Quiz3A_Solutions

Quiz3A_Solutions - determine the speed of the skier at the...

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Quiz 3A DL SEC _ _ Grade: 04/26/10 Last 6 digits of student ID _ _ _ _ _ _ Last name, first name: ________________ ________________ First three letters of your last name _ _ _ Useful formulae: Δ E TOT = Q + W; Δ E th = mc Δ T; Δ E bond = ±| Δ m Δ H|; Δ PE grav = mg Δ h; Δ KE tr = ½ m Δ v 2 ; Δ PE ms = ½ k Δ x 2 Useful constants: g=9.8 m/s 2 1. A skier skis down a mountain slope as shown below, beginning with a certain speed v i . Suppose you can entirely neglect the effects of friction and air resistance. a) Can you change the speed at the bottom by starting from a higher spot (without changing m or θ )? If so, how? If not, why not? Explain. You will be graded on the completeness and the quality of your explanation. Yes, you can. Δ PE + Δ KE = 0 mg Δ h + 1/2m(v f 2 – v i 2 ) = 0 , but v i remains unchanged. So, the bigger Δ h, the bigger v f . b) If the 70 kg skier is then initially at rest, and begins skiing down the slope from an initial height of 200 m,
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Unformatted text preview: determine the speed of the skier at the bottom of the slope. g Δ h + 1/2(v f 2 – v i 2 ) = 0 v f = sqrt[-2gDh] = sqrt[(2)(10)(200)] = sqrt[4000] ≈ 63 m/s h θ m 2. The potential energy graph shown here is similar in shape to the 2-d Higgs potential in particle physics. The horizontal axis here is given in meters and the vertical axis in Joules. a) At which point ( B or C ) would the force be greatest? Clearly explain your reasoning. Point C would have a greater force than point B. Force is related to slope, and the slope at B=0 whereas the slope at C is >0. [Note that it doesn’t matter which direction you think the force is (pos or neg slopes), just that the magnitude is bigger.] b) In which direction is the force at point A ? Explain . A C B The force at point A is to the right. Force is always in the direction to locally decrease the PE....
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