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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.

A loop-the-loop experiment is described to show how sliding friction affects motion of the ball. Conservation of energy can be used to explain the basic physics, but significant energy loss is observed in practice and expands the usefulness of this apparatus as a teaching tool.

A simple experiment is described to compare the descent time between two vertically separated points when an object slides down tracks of varying shape. A surprising result is that the descent time is shortest when it follows a circular track rather than a cycloidal track. Cycloidal tracks are usually predicted to result in the shortest descent time, but only if friction is negligible.

Experimental results are presented concerning the motion of a ball that bounces up an incline a few times then bounces back down again. The number of bounces up the incline is typically small since the speed of the ball in a direction parallel to the incline decreases rapidly, not only during each bounce but also while the ball is in the air. The ball heads back down the incline when it is incident at right angles to the incline.

An experiment is described where a pendulum bob was allowed to roll back and forth across an inclined plane. The period of oscillation is larger than that for oscillation in a vertical plane, in part because the effective value of "g" is reduced on an inclined plane. The experiment highlights the differences between all three common forms of friction, namely sliding, rolling and static friction.

A collision of one object with two or more objects is relatively complicated in general, but a simple example is provided by Newton's cradle since all the objects are identical and in line. In the present paper, an experiment is described where a heavy mallet collides head-on with two billiard balls. The two conservation equations indicate that many different outcomes are possible. The experimental results are surprising but can be explained in terms of the relative mass and stiffness of the mallet and the balls.

A 2.7 m long stringless pendulum was set up to measure the coefficient of rolling friction of various balls, at higher rolling speeds than usually observed with a short stringless pendulum. The arrangement is easy to set up and makes an impressive classroom demonstration as well as an interesting laboratory experiment.

Measurements and calculations are presented showing how the trajectory of an object sliding across an inclined plane varies with the launch and incline angles. The trajectory depends on the magnitude and direction of the sliding friction force. The experiment is easy to set up, can be analysed using a video camera, and would be suitable for a student experiment or demonstration.

The trajectory of a ball rolling across an inclined plane was recorded on video film to determine if it followed a parabolic path as others have suggested. The challenges in this case were (a) to determine the magnitude and direction of the friction force on the ball, (b) to determine the effect of the friction force on the trajectory and (c) to explain why the rotation axis of the ball rotates as it rolls up and down the incline. The experiment is a simple extension of standard inclined plane experiments but with the extra pedagogical feature that angular velocity sometimes needs to be treated as a vector quantity.

If a ball rolls in a circular path on a horizontal surface at constant speed then its horizontal rotation axis changes direction with time. A simple experiment is presented showing that the torque applied to the ball is equal to the rate of change of its angular momentum, even though the magnitudes of its angular velocity and angular momentum remain constant in time.

Calculations are presented showing that the usual 'faster than g' demonstration has a surprising property. That is, a rod hinged at its bottom end rotates at an exponentially increasing rate until it falls with maximum vertical acceleration, unlike an object that falls freely by gravity alone. If the rod is hinged at its top end and released from a horizontal position, then the vertical acceleration is a maximum at the instant it is released.

Oblique angle collisions of two penny coins on a smooth, horizontal surface were filmed with a video camera to investigate the physics of the collision process. If one of the coins is initially at rest, then the two coins emerge approximately at right angles, as commonly observed in billiard ball collisions and in puck collisions on an air table. The coins actually emerged at an angle less than 90 degrees due to friction between the coins, which also resulted in both coins rotating after the collision. At glancing angles, the friction force was due to sliding friction. At other angles of incidence the coins gripped each other and the friction force was then due to static friction.

If an empty coffee cup is spun on a horizontal surface about a vertical axis in an anti-clockwise direction, then the whole cup rotates slowly backwards, in a clockwise direction, about a vertical axis located outside the cup. Clues to this curious behaviour are presented by comparing it with other spinning objects.

A solid ball placed on a rotating turntable is known to roll slowly around a circular path, at a speed 3.5 times slower than the turnable itself. If the ball is located in a straight track across a diameter of the turntable, then it accelerates rapidly to the edge. Both effects were filmed in slow motion using a video camera and a cake decoration turntable, and are presented as supplementary videos. The results agree well with calculations based on rolling motion of the ball.

The coefficient of restitution (COR) for a collision is usually a number between 0 and 1 depending on whether the collision is completely inelastic, or perfectly elastic or something in between. That is the usual situation for colliding spheres or a ball dropped on a horizontal surface. A different situation arises when a bat collides with a ball. A simple experiment is described where a rod is dropped on another rod at right angles. In that case, two different values of the COR can be defined.

Experimental results are presented on the collision of a superball with two different wood blocks. The results are in reasonable agreement with a simple collision model where kinetic energy is conserved, but better agreement is obtained if a small loss of kinetic energy is assumed, as observed. The physics is slightly more complicated than the standard head-on collision of two balls studied in elementary mechanics since it involves rotation as well as translation of one or both of the colliding objects.

If a smooth ball is dropped vertically without spin on a smooth horizontal surface then one might expect the ball to bounce vertically without spin. If it does not then the centre of mass of the ball does not coincide with its geometric centre. An experiment is described where a billiard ball and a superball are deliberately biassed by adding a small steel ball on one side. The results agree well with theoretical calculations.

Experimental results are presented concerning a bead sliding on a rotating, horizontal rod. If the rod rotates at constant angular velocity then the sliding speed in the radial direction increases exponentially with time, provided that the rotation speed is sufficient to overcome friction. Otherwise the bead rotates in a circular path of constant radius.

Points near the top end of a falling rod hinged at its bottom end can fall faster than g. If a rod falls on a frictionless surface then the bottom end slides backwards and the centre of mass falls vertically with acceleration less than g. The effect was simulated by mounting a rod on wheels and filming the result with a video camera. The experiment provides an example of accelerated motion where the acceleration increases with time.

An experiment is described where a billiard ball and a golf ball were projected with backspin on a horizontal surface to measure the transition from sliding to rolling. During both phases, the torque on the ball is affected by an offset in the line of action of the normal reaction force.

A standard problem for physics students is to calculate or measure the acceleration of an object down an inclined plane. Additional information can be obtained by measuring the angular acceleration as well as the linear acceleration. An experiment is described where a billiard ball was filmed as it rolled down an inclined plane at different inclination angles. The transition from rolling without slipping to rolling with slipping was identified, and the relevant friction coefficients were measured independently from the linear and angular accelerations.

Newton's cradle is often discussed in science classrooms as a clear example of the laws of conservation of momentum and energy although it has been shown that this use is somewhat misleading. Approaches to understanding the behaviour of this apparatus are often over-simplified and deficient or over-complex and with little impact among teachers. In this paper, basic principles of mechanics are used to explain the behaviour of ideal cradles of various lengths and to use a simple model by which to simulate its operation. The purpose is to demonstrate that the real behaviour of Newton's cradle provides a rich source of application of those principles in coming to terms with how the apparatus behaves and experiencing the process of modelling in physics. [For "Understanding Newton's Cradle. II: Exploring a Real Cradle," see EJ1286996.]

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, then the bounce is very different. The egg can bounce forwards or backwards and it can bounce to a large height or only a very small height. The experiment would be suitable for a student project. It involves both translational and rotational motion of an object subject to an impact force and a torque.

A hula hoop rotating around a person's waist is an example of circular motion where the centripetal force on the hoop is directed towards the centre of the circle rather than the centre of the hoop. The friction force on the hoop is relatively small by comparison since the hoop rolls around the waist. A slight complication is that the hoop rotates in an elliptical path if the waist rotates back and forth in a straight line or if the waist rotates in an elliptical path. Three simple experiments are described to illustrate the physics involved.

Newton's cradle is a well-known physics toy that is commonly used by teachers to demonstrate conservation laws in mechanics. It can also be used to investigate the physics of colliding objects, by recording motion of the balls on video film. Various experiments are described using 3-ball and 5-ball cradles, showing how different types of collision can be investigated depending on whether the balls are touching or separated. The sequence of collisions was examined as a function of time, with the surprising observation that very small gaps between the balls, invisible on video film, make a significant difference to the collision dynamics. Additional experiments illustrate the effects of changes in ball stiffness and the mass of the middle ball and provide an introduction for students and teachers into possible research projects. Each experiment is illustrated with a supplementary video. [For "Understanding Newton's Cradle. I: Modelling the Ideal Cradle," see EJ1286997.]

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 a video camera. A measurement of this type involves a more extensive examination of basic mechanics, since the friction force acting at the bottom of the ball acts to change both the horizontal speed of the ball and its rotational velocity. The COR for an oblique bounce was found to be larger than that for a vertical bounce and was also found to depend on the incident spin of the ball.

The impact force and duration when one billiard ball collides head-on with another is estimated by assuming that the balls are connected by a spring. The results are compared with a measurement when a billiard ball collides with a heavy, rigid surface.

A standard physics experiment for students is to measure the terminal velocity of an object in air or in glycerine. A more difficult challenge is to measure the terminal velocity in water since the depth of the water needs to be large or the object needs to be dropped in the water at a speed close to its terminal velocity. Results obtained by filming the fall in a fish tank are presented for a range of different spheres, showing how the terminal velocity depends on the density of the sphere and its radius. The results provide a useful measurement of the drag force and the drag coefficient for a sphere in water. An additional experiment to measure the Magnus force is described where a ball was spinning when it entered the water.

A well known physics problem is to calculate the friction forces required to support a ladder against a wall. A more tractable problem is to calculate the friction forces needed to support an inclined beam on a ball or a cylinder. In the latter case there are three rather than two points of sliding support. Measurements and calculations are presented to show that a ball can support an inclined beam without slipping, provided the incline angle is not too large.

Two balls on an incline can remain at rest if friction between the balls prevents them rolling down the incline. Results with various balls are presented to show how ball mass, diameter and friction affects the outcome. A stack of four identical balls is also examined.

A ball at rest on a surface can be made to bounce up by pushing it down then releasing the downward force as fast as possible. Measurements and calculations are presented to show how it can best be done.

A method is described to measure the coefficient of restitution (COR) by dropping a ball on a piezoelectric disk. Multiple bounces can be observed at small drop heights, so the average COR over say ten bounces can be obtained from just one ball drop, without having to measure the bounce height or the bounce speed. The results show directly that the maximum impact force on the ball during each bounce decreases linearly with time until the ball stops bouncing.

If a ball rolls at constant speed on a horizontal surface about a horizontal axis then the angular velocity is easily measured. If the ball is projected with additional spin about the vertical axis, then the rotation axis is tilted and it is more difficult to measure the rotation speed. A few examples are presented to show how the separate topspin and sidespin components can be measured.

A spinning top with a spherical bottom end was constructed to examine its behaviour. Experimental results were consistent with theoretical predictions.

When a ball bounces obliquely on a horizontal surface, the bottom of the ball stretches horizontally and then vibrates backward. The resulting ball spin depends sensitively on the transverse vibration frequency. A simple model is presented to describe the effect, showing how the stored elastic energy can result in additional spin.

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]

A simple throwing task is described to illustrate various aspects of projectile motion. The task was to throw a tennis ball in a waste paper bin about 2 m away. Success depends on skill, but it also depends on the physics of the problem. If the ball is thrown underarm, then success depends primarily on the throw speed, which must be controlled to within about 2%. The launch angle can be chosen within a wide range. If the ball is thrown overarm, then the launch angle is just as important as the throw speed.

When a hard object rolls on a soft surface, or vice versa, rolling friction arises from deformation of the soft object or the soft surface. The friction force can be described in terms of an offset in the normal reaction force or in terms of energy loss arising from the deformation. The origin of the friction force itself is not entirely clear. It is investigated qualitatively in this paper by rolling a steel ball on soft foam and by rolling a foam cylinder on a hard surface. The deformation of the foam was observed visually, providing simple insights into the origin of the friction force.

A ball that rolls without sliding down an inclined plane requires a small static friction force to prevent the ball sliding down the incline. The magnitude of that force is investigated by considering the force needed to prevent the contact point from slipping.