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Trouble understanding g-forces

  1. May 19, 2007 #1
    Im having a bit of trouble understanding g-forces. I get that you divide the forces acting upon an object by 9.8 to obtain the number of g's but does that include forces like force normal? Im also having trouble applying them in situations when acceleration is in different directions such as diagonals. can anyone help explain them for me?

    Edit: I need help with the g-forces experienced from decelerating too
    Last edited: May 19, 2007
  2. jcsd
  3. May 19, 2007 #2


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    g-force is just a term to describe the force required to generate 1 g of acceleration on an object. 1 g of acceleration is defined to be 32.174 feet / sec^2, or 9.80665 meters / sec^2. Note that this takes into account that the Earth is rotating, isn't perfectly spherical, and assumes the object is at sea level (average tide height) and at 45.5 degrees latitude.

    The direction of the force doesn't matter, it's just a way of stating an accelerating force compared to the defined acceleration of gravity. In a Formula 1 race car, the cars generate downforce equal to their weight around 115mph, this could be considered "1 g" of downforce. At around 160mph, the cars can pull "4 g" turns. There's also about "1 g" of drag force at 160mph, so just lifting the throttle produces "1 g" of braking force, and with full application of the brakes, "5 g's" of iniital braking deceleration (while still close to 160mph).

    Take away the Earth's rotation and assume it to be spherical, and g force increases to about 32.224 feet / sec^2 or 9.822 meters / sec^2. This definition of g-force is the base value used for space craft, (reduced by R/(A+R)^2, where R is the average radius of the earth, and A is altitude) since they move indepedently of earth's rotation, and are not significantly affected by the non-spherical shape.
    Last edited: May 19, 2007
  4. May 19, 2007 #3
    A simpler, but less detailed, way to answer this:

    "G's" are just a unit of measurement of acceleration. Just like meters per second squared, or MPH per second, or any other acceleration unit. 1 g = 9.8 m/s/s = 22 MPH/s.

    "G-force" is not a special quantity. It just means "acceleration in units of g's". That's all.
  5. May 19, 2007 #4


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    I'm not sure if this will help, but pilots usually refer to g-force in relation to their bodies. 2 g's 'eyeballs-in', for instance, would mean a straight line acceleration (in seated position) of 2 gravities. 4 g's 'eyeballs-down' would mean entering a hard climb. It's based upon the idea that the eyeballs are somewhat free-floating in the skull and lag behind the rest of the body. (Sort of like how Wile E. Coyote's ears always hang around for a second after he falls off of a cliff.)
    Those are simply refered to as 'negative-gees'. One negative means that you're slowing at a rate of 32ft/sec^2.
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