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|