Answer to Question #105124 in Classical Mechanics for Gideon

Question #105124
1. In most calculations, we use W ≈ mg. But we know that, for large changes in altitude, we need to use W∝1/r^2. How far above the Earth's surface can we use W = mg before our systematic error reaches 1%? Use only the information given in this question, and the radius of the Earth re=6400 km. Do not explicitly use G or the mass of the Earth, and do the calculation for the pole so that we don't worry about the effect of centripetal acceleration.

Hint: Does g become larger or smaller with altitude?

Altitude = ___ km (to one significant figure)
1
Expert's answer
2020-03-16T13:06:02-0400

The law of universal gravitation and the force of gravity at earth's surface and at some height are equal:


(1)    GMmr2=mg, (2)    GMm(r+h)2=mgh.(1)\space\space \space \space G\frac{Mm}{r^2}=mg,\\ \space\\ (2)\space\space \space \space G\frac{Mm}{(r+h)^2}=mg_h.

Divide the second by the first:


ghg=r2(r+h)2.\frac{g_h}{g}=\frac{r^2}{(r+h)^2}.

The systematic error between the two values is


ϵ=(1ghg)100%= =(1r2(r+h)2)100%=h2+2rh(r+h)2100%.\epsilon=\bigg(1-\frac{g_h}{g}\bigg)\cdot100\%=\\ \space\\ =\bigg(1-\frac{r^2}{(r+h)^2}\bigg)\cdot100\%=\frac{h^2+2rh}{(r+h)^2}\cdot100\%.

Since the systematic error must be 1%, the last equation gives us the following value of height:

1%=h2+26400h(6400+h)2100%, h=32 km.1\%=\frac{h^2+2\cdot6400h}{(6400+h)^2}\cdot100\%,\\ \space\\ h=32\text{ km}.


Need a fast expert's response?

Submit order

and get a quick answer at the best price

for any assignment or question with DETAILED EXPLANATIONS!

Comments

No comments. Be the first!

Leave a comment