
Index
13. Free Fall 14. Vectors 15. Energy 16. Newton's Laws 17. Mass 17a. Measuring Mass in Orbit 17b. Inertial balance 18. Newton's 2nd Law 18a. The Third Law 18b. Momentum 18c. Work 18d. Work against Electric Forces 19.Motion in a Circle 20. Newton's Gravity 21. Kepler's 3rd Law 21a.Applying 3rd Law 21b. Fly to Mars! (1) 21c. Fly to Mars! (2) 21d. Fly to Mars! (3) 22.Reference Frames 22a.Starlight Aberration 

The MKS System and the "newton"Consider free fall due to gravity. The force of gravity is proportional to mass m, so we can write
where g is the acceleration of gravity, directed downwards. Yes, proportionality allows one to add on the right some constant multiplier, but we won't, because now we want to define some units of F. All quantitative formulas and units in physics depend on the units in which three basic quantities are measureddistance, mass. and time. Let us therefore choose from now on to measure distance in meters, mass in kilograms and time in seconds. That convention is known as the MKS system: as long as one's formulas contain only quantities derived by that system, they will be consistent and correct. But watch out... if by mistake you mix MKS units with grams or centimeters (or pounds and inches), and you may end up with some mighty strange results!
In the MKS system the effective value of g varies from 9.78 m/s^{2} on the equator to 9.83 m/s^{2} at the poles, due to the Earth's rotation (see section #24a). Equation (1) not only shows that weight is proportional to mass, butassuming it is measured in kilogramsit introduces a unit of F, named (no surprise!) the "newton." By that equation, a force of one newton acting on one kilogram of mass accelerates it by 1 m/sec^{2}, so the force of gravity on one kilogram of mass is about 9.8 newtons. Earlier this was called "a force of one kilogram of weight, " a convenient unit for rough applications (1 kg = 9.8 newton), but not for accurate ones, because of the variation of g around the globe.
Newton's Second LawWe now can express in numbers the dependence of acceleration on force and mass. Lord Kelvin, leading British scientist in Queen Victoria's era, was quoted as once saying
By Newton's second law, the acceleration a of an object is proportional to the force F acting on it and inversely proportional to its mass m. Expressing F in newtons we now get afor any acceleration, not just for free fallas
One should note that both a and F have not only magnitudes but also directionsthey both are vector quantities. Denoting vectors (in this section) by bold face lettering, Newton's second law should properly read
This expresses the earlier statement "accelerates in the direction of the force." Many textbooks write but equation (3) is the form in which it is usually usedF and m are the inputs, a is the result. The example below should make it clear.
Example: the V2 RocketThe V2 military rocket, used by Germany in 1945, weighed about 12 tons (12,000 kg) loaded with fuel and 3 tons (3,000 kg) empty. Its rocket engine created a thrust of 240,000 N (newtons). Approximating g as 10m/s^{2}, what was the acceleration of the V2 (1) at launch (2) at burnout, just before it ran out of fuel?Solution Let the upwards direction be positive, the downwards direction negative: using this convention, we can work with numbers rather than vectors. At launch, two forces act on the rocket: a thrust of +240,000 N, and the weight of the loaded rocket, mg = –120,000 N (if the thrust were less than 120,000 N, the rocket would never lift off!). The total upwards force is therefore
and the initial acceleration, by Newton's 2nd law, is
The rocket thus starts rising with the same acceleration as a stone starts falling. As the fuel is used up, the mass m decreases but the force does not, so we expect a to grow larger. At burnout, mg = –30,000 N and we have
giving The fact that acceleration increases as fuel is burned up is particularly important in manned spaceflight, when the "payload" includes living astronauts. The body of an astronaut given an acceleration of 7 g will experience a force up to 8 times its weight (gravity still contributes!), creating excesive stress (34 g is probably the limit without special suits). It is hard to control the thrust of a rocket, but a rocket with several stages can drop the first stage before a gets too big, and continue with a smaller engine. Or else, as with the space shuttle and the original Atlas rocket, some rocket engines are shut off or dropped, while others continue operating. 
Next Stop: (18a) Newton's 3rd Law
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Author and Curator: Dr. David P. Stern
Mail to Dr.Stern: stargaze("at" symbol)phy6.org .
Last updated: 6.6.2004