16lecture2a08 - Force & Power Work done is force...

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1 Physics 2A Lecture 16: Feb 13 Vivek Sharma UCSD Physics Force & Power Work done is force applied over a distance. No mention of time in def. ave 0 Power is time rate at which work is done. When a work W is done over time t define average Power and Instantaneous pow WW PP = l i m er tt t dW dt Δ→ Δ Δ Δ ΔΔ == Δ 36 SI Unit of power: 1W = 1 J/s More useful unit is KW=10 W or MW=10 W 1 hp = 550ft.lb/s= 746W Unit of power can be used to define new unit of Work or Energy Kilowatt-hour (kWh) | ave | ave F Δ s Δ W And finally, P = = = F . Δ V Δ t Δ t P = F.v G G Power ! (Girl) Red Rocks, Nevada Woman, m=50kg, climbs up cliff h=400m in 15 minutes ! What’s her power output in Watts & hp? ave Work Done Δ W P = = Time Taken Δ t 2 5 ave =(50.0kg)(9.8m/s )(400m) Δ W Δ W=1.96×10 J & P = =0.218kW Δ t Since 1hp = 0.746kW Δ W = F.d = (mg)h Girl power=0.29hp G G Conservative Force Work done by a conservative force on an object moving from one point to another depends only only on the initial and final positions and is independent of path taken. e.g: Gravity 2 1 2 1 2 Gg 1 G 2 1 W=F . co s θ Use cos cos () W y y dl mg dl dy dl dl mg mg y mg dy yh φθ −− = ⇒= = =− ∫∫ G G Conservative Force: Alternate Def. A force is a conservative force IF the net work done by the force on the object moving around any closed path is zero Work Done By Conservative Forces Work done by conservative force : 1. Can be expressed as difference between initial and final values of the potential energy functions 2. It is reversible 3. Independent of path of body, depends on start/end point only 4. When start/end point are same, work done is zero
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2 Non-Conservative Force
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16lecture2a08 - Force & Power Work done is force...

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