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Tough power/work problem

  1. Feb 27, 2008 #1
    1. I have this tough question on power and work.
    "Calculate the power required of a 1400 kg car under the following circumstances a) the car climbs a 10 degree hill at a constant velocity of 80km/hr. b) the car accelerates along a level road from 90 to 110 km/hr in 6 seconds. Assume in both cases that the retarding force on the car is 700 newtons.(air resistance and friction). Calculate the power in HP. Assume that only 60% of tehcars engine delivers power to the wheels. If the cars engine is rated a 150 HP, does it have enough power to accomplish both tasks?

    2. W=FD P=W/T P=VF KE=.5m(v(squared)

    3. I don't really know how to start. I can't seem to get the force in the first scenario of the hill. Would the force be 700 newtons? as there is a -700 newton force acting on it?
    so 700*cos10*d = work. I don't know what D is though. Or can I use the Ke equation to get the work of the first situation? Would the angle affect that answer in anyway?
  2. jcsd
  3. Feb 27, 2008 #2
    For the first one the car has a mass and is climbing a hill, so pesky gravity is gonna be trying to drag it down the hill(ALSO with that 700 newton force). You can calculate the force of gravity exerted on the car parallel to the slope like I'm sure you've done 5 dozen times in previous problems. You know the car is travelling at a constant velocity, so the NET force must be 0.

    For the second one, you know its kinetic energy changes, so you know Ki and Kf so you can find _____ and you know time so you can find ____
    Last edited: Feb 27, 2008
  4. Feb 28, 2008 #3
    Ok, for the first part, i got Fa= 3131 N. I did Fa-700-mgsin10=0, than solved. I don't know the d however, so I don't know how to solve for work, which is what I need.

    Also, is the 60% engine part extra information I don't need? For the final part do I combine the total work of both situations than divide it by t? I don't know what t is of the first part however.
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