Final answer:
The question requires the calculation of a horizontal force to move a steel beam on rollers, but lacks clear and correct information on the mass of the steel beam. Additional information is needed for an accurate calculation.
Step-by-step explanation:
The student has provided a scenario involving a 1.2 mg steel beam moved over a level surface using 30 mm rollers, with varying coefficients of rolling resistance for different surface types. The task is to determine the horizontal force P required to move the beam at a constant speed. The information provided in the question does not contain the necessary details to calculate the horizontal force (such as the actual beam mass or a typo in the unit for mass), and the other examples, while related to physics and force calculations, are not directly linked to answering this specific scenario.
To provide assistance, we would need additional clarification on the steel beam's mass, since 'mg' typically denotes milligrams, which is not a realistic mass for a steel beam. Assuming mg stands for kilograms or another unit, the force can then be computed by understanding the concept of rolling resistance, which can be factored using the coefficients provided. Nonetheless, without the correct mass and clear parameters, an accurate answer to this problem cannot be given.