Rotational motion problems

When you try to solve problems of Physics in general and of rotational motion in particular, it is important to follow a certain order. Try to be organized when you solve these problems, and you will see how it gives good results. It is worth spending a bit of time on the analysis of a problem before tackling it.

When solving a rotational motion problem, follow these steps:

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          • Read carefully the problem statement.
          • Draw a picture of the physical situation described in the problem.
          • Write in your notebook the givens in the problem statement.
          • Identify the elements that constitute the system in motion: if it is a single body or if on the contrary you must take into account several objects. This step is important because it will allow you to distinguish the internal and external forces that act on the system.
          • Draw the external forces that act on the system.
          • Remember that the forces are physical interactions. Therefore, if a system is supported on another system, a normal force will act on it; if it is close to the surface of the Earth, the weight will act on it, and so on with the rest of the interactions that the system experiences.
          • In this link you can see how the forces exerted by the different types of support are represented.
          • Add a Cartesian coordinate system on the drawing, indicating which is the positive direction of each of the axes. you should also indicate which point you are going to use as an origin to calculate the torque of the external forces acting on the system.
          • If the center of mass of the system moves, write Newton’s second law for it as a vectorial equation, including in the vectorial sum for the different external forces you have identified.
          • Project the resulting equation on the Cartesian axes. Take into account the sign of the different projections.
          • If the system rotates, write Newton’s second law for rotation in a vectorial form.
          • Calculate the torques of the different external forces with respect to the point you have chosen as origin.
          • Project the resulting equation on the Cartesian axes. Take into account the sign of the different projections.
          • Solve the system of equations that you have obtained after projecting the different equations on the axes. In this way you will be able to determine the magnitudes asked in the problem.
          • Do not forget to include the units in your results.
          • Review the problem and check that the results you have obtained make sense.


        On the following pages you will find solved problems of rotational motion. Try to do them before looking at the solution.</ li>

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