Objects falling due to the force of gravity in a free fall increase in speed with time and distance. The exact acceleration rate varies slightly depending exactly where on earth the object is falling. The earth is not a perfect sphere and gravity varies slightly depending on location. By International definition 9.80665 meters per second per second (s^{2}) is the gravitational acceleration constant, 1g = 9.80665 meters per second per second (m/s^{2}) exactly.
Gravitational Constant (g)
9.80665
m/s^{2}
32.17405
ft/s^{2}
21.93685
mph/sec
35.30394
kph /sec
19.06260
knots/sec
Acceleration compared to free fall gravity acceleration g's is not a force. Force is mass multiplied by acceleration. The term "g" is a fixed acceleration baseline for comparison purposes.
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Vehicle Acceleration and Braking
Maximum braking depends on vehicle weight and tire traction, width and diameter. Maximum vehicle acceleration depends on tires and horsepower. Top of the line production muscle cars can go from 0 to 60 mph in 5 seconds. Average acceleration is 60 mph per 5 seconds which equals 12 mph per second and equates to +0.55 g's.
0 to 60 mph in 5 seconds (+0.55 g's)
Time
0
1
2
3
4
5
seconds
Speed
0
12
24
36
48
60
mph
Distance
0
9
35
79
141
220
feet
Adaptive Cruise Control systems, vehicle radar, automatically brake when approaching other vehicles. Maximum braking varies from -3 to -5 meters/sec2 (-7 to -11 mph/sec), or about -0.3 to -0.5 g's. Hard braking occurs around -0.55 g's
Deceleration g's
g's
mph / sec
Braking
0.30
6.6
Safe
0.35
7.7
Safe
0.47
10.3
Average Driver Max
0.62
13.6
Reasonably Skilled Driver Max
0.66
14.3
Skilled Driver Max
0.70
15.4
Vehicle Max
1
21.9
Maximum Braking for select Vehicles
Productions years 1991 - 1995. Braking to a stop
measured on a dry flat surface starting at 60 mph.
Vehicle
g's
ft/s^{2}
mph/s
BMW M3
1.0
32.3
22.0
Toyota Celica GT
0.94
30.2
20.6
Lincoln Continental
0.92
29.6
20.2
Nissan Maxima
0.85
27.3
18.6
Chevy Blazer
0.76
24.5
16.7
Dodge Colt GL
0.72
23.2
15.8
Convert Acceleration Units
g's,
ft/s*s,
m/s*s,
mph/s,
kph/s,
knots/s.
Many safety experts use 15 ft/sec^{2} (0.47 g's) as the maximum deceleration that is safe for the average driver to maintain control, good to excellent tires, dry surface. A reasonably skilled driver can stop at 20 ft/sec^{2} (0.62 g's). Most production street vehicles have a maximum braking around 0.8 g's.
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General Equations
Speed and Distance given Acceleration
v = v_{o} + a t
d = v_{o} t + 0.5 a t^{2}
t = time
v = speed at time t
v_{o} = speed when t = 0
a = acceleration (-a for deceleration)
d = distance traveled at time t
CopRadar.com
Police Radar Information Center
Acceleration Parameters
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