Friction is one of the most important forces studied in classical mechanics, and still is the subject of pedagogical literature. In a small series of problems stated below, we consider a particle sliding down a curve under the actions of gravity and kinetic friction. Unlike many of the referenced sources, we neglect the centripetal force arising…

A rocket must carry the fuel it expels in order to accelerate its structure and payload. The rocket equation relates the change in speed to the fuel mass expelled. To launch a spacecraft into Earth orbit requires a multi-stage rocket, since otherwise the mass of fuel required would be prohibitive. While the details vary among historical and…

A loop-the-loop experiment usually involves a ball rolling around a vertical loop. A different version of the experiment is described where a nut was allowed to slide around a vertical loop. In both experiments there is a large decrease in kinetic energy when the ball or the nut first enters the loop.

The momentum is often used to analyse the dynamics of the motion of an experimental interaction between objects. Meanwhile, the interaction force tends to be challenging to observe and obtain. In this study, a simple video-assisted experiment was used to observe the interaction forces during the interaction of two magnetic objects. The interaction…

A rocket in free space accelerates from rest by continuously expelling fuel; as its speed increases, its mass decreases. At what speed (and remaining mass) does the rocket carry maximum momentum? Maximum kinetic energy? The answers provide insights into the dynamics of variable-mass systems, and have applications to planetary defense that are…

The question from the title is raised because in almost all introductory physics courses/textbooks the atmospheric pressure has been attributed to the weight of the column of air from a given level in the atmosphere up to its top. "Air is pressing on air." However the same textbooks, in the chapter on the kinetic theory of gases, tell…

The principles of chemical kinetics comprise one of the core topics that appear throughout chemistry. Standard kinetics lessons typically cover reaction rates and relative rates, rate laws, integrated rate laws, half-lives, collision theory, and the Arrhenius equation. They can also introduce a discussion of mechanisms as well, which may be the…

The vertical bounce of a plastic egg was investigated by dropping the egg on a horizontal surface and filming the result with a video camera. If the egg is dropped on one end then it bounces just like a spherical ball. If the top end of the egg is pointing forwards or backwards when it lands on the surface, or if the egg is spinning when it lands,…

A simple experiment for students is to measure the coefficient of restitution (COR) for a vertical bounce on a horizontal surface. In this paper, measurements are presented of the COR for a tennis ball bouncing at an oblique angle on a horizontal surface. Changes in the horizontal and rotation speeds were also measured, by filming the bounce with…

In this paper we describe a simple alternative to the telephone book friction experiment, a classic demonstration where a small force (generated for example by the bending of the outer pages) is amplified by the large number of surfaces in contact, resulting in a huge maximum static friction force that has to be counterbalanced in order to…

The mechanical energy change of a system in an inertial frame of reference equals work done by the total nonconservative force in the same frame. This relation is covariant under the Galilean transformations from inertial frame S to S', where S' moves with constant velocity relative to S. In the presence of nonconservative forces, such as normal…

Newton's first and second laws have implications for the kinetic energy as well as the momentum of a body. It is recommended that this should be made explicit at an appropriate point in a course.

My favorite exam question comes from the final exam in an introductory mechanics course: "A rolling 31 ton railroad boxcar collides with a stationary flatcar. The coupling mechanism activates so the cars latch together and roll down the track attached. Of the initial kinetic energy, 38% dissipates as heat, sound, vibrations, mechanical…

This "Science Note" explores the new adaptation of Newton's Second Law of Motion, "F = ma." In older physics and applied mathematics textbooks this expression appears as "P = mf." The author examines why "f" is now favored over "a" and why practitioners write "P = mf" rather than…

The paper is about the appearance of space charge in an ohmic conductor moving in a magnetic field, as pointed out in this journal by Lorrain (1990 "Eur. J. Phys." 11 94-8) and earlier by van Bladel (1973 "Proc. IEEE" 61 260-8). The phenomenon is reinvestigated here in the light of energy balance considerations, in the particular case of a…