In our daily lives we can say that we are constantly making use of force, whether when we are pulling or pushing objects. When we lift a bag full of cleaning products off the floor, for example, we are exerting force on the bag. When pushing an automobile, be it big or small, we are pushing.
Therefore, we can conclude that the force always acts between two bodies causing a change in velocity or also causing a deformation. And we cannot forget that force is a vector quantity, so the force has magnitude, direction and sense.
Let’s see then a basic strategy that can be used in solving problems involving bodies subject to forces:
1) carefully read the entire statement and identify what is being requested;
2) sketch the situation;
3) identify the objects involved in the situation;
4) draw all the forces acting on each object;
4.1) initially place the weight force downwards;
4.2) identify contact forces with other objects. These forces always act perpendicular to the contact surface;
5) make a drawing, separately, for each object. Put on this drawing a schematic of all the forces acting on the object. This scheme is known as a free-body diagram;
6) choose a reference system;
7) apply Newton’s Second Law to each of the objects and to the x and y components of the forces.
Fr = ma
∑F x =F 1x +F 2x +⋯=ma x
∑F y =F 1y +F 2y +⋯=ma y
If necessary, use the kinematics equations to find the quantities required by the statement.