Posted by **Jessy** on Monday, January 15, 2007 at 11:29am.

A projectile is shot directly away from Earth's surface. Neglect the rotation of Earth.

(a) As a multiple of Earth's radius RE, what is the radial distance a projectile reaches if its initial speed is one-fifth of the escape speed from Earth?

____ times RE

(b) As a multiple of Earth's radius RE, what is the radial distance a projectile reaches if its initial kinetic energy is one-fourth of the kinetic energy required to escape Earth?

_____ timesRE

(c) What is the least initial mechanical energy required at launch if the projectile is to escape Earth?

These problems all can be solved by using the Conservation of Energy relationship

Total Energy = KE + PE =

(1/2) m V^2 - GMm/R = constant

wher M is the mass of the earth, m is the mass of the projectile and R is the distance from the center of the Earth.

The "escape speed" Ve is the launch velocity that allows V to be zero when R = infinity. Thus

(1/2) m Ve^2 - G M m/Re = constant

Ve = sqrt (2 G M/Re)

Here is how to do (a):

If V = (1/5) Ve at R = Re, then

(1/2) m V^2 - G M m/R = constant

= (1/50) m Ve^2 - G M m/Re

When V = 0 (maximum value of R),

(1/50) Ve^2 = GM [(1/Re - (1/R)]

(1/25) G M/Re = G M [(1/Re - (1/R)]

GM/R = (24/25)(GM/Re)

R/Re = 25/24

Proceeds imilarly for (b)

For (c), the least mechanical energy (including potential energy), is zero, since potential energy is defined as zero at infinite distance, as is almost always done in inverse-square law situations. This is the case when PE = - G M m/R

Your explanaion is very clear to me. Thank you.

- Physics Question......many thanks -
**Max**, Tuesday, November 9, 2010 at 11:35pm
Sorry, your explanation made too many logical leaps. How did you get from 1/5 to 1/50??? It's garbage like this that makes you want to never see physics problems again.

- Physics Question......many thanks -
**Jon**, Saturday, November 20, 2010 at 1:08am
Amen Max!

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**.**, Thursday, March 31, 2011 at 11:21pm
1/2(1/5Ve)^2 = 1/2 * 1/25 * Ve^2 = 1/50Ve^2

- Physics Question......many thanks -
**edward**, Wednesday, June 18, 2014 at 9:40am
why is it 1/50??

- Physics Question......many thanks -
**Chris**, Wednesday, November 4, 2015 at 11:04pm
Because (1/5Ve)^2 = 1/25Ve^2 You then multiply the 1/25 by the 1/2 to get 1/50.

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