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Homework Help: Newton's Second Law: Car and Rock Problem

  1. Mar 4, 2017 #1
    • Member advised that each homework problem requires a separate thread.
    Hi Guys, I solved two questions below. Just need some feedback to make sure I solved them correctly. Thanks a bunch.

    1. The problem statement, all variables and given/known data

    1. A car of mass 1.4 tonnes (1400 kg) traveling down the highway at 120 km/h has a F air of 1800 N acting against it. What force must be applied by the engine if the car's speed is to increase to 130 km/h in 5.5 s?
    2. A 786 kg rock is being towed on a frictionless surface by a 45 N force acting north. A second force acts at 32 N west. What is the acceleration of the rock? (Note that the forces are not acting directly opposite to one another.)

    2. Relevant equations

    Fnet = m.a

    3. The attempt at a solution


  2. jcsd
  3. Mar 4, 2017 #2


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    In the first problem, you forgot about the air resistance.
    In the second problem, you got the magnitude of acceleration, but the acceleration is a vector, you need to give the direction, too.
  4. Mar 4, 2017 #3
    If I take air resistance into consideration, wouldnt this give me Fnet? Whereas we are only interested in Fengine?

    For the second question would the direction be NW or WN?
  5. Mar 5, 2017 #4


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    The acceleration is determined by Fnet =ma. Fnet = F(engine)+F(air) and the direction of F(air) opposes the force of the engine.

    You have to calculate the angle of the acceleration with respect to North, and it is not 45°.
  6. Mar 5, 2017 #5
    I get it. Thank you!

    So basically, F(net) = F(eng)+F(air)
    707N = F(eng) - 1800N
    707N + 1800N = F(eng)
    F(eng) = 2507N

    And for the second question I found the angle to be 54 NW
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