lecture10

# lecture10 - θ =f s(a=0 µ s N=mgsin θ N=mgcos θ µ s...

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Physics 131 Spring 2008 Dongping Zhong Lecture 10 (Monday, Apr. 14) Force and Motion Page 1 Apparent Weight, Friction

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P2 Apparent Weight w=mg F sp F net F net =F sp -mg=ma w app =Fsp=mg+ma=mg(1+a/g) a>0, w app >mg; a<0, w app <mg + An elevator that has descended from the 50th floor is coming to a halt at the 1st floor. As it does, your apparent weight is 1. More than your true weight. 2. Less than your true weight. 3. Equal to your true weight. 4. Zero. Example:
P3 Friction f= µ N Static/kinetic friction: W=mg N F f If a=0, f s =F= µ s N= µ s mg If a=0 (v 0), f s =F= µ k N= µ k mg

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P4 Example T (with friction) θ Known: constant motion with v, mass m, θ , µ k Find: T=?
P5 Inclined Plane m θ (with friction) W=mg f s F N θ Ex: F=mgsin

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Unformatted text preview: θ =f s (a=0) µ s N=mgsin θ N=mgcos θ µ s mgcos θ =mgsin θ µ s =tan θ P6 Inclined Plane m θ (with friction) W=mg f k F N θ Ex: If a ≠ F-f k =ma mgsin θ-µ k mgcos θ =ma θ µ cos sin g a g k − = P7 Drag force and terminal speed 2 4 v A mg dt dv m ma − = = Initially, v=0, a=g (downward) ⇒ v increases ⇒-Α v 2 /4 important ⇒ finally, -Av 2 /4=mg ⇒ a=0 , 4 1 2 Av F D ≈ A=cross-section area of the object direction opposite the motion P8 ⇒ A mg V T 4 ≈ V T T t t T V g e e V V 2 , 1 1 = + − = α V t → 0, ⇒ V → t → ∞ , ⇒ V → V T...
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lecture10 - θ =f s(a=0 µ s N=mgsin θ N=mgcos θ µ s...

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