Model Identify which objects form the system.

Visualize Draw a pictorial representation.

• Define symbols and coordinates.

• Identify acceleration constraints.

• Draw an interaction diagram.

• Draw a separate free-body diagram for each object.

• Connect action/reaction pairs with dashed lines.

Solve Write Newton’s second law for each object.

• Use the free-body diagrams.

• Equate the magnitudes of action/reaction pairs.

• Include acceleration constraints and friction.

Review Is the result reasonable?

Objects whose motion is of interest are the system.

Objects whose motion is not of interest form the environment.

The objects of interest interact with the environment, but those interactions can be considered external forces.

Objects that are constrained to move together must have accelerations of equal magnitude: $a_{\text{A}}=a_{\text{B}}$.

This must be expressed in terms of components, such as $a_{\text{A}x}=-a_{\text{B}y}$.

The tension in a string or rope pulls in both directions. The tension is constant in a string if the string is:

• Massless, or

• In equilibrium

Objects connected by massless strings passing over massless, frictionless pulleys act as if they interact via an action/reaction pair of forces.