Unformatted text preview: to the block, we have ΔE = W f + W T Since the speed of the block is constant, change in kinetic energy is zero; the gravitational potential energy does not change either because the block is moving on a horizontal surface. Therefore, the change in mechanical energy is zero. Thus we have 0 = W f + W T ΔE ⇒ th ≡  W f = W T = 45.3 J c) What is the coefficient of kinetic friction between the block and floor? Taking +y axis vertically up, we apply Newton’s 2 nd law along the yaxis to get the normal force. 0 = N + Fsinθ – mg N = mg  Fsinθ = 35.0 N ⇒ Expressing the change in thermal energy in terms of normal force and displacement, we get ΔE th =  W f = +μ k Nd μ ⇒ k = ΔE th /Nd = 0.259...
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 Fall '08
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 Physics, Energy, Force, Potential Energy, Work, Thermal Energy, Tomoyuki Nakayama

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