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Unformatted text preview: PHYS115 ‐ Spring 2013 1 Test 1 ‐ 4 March 2013 NOTES: This test contains 22 multiple‐choice questions; All questions have equal weight; For every question there is one, and only one, correct answer; Always ignore friction and air resistance; Use g = 9.8 m/s2 unless the question mentions a differ ent value. r V/c A Question 1 – The wheel has 5 spokes. What is the angle between the spokes A and B? B a. 72 rad b. 0.20 rad c. 0.63 rad d. 1.3 rad Question 2 – When was Sir Isaac Newton born? a. around 200 BC b. in 1642 c. in 1861 d. in 1944; last year, he gave a public lecture at UNL Question 3 – The rotating pr airie windmil makes two full turns every 4 ll
seconds. What is its angular speed? a. 3.1 rad/s b. 90 rad/s c. 0.50 rad/s d. 0.79 rad/s PHYS115 ‐ Spring 2013 Test 1 ‐ 4 March 2013 2 Question 4 – Where approximately is the center of mass of a boomerang? a. point A b. point B c. point C d. point D Question 5 – Wile E. Coyote drops a hand grenade from a 50 meter high cliff. (Of course, It never explodes.) What speed will it hit the ground with? a. 5.1 m/s b. 22 m/s c. it depends on the grenade’s mass d. 31 m/s A B C D PHYS115 ‐ Spring 2013 3 Test 1 ‐ 4 March 2013 PHYS115 ‐ Spring 2013 4 Test 1 ‐ 4 March 2013 Question 8 – The builders of the pyramids used a long ramp t o lift 30,000 kg blocks. Traveling at constant velocity, a block rose 2 m in height while traveling 3 0 m along the ramp’s surface. For Questions 6 and 7: A cannon fires three cannon balls in a row. The first one is fired at an angle of 20, the second one is fired at 45, and the third one is fired at 60. The balls are all identical and fired with the same launching speed. How much uphill force was needed to push it up the ramp? a.
d. 590,000 N 3,100 N 20,000 N 60,000 N Question 9 – As you jump across a small stream, does a horiz ontal force keep you moving forward? a.
d. canon Question 6 – Which of the three balls is in the air for the long est time? a. the one lauched at 60 b. the one lauched at 45 c. the one lauched at 20 d. this time is the same for all three balls Question 7 – Which of the three balls hits the ground with th e greatest speed? a. the one lauched at 60 b. the one lauched at 45 c. the one lauched at 20 d. this speed is the same for all three balls no, there is no horizontal force yes, but because of your inertia this force gradually e xpires yes, there is a horizontal force: your weight no, because your horizontal acceleration is constant Question 10 – Plumbers sometimes fit a length of PVC pipe o nto the end of a pipe wrench. Why? a. to increase the torque they can apply b. to increase the friction between their hand and the wrench handle c. to reach high and out of the way plumbing d. to reduce the torque on fragile fittings u
Question 11 – A projectile is thrown directly upward and cau ght again. At the top of its path … a.
d. … its vertical velocity is zero … its horizontal velocity changes … its acceleration changes … it stops accelerating PHYS115 ‐ Spring 2013 5
Test 1 ‐ 4 March 2013 Question 12 – A gymnast jumps upward with an initial speed of 10 m/s. For this question, use g = 10 m/s2. She is in the air for a total time of … a. … 10 s b. … 5 s c. … 2 s d. … 1 s Question 13 – During its explosive demolition, a chimney fall s to the left, as shown. What is the direction of the gravitational torque that acts on it during its fall? a. b. c. d. Question 14 – On the planet Mars, the acceleration due to gr avity is 3.7 m/s2. How far would a rock fall in 4 seconds if you dropped it on Mars? (Ignore the friction o f the thin Martian atmosphere) a.
d. 15 m 30 m must know the rock’s mass 59 m PHYS115 ‐ Spring 2013 6 Test 1 ‐ 4 March 2013 line of initial angular action speed = ? rotational mass I = 4 kg∙m2 P radiu s u
string 1 m Questions 15 and 16 both refer to the ad‐
constant force jacent figure of a spinning disk. The initial 48 N angular speed is unknown. P and Q indicate directions along the dotted arrows. Question 15 – What are the magnitude and direction of the t orque vector? a. 12 N∙m, direction Q b. 12 N∙m, direction P c. 48 N∙m, direction Q d. 96 N∙m, direction Q After 2 seconds, the disk temporarily stops spinning. Question 16 – What were the magnitude and direction of the initial angular velocity? a. 96 rad/s, direction P b. 24 rad/s, direction P c. 48 rad/s, direction Q d. 48 rad/s, direction P Q PHYS115 ‐ Spring 2013 7 Test 1 ‐ 4 March 2013 Question 17 – Why does an archer aim somewhat above her target? Why can’t she simply aim directly at the bull’s‐eye to hit it? a. otherwise she cannot see her target b. the arrow will fall in flight, so she must compensate for its loss of height c. to increase the acceleration of the arrow because leaning back gives a (small) ramp force d. to make the arrow hit the target with more speed Question 18 – One type of home coffee grinder has a small blade that rotates very rapidly and cuts the coffee beans into powder. Nothing prevents the coffee beans from moving. Why don’t the beans get out of the way when the blade begins to push on them? a. gravity holds them in place b. the beans are stuck together c. they have inertia d. the blades aren’t strong enough to make them move Question 19 – Which of a car’s devices can be used to cause a car to accelerate? ( = yes, = no) steering brake gas pedal wheel pedal a. c. d. b. PHYS115 ‐ Spring 2013 8 Test 1 ‐ 4 March 2013 Question 20 – Two ants, Bert and Ernie, B ert
are riding on a record that rotates at 33⅓ rpm. They don’t walk. Ernie is closer to the center than Bert. 33⅓ rpm Who has the greatest angular speed? a. Bert b. Ernie c. it depends on the record’s rotational mass d. their angular speeds are equal ive
Question 20 – You have two cars, and the larger one is five ti mes as massi as the smaller one. You and a friend push on them so that their accelerations are equal. The force on the smaller car is ___________ that on the larg er car. a. equal to b. 1 25 times c. 1 d. 4 5 times 5 times Question 22 – Why is your velocity continuously changing as you ride on a carousel? a. because your direction of motion is continuously cha nging a
b. it doesn’t, unless you are on a carousel animal which is constantly moving up and down c. it doesn’t, because your speed is not changing d. because your speed is continously increasing and dec reasing as your direction changes PHYS115 ‐ Spring 2013 V/c 1d 2b 3a 4c 5d 1 Test 1 ‐ 4 March 2013 angle between two spokes is 360/5 = 72 degrees this is 72 / 360 * 2 pi = 1.3 rad (or just 2*pi/5 = 1.3 rad ) 2 rot/ 4 s = 2 *2 pi / 4 = pi = 3.142 rad/s See below: Imagine you suspend the boomerang; only the third diagram makes sence time from ½ g t2 = height so time = sqrt(2 x height /g ) = 3.194 sec. In this time speed grows to g x time = 31 m/s 6a it has the greatest vertical speed 7d because of the symmetry of the parabolic path: launch speed = landing speed hing
8c work = force x distance = change in potential energy = mass x g x height risen force = mass x g x height risen / distance = 30,000 x 9. 8 x 2 / 30 9a Newton’s first 10a torque = force x lever arm 11a the horizontal velocity remains zero and the accelerat ion is always g (down) 12c Symmetry: her velocity must change from 10 m/s (up) to 10 m/s (down). Change in velocity is 20 m/s (down). Now g = 10 m/s for every second (down). So this takes 2 sec. 13a Right‐hand rule 14b height = ½ x gMars x t2 nd
15c force x lever arm = 48 x 1 = 48 N∙m; leaves no other ch oice than c. Direction Q from right‐han rule 16b Newton’s second for rotation: angular accel. = torque / rot. mass = 48 / 4 = 12 rad/s2 direction Q. Every second adds 12 rad/s in dir. Q, so after 2 sec. ha s added 24 ra d/s dir. Q Intial angular velocity + 24 rad/s (Q) = 0 in. ang. vel oc = 24 rad/s (P) 17b parabolic path 18c 19b If speed, or direction, or both change, the velocity cha nges so there is acceleration 20d They need the same amount of time for a full turn 21c Newton’s second 22a Velocity involves both speed and direction ...
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