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Unformatted text preview: Physics 101 Classical Physics Spring 2010 Homework 1 Solutions DUE IN CLASS ON JANUARY 21 1. Which of the following does the Laws of Physics not allow us to do: (a) Predict the position of Mars a million years from now (b) calculate the range of projectiles (c) forecast the time of high and low tide (d) Debunk the idea that people can talk to the spirits of dead people (e) derive an equation for the position of a particle forever 2. A pharmaceutical study tests the effects of a cancer drug on 100 patients, finding that 63 are cured by the drug. They claim that the cure rate of the drug is therefore 63 4 . 8%. Which of the following might be a source systematic error on this claim? (a) The number of patients (b) The number of people that are cured (c) The number of people that are not cured (d) The fact that all the patients were male (e) The type of cancer being studied We will also accept (e) since if you study a particular cancer but then say your drug cures all cancer this would certainly be a systematic error. 3. Here we see how dimensional analysis can let us figure out a formula, which is very useful if you cant remember it! A rope has a cross section A = 10m 2 and a density = 2000kg/m 3 . The linear density of the rope, , which is defined as the mass per unit length , can be written as a product of the density and the area, each raised to some power: = x A y . Which of the following relations shows the correct powers x and y for this formula? (a) = A , (b) = A , (c) = A 2 , (d) = A 2 , (e) = A 2 2 [ ] x [ A ] y = [kg/m 3 ] x [m 2 ] y = [kg] x [m] 2 y- 3 x which we want to have units of mass per unit length, so x = 1 and 2 y- 3 x =- 1 y = 1. Therefore = A . 4. An object moves along the x axis. Its position as a function of time is given by x ( t ) = 2 . 0m+7 t- 3 t 2 , where x is in meters and t is in seconds. Its average velocity over the interval from t =0 to t = 4 is: (a) 5.0 m/s, (b) -5.0 m/s , (c) 11 m/s, (d) -11 m/s, (e) -14.5 m/s v = ( x (4 s)- x (0 s)) / (4 s- 0 s) = (- 18 m- 2m) / (4 s- 0 s) =- 5 . 0 m/s. 5. You toss an object upwards. Where in its trajectory is its acceleration equal to zero? (a) At the instant it leaves your hand (b) In middle of its trajectory, either on the way up or on the way down (c) At the very top of its trajectory (d) At the bottom, just before it hits your hand (e) Never Even though the velocity of the object is instantaneously zero at the top of its trajectory, gravity is still acting on the object, so there is nonzero acceleration. 1 6. A neutrino is a particle that can travel more easily through matter than light does through glass. A neutrino created in the center of the Sun travels at nearly the speed of light, 3 10 8 m/s. If it takes 2 seconds for a neutrino to get from the center of the Sun to the surface of the Sun, what is the radius of the Sun measured in miles? Use any conversion factors you might need....
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