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F=MA questions.

  1. Mar 5, 2009 #1
    so if we wanna find the force of a car with a mass of 1500 and going a constant velocity of 14m/s would it just be 0?? but if velocity is constant then A=0 so then force would equal zero too!? how can that be,because if it hits something it obviously exerts a force?? or would you just use the acc of gravity?? Is weight measured in newtons? does weight = force? if a car accelerates 2 m/s^2 and has a mass of 2 kg then its force=4 N right? but what exactly does it mean to have a force of 4N??? does that mean it will take 4N to stop it??

    thanks for the help!
     
  2. jcsd
  3. Mar 6, 2009 #2
    When you say phrases like "its force" and "the force of" I think you're unclear about the difference between force exerted BY an object and force exerted ON an object. F=ma is used to describe the force F exerted ON a mass m, which causes it to undergo an acceleration a.

    A car moving with constant velocity has zero acceleration, and there is no net force acting it, so F=ma is the true statement (0)=(m)(0).

    What a car of mass 1500 kg and a velocity of 14 m/s has is a momentum of (1500)(14) in units of kg m/s.

    If and when the car hits another object, then the car and the other object will, for a short period of time, exert forces on each other.

    The weight of an object is the gravitational force that the earth exerts on the object, and it's measured in Newtons. But gravity has nothing to do with your car example.

    If a thing with mass 2 kg is observed to be undergoing an acceleration of 2 m/s^2 right now, meaning that the rate of change of its velocity at this moment is an increase of 2 m/s every second, then you know that a force of 4 N must be acting on it at this moment.

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    edited typo: momentum units kg m/s, not km m/s
     
    Last edited: Mar 6, 2009
  4. Mar 6, 2009 #3

    russ_watters

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    Staff: Mentor

    If it hits something, it decelerates and thus f=ma applies and you can calculate the force. If it isn't hitting anything, there is no acceleration and no force (as you indicated). I'm not sure why you would see a contradiction there - they are two completely different scenarios.
    Yes, but that's an unrelated issue to what you are asking. Weight is a force, but it isn't the force you're discussing here.
    Yes.
    It will take 4N to stop it in the same amount of time it took for 4N to accelerate it, yes.
     
  5. Mar 6, 2009 #4
    Force is a vector because it always has a direction associated with it. Forces in the horizontal plane are independent of the vertical forces, e.g., gravity. Mass, on the other hand, is a scalar because it has no direction associated with it. So a car undergoing acceleration or deceleration (e.g., collision) due to horizontal forces in the horizontal plane do not increase or decrease the vertical forces. F = ma is a vector equation, so the acceleration and the resultant force are both in the same direction. F and a can each have components in all three orthogonal directions. In your example above, The vertical force is Fv = mgv, where gv is a vertical vector (9.81 m/sec^2). The horizontal force is Fh=mah, where ah is a horzontal vector (2 m/sec^2). The vector sum is 10.01 m/sec^2. If you had a spring scale in the car measuring the weight of a 1 kg mass, you would measure 10.01 Newtons.
     
    Last edited: Mar 6, 2009
  6. Mar 6, 2009 #5
    Why did you include a vertical force? There is no net vertical force when the car is driving down the road(unless I missed something above)

    You would measure 10.01 N assuming that your scale is in the direction of the force and there is no net vertical force like above.
     
  7. Mar 6, 2009 #6
    The only vertical force I included is the omnipresent gravitational force. It holds the car on the road, and the driver in the seat. For a 1 kg mass, the downward vertical gravitational force is 9.81 Newtons.
     
  8. Mar 6, 2009 #7
    But there is an equal and opposite force by the road. If the car isn't moving in the vertical direction then there is no net force which is what f=ma applies to. So the acceleration is just 2 m/s2 in the horizontal direction
     
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