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What is G force?
• G-force stands for either the force of gravity on a particular extra terrestrial body or the
force of acceleration anywhere.
• It is measured in g's, where 1 g is equal to the force of gravity at the Earth's surface,
which is 9.8 meters per second per second.
• Moving on with the explanation of G force, the g-force on an object is its acceleration
relative to free-fall. The object experiences this acceleration due to the vector sum of
non-gravitational forces acting per unit of the object's mass. These accelerations, also
known as "proper accelerations,” are not the result of gravity itself.
• Because of the stresses and strains on objects, sufficiently large g-forces may result which
can be highly destructive to objects and organisms.
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• Today the analysis and study of g-forces is significant in a variety of scientific and
engineering fields, such as planetary science, rocket science and astrophysics.
• It is equally important in the engineering fields of various machines such as race cars,
fighter jets, and large engines.
• G-force can vary on different planets or celestial bodies.
• A body having a bigger mass will produce a higher gravitational field, thus resulting in
higher g-forces.
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Examples of G force
• For example, the g-force on the Moon is about 1/6 g, and on Mars it is about
1/3 g. Humans are able to bear localized g-forces in the 100s of g's for a split
second, such as a slap on the face.
• But continued g-forces above about 10 g can lead to permanent injury and are
deadly.
• It has been seen that there is significant disparity among individuals on the
tolerance to g-force.
• For example, Race car drivers have survived instant accelerations of up to 214 g
during accidents. Some rocket sled experiments are designed to examine the
effects of high acceleration on the human body. In 1954 Colonel John Stapp
experienced 46.2 g for several seconds.
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Unit of “G”
• International System of Units (SI) for the g-force, which is m/s2.
• However, for easy comparison the unit g is also often used, which stands for the
acceleration due to gravity at the Earth's surface.
• It is written g, or G.
• Generally, accelerations beyond 100 g are lethal even if momentary.
• It would be technically incorrect to look at the term g-force as force, as it is not
force but a measure of acceleration
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Acceleration and G force
• It is interesting to note here that while acceleration is a vector quantity, g-forces
are often expressed as a scalar.
• G-force can also be expressed as vector acceleration, with the positive g-forces
working towards the bottom of a vehicle and negative forces towards the top.
• G forces, when multiplied by a mass upon which they are acting, are related to a
certain type of mechanical force.
• It is this force which creates compressive stress and tensile stress.
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Example of application of G force
• For example, in the case of is vertically upward G-force, applied by the ground or
the floor of an elevator to a standing person, most of the body will feel
compressive stress.
• This stress at any height, if multiplied by the area, is the related mechanical force,
which is the product of the g force and the supported mass.
• At the same time, the arms will experience a tensile stress, which at any height, if
multiplied by the area, is again the related mechanical force, which is the product
of the g-force and the mass suspended below the point of mechanical support. It is
seen that for a given g-force the stresses are the same, in spite of of whether this g
force is a result of gravity, acceleration, or a combination. Hence, for people it will
feel exactly the same
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Vertical Axis G force
• The tolerance of G forces by human body depends on the magnitude of the g-force,
the length of time for which it is applied, its direction and location of application
and as well as the posture of the body
• The human body, as we know is flexible and supple. To some extent, g-tolerance
can be trainable, but some illnesses, particularly relating to cardiovascular
problems, reduce the g-tolerance.
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Horizontal Axis G force
• It has been seen that the human body is much better at surviving g forces that are vertical
to the spine. So, when the acceleration is forward, the g-force pushes the body backwards,
which is known as "eyeballs in".
• When the acceleration is backwards, the g-force pushes the body forwards, which is
known as "eyeballs out".
• The human body shows much higher tolerance when the acceleration is backwards. The
blood vessels in the retina show as being more sensitive in the latter direction.
• The endurance of g-force is also known to depend on time period and the rate of change in
acceleration.
• This is known as jerks and is expressed as m/s3 in SI units.
• In non-SI units, jerk can be stated simply as gees per second (g/s).
• There are no jerks without push. Very short durations or high jerk forces of 100g have been
claimed.
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Measuring of G force
• An accelerometer is the main tool used for measuring G Force
• An accelerometer is a damped mass on the end of a spring in its simplest form.
• It is capable of measuring the distance the mass has moved on the spring along the axis
or in a particular direction. Accelerometers are often standardized to measure g-force
along one or more axes
• For example, a stationary, single-axis accelerometer is adjusted so that its measuring axis
is horizontal.
• Therefore, its output will show G Force measurement to be 0 g, and will continue to be 0
g if it is placed in a vehicle moving at a constant velocity on a level road.
• But if the automobile driver brakes sharply, the accelerometer will give a reading of
about −0.9 g, which corresponds to a deceleration. However, the jerk due to a change in
motion in the vehicle and gravity pull of the ground on the accelerometer should not be
looked at as the same thing
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• But if the accelerometer is turned around by 90°, so that its axis points upwards, it will
calculate G Force to be +1 g upwards even though the vehicle is still stationary.
• Here, the accelerometer is exposed to two forces: the gravitational force and the ground
reaction force of the surface it is placed on.
• Remember that the accelerometer can measure only the latter force, due to mechanical
interaction between the accelerometer and the ground.
• During a free fall in an airplane the accelerometer does not calculate the earth’s force of
gravity. The reading given by the device is the acceleration it would have if it were solely
subject to that force. The accelerometers are designed to measure only the mechanical
components of accelerations, and thus calculate G Force directly
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• The three-axis accelerometer, for calculating G Force.
• It will give the output zero-g on all three axes if it is dropped. Otherwise it is put into a
ballistic trajectory, which is also known as an inertial trajectory, so that it experiences
"free fall”.
• This is what the astronauts experience while in orbit. Some popular amusement park
rides offer several seconds at near-zero g force. “Vomit Comet” of NASA also offers near-
zero g for about 25 seconds at a time.
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• Measuring G Force in a single-axis accelerometer adjusted in an airplane so that its
measurement axis is vertical, will give a reading of +1 g when the plane is stationery.
• This is the "g-force" applied by the ground. But when the airplane is flying at a constant
altitude, the accelerometer will continue to give the G Force measurement of 1 g. Now,
the g-force is provided by the aerodynamic lift, acting in place of the ground to prevent
the plane from free-falling.
• Under these conditions, the upward force which acts upon the pilot’s body is the normal
value of about 9.8 newton's per kilogram (N/kg). This is provided by his seat, which is
supported by the lift of the wings. If the pilot pulls back on the stick so that the
accelerometer gives a reading of 2 g, the g force calculation acting upwards on him will
become double to 19.6 N/kg.
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Examples of G forces
Some Typical Examples Of G Forces
• In order to understand G forces better, it is a good idea to see them
• manifesting in practical life situations as G Forces examples.
Given below are some typical examples of G Force:
• Standing on the Earth at sea level, where g force = 1 g
• A ride in the Vomit Comet , with g force = 0 g
• Standing at its equator on the Moon, with g force = 0.1654 g
• The gyro rotors in Gravity Probe B and the free-floating proof masses in the TRIAD I
navigation satellite, with g force equal to 0 g
• Saturn V moon rocket just after launch , g force = 1.14 g
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• Space Shuttle, maximum during launch and reentry, g force =3 g
• Luge, maximum expected at the Whistler Sliding Center, g force equal to 5.2 g
• Formula One car, maximum under heavy braking, g forces = 5 g
• Apollo 16 on reentry, g forces = 7.19 g
• Standard, full aerobatics certified glider , g forces = +7/-5 g
• Death or serious injury likely, g force can be greater than 50 g
• Max. turn in an aerobatic plane or fighter jet, g forces ranging from 9–12 g
• Maximum g force for human on a rocket sled = 46.2 g
• Sprint missile, g forces = 100 g
• Brief human exposure survived in crash has g forces > 100 g
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• Bugatti Veyron from 0 to 100 km/h in 2.4 s, has g force = 1.18 g
• High-g roller coasters have g forces ranging from 3.5–6.3 g
• Top Fuel drag racing world record of 4.4 s over 1/4 mile, g force = 4.2 g
• Shock capability of mechanical wrist watches g forces > 5,000 g
• Rating of electronics built into military artillery shells, g forces equal to15,500 g
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Application of G in analysis
• Note: The information in this section applies to all linear, nonlinear, and fluid flow
analyses that support gravity/acceleration loads.
• A gravity or acceleration load applies an acceleration value to any part that has a mass
density defined. The acceleration can be applied along any direction.
• In the case of Linear analyses : Static Stress, Modal with Load Stiffening, and Critical
Buckling and Flow through Porous Media analysis, the acceleration value is constant.
• For Nonlinear analyses : Static Stress with Nonlinear Materials and Fluid Flow analyses,
Steady, Unsteady, and Open Channel Flow, the acceleration is controlled by a load curve
and can be increased gradually and/or otherwise varied over time
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Standard Earth Gravity in ANSYS
• This boundary condition simulates gravitational effects on a body in the form of an
external force.
• Gravity is a specific example of acceleration with an opposite sign convention and a fixed
magnitude.
• Gravity loads cause a body to move in the direction of gravity.
• Acceleration loads cause a body to move in the direction opposite of the acceleration.
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Analysis Types
Standard Earth Gravity is available for the following analysis types:
- Explicit Dynamics
- Rigid Dynamics
- Static Structural
- Transient Structural
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Applying G or Acceleration
- To apply a gravity or acceleration load to a model, right-click the Gravity/Acceleration
heading under the Analysis Type heading in the tree view and select the Edit command.
- Note that the text of this command will be gray before gravity is activated and defined.
- However, it is not grayed-out. That is, the heading is still right-clickable and
the Edit command is available. The heading text becomes black once gravity is set up.
Note: You can also click the Gravity command within the Loads or Fluid Flow Loadspanels
of the ribbon Setup tab. Either method displays the Gravity/Acceleration tab of the
Analysis Parameters dialog box.
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- To apply the acceleration due to gravity on Earth, press the Set for standard gravity
button. The standard value for the acceleration of gravity is applied in the units of the
model.
- To apply a different acceleration magnitude, specify this in the Acceleration due to body
force field. Next, use the X multiplier, Y multiplier, and Z multiplier fields to define the
vector along which the acceleration is applied.
- Specifying a value in only one of these fields applies the acceleration in that direction.
Specifying values in more than one of these fields applies the acceleration along an
arbitrary vector.
- The value in the Acceleration due to body force field is multiplied by the values in these
three fields before it is applied to the model in that direction
