During my introductory physics classes, I find myself asking my students this question a lot. I realize that the answer isn't always evident, and created this non-exhaustive (I will add things as they come to mind or come up in class/conversation) list of how a physics problem might use words to indicate how to solve it.
To clarify, I am referring to inferences made on any provided information or results. The inferences listed may be useful in a variety of problems, but not all inferences can be associated with all problems. Therefore, it is still important to think through each problem yourself and use this tool only as a guide.
| "at rest" | (initial) velocity = 0 (therefore, acceleration is also zero) equilibrium (sum of forces = 0 and no change in energy) static friction (as opposed to kinetic) |
| "rough surface" | consider friction (if moving, kinetic; otherwise, static) |
| "smooth surface" | no friction |
| "constant velocity" | acceleration = 0 (simplifies kinematic equations) no change in force (implied from above equation and Newton's 2nd Law) |
| "elastic" collision | conservation of momentum and kinetic energy; use two conservation equations and simplify to solve for unknown variables |
| "inelastic" collision | only momentum is conserved; the conservation equation should be sufficient (with other knowledge) to solve the problem |
| "free fall" | acceleration = gravity (be consistent with your use of the negative sign) only concerns y-component (x-component added for parabolic motion) |
| "at an angle" | break apart vector (velocity, force, etc.) into x- and y-components using trigonometry |