Power Practice

Sample questions

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1. Sample 1difficulty 1/5

   Two motors do the same total work but motor B takes half the time of motor A. The ratio $P_B/P_A$ is

   • A

     $1/2$

   • B

     $1$

   • C

     $2$

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   • D

     $4$

   Why

   $P = W/t$. Same work, half time → twice the power.

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2. Sample 2difficulty 1/5

   An engine performs $6000~\text{J}$ of work in $30~\text{s}$. What is its average power output?

   • A

     $1800~\text{W}$

   • B

     $50~\text{W}$

   • C

     $200~\text{W}$

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   • D

     $500~\text{W}$

   Why

   $P = W/t = 6000/30 = 200~\text{W}$.

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3. Sample 3difficulty 2/5

   Engine A delivers $200~\text{W}$ for $10~\text{s}$. Engine B delivers $400~\text{W}$ for $10~\text{s}$. Engine B does work

   • A

     Four times A

   • B

     Twice A

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   • C

     Same as A

   • D

     Half of A

   Why

   $W = P t$. Same time, double power → double work.

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4. Sample 4difficulty 2/5

   A motor lifts $50~\text{kg}$ at constant $0.40~\text{m/s}$. Using $g \approx 10~\text{m/s}^2$, the motor's mechanical power output is

   • A

     $2000~\text{W}$

   • B

     $20~\text{W}$

   • C

     $500~\text{W}$

   • D

     $200~\text{W}$

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   Why

   $P = F v = m g v = (50)(10)(0.40) = 200~\text{W}$.

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5. Sample 5difficulty 2/5

   A locomotive applies $40{,}000~\text{N}$ tractive force at constant $25~\text{m/s}$. Its power output is

   • A

     $1600~\text{W}$

   • B

     $10^6~\text{W}$

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   • C

     $10^9~\text{W}$

   • D

     $10^5~\text{W}$

   Why

   $P = F v = (40{,}000)(25) = 1.0 \times 10^6~\text{W}$.

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