. You draw (pull back) an ordinary bow and hold it steady a you aim. Holding it drawn at 24 inches as
requires a force of 40 lbf. Holding it in place when drawn at ju st 12 inches would take s a. 1/4th of that: 10 lbf b. half of that: 20 lbf c. the same force: 40 lbf d. twice that force: 80 lbf force = (spring constant) x extension so the force goes up/down with the extension going up/d
down half the extension (12 instead of 24 inches) gives half the force 23. The (downward) speed of the branch continuously incre ases with time as in speed = (acceleration of gravity) x time = g x time At time zero when it just broke off the tree its speed is ze ro. One second later its speed is g x time = 9.81 x 1 = 9.8 1 m/s Two seconds (right before it hits the ground) its speed is g x time = 9.81 x 2 = 19.62 m/s S...
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