WOE - Power

Power

Instantaneous power is the rate at which energy is transferred. The average power is the total work done divided by the time it took. For something to be powerful, it does work at a high rate. Power can also be calculated as force times the velocity because displacement divided by time is velocity, and work is force times displacement. The unit for power is a joule per second which is called a Watt. The common unit for power in the US is a horsepower, which is 746 watts. Please watch the following presentation that will discuss Power.

Power Practice

Now we will do some practice problems apply the definition of power to different situations. Please watch the following presentation of some example problems.

Power Self-Assessment

Now it is your turn. Complete the following Power problems to verify that you understand how to apply the power equations to different situations. These will also give you additional practice with the concepts of conservation of energy.

  1. A 75kg man gets on a 500 kg elevator. The elevator accelerates from rest for 4.7. seconds until it reaches its maximum speed of 2.2 m/s.
    1. What is the average power output of the electric motor moving the elevator as it accelerates? 
    2. What is the instantaneous power output of the motor when the elevator is moving at maximum speed? 

SOLUTIONS Links to an external site.

  1. A new sports car with a mass of 2000kg experiences a constant acceleration from 0 to 27 m/s 5.1 s.  
    1. Write the equation for the instantaneous power as a function of time, assume that here is no measurable friction acting on the car. SOLUTION
    2. What is the power output of the car at
      1. t=2.55s 
      2.  t=5.1s 

SOLUTIONS Links to an external site.

  1. A box of mass M is on a frictionless slope that makes an angle of q with the horizontal. A constant force F is applied to the box causing it to move up the slope a distance x.
    1. How much work is done by the force F as it moves the box a distance x? 
    2. What is the net work done on the box in the distance x? 
    3. Find the speed of the box as a function of x. 
    4. Find the power delivered to the box by the force F as a function of x. 

SOLUTIONS Links to an external site.

Power Practice Problems

Download Power Practice Problems Links to an external site. for more practice. 

Download solutions Links to an external site. to the practice problems. 

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