# Homework Help: Work done by external force in a gravitational field.

1. Aug 10, 2012

### moatasim23

1. The problem statement, all variables and given/known data

Spheres of masses m1 2.53kg and m27.16kg are fixed at a distance 1.56m apart centre to centre.A m3 212 g sphere is positioned 42cm from the 7.16 kg fromcentre of 7.16 kg sphere along the line of centres.How much work must be done by ext agent to move thesphere of 212 g sphere along the line of centres and place it 42 cm from the centre of sphere of mass 2.53 kg sphere?Assume the 212 g sphere to be moved bw the two spheres.

2. Relevant equations

Wext(External)=-Wg(gravitational)

3. The attempt at a solution
Wext=Gm3m2(1/.42-1/1.14)-Gm3m1(1/1.14-1/.42)
Which is not giving the desired result of 98.5pJ.
But if I take
Wext=Gm3m2(1/.42-1/1.14)+Gm3m1(1/1.14-1/.42) the answer gets right.But why should it be a positive sign?

2. Aug 10, 2012

### tiny-tim

hi moatasim23!

(try using the X2 button just above the Reply box )
so m3 starts 0.42 from one sphere and 1.14 from the other sphere, and finishes the other way round
why not?

one is the work done moving it away from one sphere, the other is the work done moving it towards the other sphere, so the work done should have opposite signs, so your formula should have a +

3. Aug 10, 2012

### moatasim23

That is what Im saying work done should have opposite signs.Taking both works to be positive does this means they hv opposite signs?How?

4. Aug 11, 2012

### tiny-tim

(just got up :zzz:)

not following you

your first bracket is negative, and your second bracket is positive, so they have opposite signs, as expected (and so you add them)