Begin with a sketch. Isolate each object in the problem and draw a free-body diagram for each object of interest, showing all external forces with their directions.

Establish a convenient coordinate system. Find sum of forces for each component. Apply Newton's second law of motion separately for each component.

Action-reaction forces always act on two different bodies, and they never cancel each other.

Remember that the normal force is always perpendicular to the surface. The force of gravity always acts vertically downward.

When a surface is involved, there is typically no acceleration in the normal direction. The sum of the normal components of the forces must be zero.

For an object moving on a flat surface, balance the forces in the normal direction to find the normal force. Sometimes, but not always, this is equal to the weight of the object.

If you need to resolve forces, keep in mind that the horizontal and vertical directions may not always be the simplest. For inclined surfaces, it is convenient to choose x and y axes parallel and perpendicular to the inclined surface respectively.

Find the normal force in order to calculate the force of friction (when present). If the object is at rest, calculate the force of static friction. If the object is moving with constant velocity, calculate the force of kinetic friction.

If the forces are neither parallel nor perpendicular to the actual or impending motion, resolve the forces into x and y components. Find the net force in the x and y directions.

Pulleys are used to change the direction of tension. If the mass of a string is negligible, the tension in the string has the same magnitude throughout its length.

If an object is in equilibrium (that is, at rest or moving at a constant velocity) the net acceleration is 0. This means that it is 0 in both directions.