Search

If a ball is incident obliquely on a horizontal surface and is allowed to bounce more than once, then it is likely to bounce many times before it starts rolling along the surface. The number of bounces before rolling commences depends on the initial vertical speed and the normal coefficient of restitution. The transition from bouncing to rolling is examined using a simple theoretical model and is compared with experimental data obtained by filming the process with a video camera. We find that the final rolling speed is proportional to the initial horizontal speed of the ball and depends on the initial ball spin, but is independent of the tangential coefficient of restitution. Representative videos for different balls are included as supplementary material, including a superball thrown with a backspin that creates a back and forth motion. Instructors could use the experiment and/or analysis for an advanced undergraduate lab or use a simplified observational exercise for non-majors. Editor's Note: When a ball is dropped on a horizontal surface with no initial spin, previous studies have found that its bouncing behavior can be simply described using a coefficient of restitution, which gives the ratio of the velocity after the bounce to before the bounce. The value of this coefficient is −1 for a perfectly elastic ball/surface, with a smaller magnitude for any real ball. In many sports, like golf, basketball, or bowling, balls are thrown at an angle and are often given some initial spin by the player. Depending on initial conditions, these balls can bounce, roll, or start by bouncing and then transition to rolling. Here, the transition from bouncing to rolling is shown to be described by using both a vertical and horizontal coefficient of restitution, with the horizontal velocity defined at the spinning edge of the ball rather than its center. Videos of undergraduate level experiments are included, with results used to validate the model. [ABSTRACT FROM AUTHOR]

An experiment is described where a metal nut was allowed to slide from the top to the bottom of a vertical wire loop. An interesting feature is that the normal reaction force on the nut decreases to zero and reverses direction before the nut reaches the bottom of the loop. [ABSTRACT FROM AUTHOR]

Li et al. conducted an experiment on the bounce of a capsule dropped vertically onto a horizontal surface. They found that if the capsule has spin, it can bounce to a slightly greater height than the drop height. This is because rotational kinetic energy is converted to vertical translational kinetic energy during the bounce. The effect is more pronounced when the capsule is incident obliquely with spin. Similar effects have been observed with other elongated objects, such as a spinning plastic egg or two golf balls joined together. This phenomenon can be visually appealing and is often used in lecture demonstrations. [Extracted from the article]

This article discusses a claim made in a previous journal article about kinetic energy conservation in elastic collisions. The authors of the current article argue that the coefficient of restitution (COR) should be defined in terms of the normal components of the incoming and outgoing speeds of the objects at the point of impact, rather than just the translational motion. They argue that this definition would result in a COR of unity in a perfectly elastic collision, regardless of the point of impact. The issue of impact point dependence is recognized by U.S. Major League Baseball, which specifies the performance of baseball bats in terms of the COR at the point of impact. [Extracted from the article]