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Unformatted text preview: ection of the support force) the force of friction is zero because the book stays with the car Answer: Book rides with car: stays where it is on the roof. So only static friction is possible. Book accelerates together with car, so in forward direction. This requires a forward force to act on the book. So there is forward, static friction. Question 17 – A girl slides down a slide. What kind of forces act on her? ( = yes, = no) gravitational force support force sliding frictional force a. b. c. d. ANSWER: Weight is there: the gravitational force of the Earth pulls on her…; this excludes answers (a) and (b). There must also be a support force, as she doesn’t fall through the slide. This excludes answer (d), and this leaves us with answer (c). There must be sliding friction, because the girl is in contact with the slide, and, well, it’s a SLIDE, so there’s SLIDING friction! PHYS115  Fall 2012 9/8 Test 2  November 28, 2012 Question 18 – Your 5‐kg rifle fires a 6‐gram bullet forward with muzzle velocity 900 m/sec. The rifle recoils in your arms backward at 900
a. 36 m/s 5 5 (0.006) 900 1.1 m/s b.
5 1
5 154 m/s c.
2
(0.006) 900 d. ( 0.006) 900
5 0.080 m/s ANSWER: Momentum before shot (zero) equals momentum after shot, bc. mometum is conserved. So total mom. after shot is zero. Momentum after shot is (rifle’s momentum, pointing to the shooter), plus (bullet’s momentum, poiting away from shooter) We have rifle’s momentum is (rifle’s mass) x (its speed pointing to the shooter) = 5 x (unknown speed) bullet’s momentum is (bullet mass) x (its speed, pointing away from shooter) = 6E–3 x 900 We get 5 x (unknown speed) = 6E–3 x 900 and so (unknown speed) = 6E–3 x 900 / 5, answer (b). PHYS115  Fall 2012 10 / 8 Test 2  November 28, 2012 Question 19 – A toploading washer is in its spinning cyc Its tub cle.
spins about a vertical axis and you look at the tub from a bove. It spins counterclockwise, as indicated. How (dotted arrows) does the water from the wet clothes leave the tub? (a) (b) (c)
(d) ANSWER: Once the water has left the spinning tub, there are no ho rizontal forc acting anymore. ces
Therefore, it must fly in a straight line, which excludes a nswers (a) and (b). The moment the force becomes zero, it continues with the vel...
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 Spring '13
 Cornelis(“Kees”)J.Uiterwaal
 Physics, Force

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