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Homework Help: Δp/Δt = −ΔEP/Δs why a negative?

  1. Oct 12, 2009 #1
    Why is there a minus sign in the formula Δp/Δt = −ΔEP/Δs? Is it because the objects are accelerated toward or away from a higher potential, or is it something else? If anyone could help me, thank you!
  2. jcsd
  3. Oct 13, 2009 #2

    Andrew Mason

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    If energy is conserved, there is no overall change in energy. So, when potential energy is lost, the same amount of kinetic energy is gained and vice versa: ΔKE + ΔPE = 0 or ΔKE = - ΔPE.

    Since the magnitude of the force can be thought of as the rate of change of kinetic energy with distance (ie since ΔKE = work done = |F|Δs, |F| = ΔKE/Δs -- Δs being the change in distance in the direction of the force) and ΔKE = -ΔPE, then |F| = -ΔPE/Δs.

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