The gravitational force between two objects of masses m_1 and m_2 that are separated by distance r is:

Explanation:

Gravitational force is the force of attraction that exist between two bodies having certain mass . It is proportional to the product of mass of two objects interacting with each other and inversely proportional to the square of the distance between them.

if and  are the mass of object then Gravitational force is given by

where r=distance between them

G=gravitational constant  

adding a(n) to an electric circuit would increase the magnetic field around a wire;

b. solenoid

a solenoid is a type of electromagnet whose purpose is to generate a controlled magnetic field.

if the purpose of the solenoid is instead to impede changes in the electric current, a solenoid can be more specifically classified as an inductor rather than an electromagnet.

the total distance that martina travels that day is

(distance from home to mom's office) plus (distance from mom's office to school)

plus (distance from school to library) plus (distance from library to friend's house)

plus (distance from friend's house to martina's house).

(we don't know what any of these distance actually is.)