Magnitude of the maximum gravitational force

[iceT]

Hello all..I need some help on these 2 questions..
Thanx


-A bowling ball (mass = 7.2 kg, radius = 0.15 m) and a billiard ball (mass = 0.47 kg, radius = 0.028 m) may each be treated as uniform spheres. What is the magnitude of the maximum gravitational force that each can exert on the other?

I know its the gravitational force ...so is it going to be this way?..

F= G m1m2/r^2 ...where G= 6.673x10^-11 N
?




-A cup of coffee is sitting on a table in an airplane that is flying at a constant altitude and a constant velocity. The coefficient of static friction between the cup and the table is 0.20. Suddenly, the plane accelerates, its altitude remaining constant. What is the maximum acceleration that the plane can have without the cup sliding backward on the table?

for this 1 I'm totally lost...

 

[andrevdh]

The gravitational force between the balls (they will experience the same force, but in opposite directions) will be a maximum when the distance between the balls are a minimum.

For the cup of coffee - the frictional force on the cup will accelerate it in the direction of the plane's acceleration.

 

[HallsofIvy]

Yes, that is the formula for gravitational force. r is the distance between their centers. What is the distance between the two centers when the balls are touching?

The maximum possible force on the cup before it starts to slide is 0.2 times the weight of the cup= 0.20mg where m is the mass of the cup. Also, F= ma. Solve for a.