(adsbygoogle = window.adsbygoogle || []).push({}); 1. The problem statement, all variables and given/known data

In each of the following, an order of magnitude estimate is required. Estimate

i) the gravitational energy of a 100kg satellite in low earth orbit, the velocity of the satellite, and the period of the orbit.

ii) the kinetic energy of a car travelling at 70mph and the power output of

a car engine.

2. Relevant equations

3. The attempt at a solution

Solution:

i) U = GMm/r^{2}, where r = radius of (low earth) orbit radius of earth = 6400 km and mass of earth = 6*10^{24}kg.

Plugging numbers gives U = 6.3*10^{9}J.

Such a big number looks a bit odd. Is this a realistic number? (or am I worng somewhere?)

v = sqrt(GM/r), so v = 750 km/s.

Assuming a circular orbit, v = 2*pi*r/T, so the time period T = 52s. !!!!!!!!!

I think this means my velocity is wrong, which U is wrong, but where IS the mistake?

ii) 1 mile = 1.6 km.

mass of (typical) train = 2000 kg.

So, K = 0.5mv^{2}= 3.1*10^{4}J.

I don't know how to work out the power.

Any help I would greatly appreciate.

**Physics Forums | Science Articles, Homework Help, Discussion**

Dismiss Notice

Join Physics Forums Today!

The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

# Homework Help: Classical Mech. prob.

**Physics Forums | Science Articles, Homework Help, Discussion**