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Forces Acting on Accelerating Car

  1. Jan 14, 2013 #1
    Good day to all. I'm working on something that requires me to determine the forces acting on an accelerating vehicle. I have been receiving some prior instruction from someone I know and he tells me that quite simply, an accelerating car would experience forces (1) due to the aerodynamic drag, (2) due to gravity, (3) due to rolling friction and finally (4) a force due to the inertia of the vehicle. I understand the first three quite well but the last one has been troubling me all day. This force, as I'm told, is equal to the product of the mass of the vehicle and its acceleration, as explained by the second law of motion. Is it correct that there is in fact an actual non-fictitious force that acts on the accelerating vehicle that opposes its change in velocity, and thus acts on the opposite direction of the motion of the vehicle? Or if you see something's a miss with what I have here to begin with just hit me up.

    I'm trying to develop a simple simulation and I want to get as many of the forces as I can right. Any help would be greatly appreciated. By the way, I'm new here so please let me know if I did anything wrong.
    Last edited: Jan 14, 2013
  2. jcsd
  3. Jan 14, 2013 #2


    Staff: Mentor

    this may help:

    http://en.wikipedia.org/wiki/Reaction_(physics [Broken])

    there's a section on misconceptions when people attempt to apply the for every action there is an equal and opposite reaction.
    Last edited by a moderator: May 6, 2017
  4. Jan 14, 2013 #3


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    Gold Member

    That is the equation F=ma
    To change the velocity of a mass m , one provides a force F to the mass, giving an acceleration a.
    http://www.dynamicscience.com.au/tester/solutions/flight/velocity/force.htm [Broken]

    Of course if you push on your car ( ie motor ), the force to accelerate your car would be Fnet which is the force after subtracting wind resistance and rolling resistance.
    Last edited by a moderator: May 6, 2017
  5. Jan 15, 2013 #4
    I was once in a car accident where I ran into another car in my path. The force due to the accident on the FRONT of my car was severe enough that the BACK, behind the rear wheels, was actually buckled. This is because even though the front has stopped, the back has inertia: it wants to keep going. If you try to accelerate it (or decelerate, in my case) hard enough, its inertia will push back just as hard...to the point of buckling in this case.

    Of course, the inertia of the whole car is also the reason why the entire front end was smashed. The acceleration on the car was so high that it probably required tens of thousands of pounds of force to overcome the inertia. All this force was applied to the bumper, and so of course it and most of what it was attached to has been severely warped, me being a notable exception :approve:
  6. Jan 15, 2013 #5


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    Inertial forces are make-believe. It is sometimes helpful to use them in certain problems, and they stem from Newton'd 2nd law, where F_net = ma. Rearranging that equation , F_net - ma = 0, where here, the term '-ma'...equal and opposite to the net force..., is called the inertial force. I tend to stay away from these imaginary forces most of the time.

    So, anyway, let's look at the real forces acting on the car accelerating from left to right. In the vertical direction, there is gravity acting down and the normal force of the ground on the tires acting up. In the horizontal direction, there is the drag and rolling friction acting leftward, which you have identified. But since the car is moving and accelerating to the right, there must be a rightward force acting on it, and a real one at that. This force is sometimes called a traction force , and is nothing more than the driving force of friction between the road and tires. The friction acts rightward as the engine spins the wheels clockwise causing them to push backwards on the ground..hence the forward direction of the friction force. No friction, no acceleration, as the wheels would spin hopelessly in place, and the car would go nowhere.
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