Answer:
An object in equilibrium has a net force of zero
Static equilibrium describes an object at rest having equal and balanced forces acting upon it.
Dynamic equilibrium describes an object in motion having equal and balanced forces acting upon it.
Explanation:
An object is said to be in equilibrium when a net force of zero is acting on it. When this condition occurs, the object will have zero acceleration, according to Newton's second law:

where F is the net force, m the mass of the object, a the acceleration. Since F=0, then a=0. As a result, we have two possible situations:
- If the object was at rest, then it will keep its state of rest. In this case, we talk about static equilibrium.
- If the object was moving, it will keep moving with constant velocity. In this case, we talk about dynamic equilibrium.
(a) For the work-energy theorem, the work done to lift the can of paint is equal to the gravitational potential energy gained by it, therefore it is equal to

where m=3.4 kg is the mass of the can, g=9.81 m/s^2 is the gravitational acceleration and
is the variation of height. Substituting the numbers into the formula, we find

(b) In this case, the work done is zero. In fact, we know from its definition that the work done on an object is equal to the product between the force applied F and the displacement:

However, in this case there is no displacement, so d=0 and W=0, therefore the work done to hold the can stationary is zero.
(c) In this case, the work done is negative, because the work to lower the can back to the ground is done by the force of gravity, which pushes downward. Its value is given by the same formula used in part (a):

The best answer is D. field lines should always be crossing each other.
F(g)= Gm1m2/ r^2
If mass is increased, so will the force of gravity because it is in direct relationship with the gravitational force, but if distance is increased, the force of gravity will decrease because it is indirectly related ( since it is on the bottom of the equation)