OTHER PHYSICS MOVIES

Slow motion video cameras are relatively inexpensive these days, and can be used with motion capture software (eg Tracker) to analyse almost anything that moves. Here are a few examples.

NEWTON’S CRADLE (actually MARIOTTE’s CRADLE)

It is commonly assumed that when one ball is incident on a row of balls in Newton’s cradle, then only one ball emerges at the far end and the other balls come to rest. That might be the case if all the balls are initially separated, but not if they are touching. In that case, the ball at the far end emerges at about the same speed as the incident ball but all other balls are set in motion, at least at low speed. The balls behave as if they are connected by springs, the springs being the ends of the balls themselves. The effect is shown in the following videos filmed at 600 frames/s, and is especially clear with the hollow rubber balls. The end of each ball compresses in turn until the ball at the far end is eventually ejected. It doesn’t happen instantly or even at the speed of sound through the balls. See Flansburg L and Hudnut K 1979 Dynamic solutions for linear elastic collisions Am. J. Phys. 47 911–4 and Cross R 2008 Differences between bouncing balls, springs, and rods Am. J. Phys. 76 908–15.

WHY CHALK BREAKS INTO 3 PIECES WHEN DROPPED

Most teachers know that chalk breaks when it is dropped. Some have observed that it nearly always breaks into three pieces. Here are two QuickTime videos at 1200 fps showing why. It is best to advance one frame at a time. First, the chalk breaks in half. Then the half still in the air falls and it breaks in half. Of course, if you drop chalk from a height of only a few cm, it won’t break at all. Dropped from a height of about 30 cm, it will break into two pieces, as shown in the second video. But teachers always drop chalk from a greater height. Richard Feynman noticed that spaghetti sticks also break into 3 pieces when they are bent far enough, but that’s a different effect. Check it out on the web. See also R. Cross, Why chalk breaks in 3 pieces when dropped, The Physics Teacher, 43, 13-14 (2015).

TOP AND EGG ON INCLINED PLANE

A spherical object placed on an inclined plane tends to roll down the incline. How about a spinning top or a spinning egg? Check out these two videos. Spinning tops and eggs roll sideways across the incline !! It’s not a magic trick. It is the same effect as the E x B drift in plasma physics. As a top or egg speeds up (downhill) the precession radius increases. As it slows down (uphill) the radius decreases. So the top or the egg walks sideways across the incline. Discovered independently by David Featonby and myself in 2014. See R. Cross, Surprising behavour of spinning tops and eggs on an inclined plane, The Physics Teacher 54, 28-30 (2016).

IMPACT WITH WATER

Here is a squash ball dropped into a fish tank from a

height of 40 cm. The result is spectacular at 300 fps.

See The Physics Teacher, 54, 153-155 (2016).

300 fps results with a golf ball dropped from a height of

10 cm (laminar flow) low Reynolds Number, Drag coeff = 0.5

70 cm (turbulent flow) high Reynolds Number, Drag coeff = 0.2

See Eur J Phys 37, 054001 (2016)

SLAP SHOT IN ICE HOCKEY

Click to see a simulated slap shot where a steel ruler impacts

a brass puck. The initial bend adds PE to the stick with

the result that the total KE after the collision > total KE before the collision.

This is a superelastic collision with a coefficient of restitution > 1

CENTRIFUGAL FORCE

A small steel ball is attached with Blu-Tack to a 2inch diam steel ball

that is spinning rapidly. Filmed at 300 fps. The Blu-Tack exerts a centripetal force

radially inwards on the small ball, so the small ball exerts a radially outwards

force on the Blu-Tack.

ROLLING FRICTION

The coefficient of rolling friction for a steel ball on a hard surface is typically between 0.0001 and 0.001 and decreases as the ball diameter increases. Here are two ways to measure the COF, by measuring the decrease in speed over time in a groove or in a concave lens. The speed decreases very slowly due to small collisions between the slightly rough ball and the slightly rough surface. You can hear the collisions if you turn up the volume. See R. Cross, Coulomb’s law for rolling friction, Am. J. Phys. 84, 221-230 (2016)

WATER BOTTLE WITH TWO HOLES

A water bottle has two holes in the side. The holes are blocked, the bottle is filled almost to the top

with water and then sealed with a lid. What happens when the bottom hole is unblocked?

What happens when both holes are unblocked? Does water flow out both holes?

If water covers the top hole, can air enter the bottle?

Click the bottle to see if you got the correct answers.

ELASTIC PENDULUM

A mass on the end of a spring can vibrate vertically or horizontally

or both. If the up and down vibration frequency is about equal to

twice the horizontal pendulum frequency, then the up and down motion

gradually changes to horizontal motion and vice versa – see here.

HUMAN DOUBLE PENDULUM

Here is an elegant example of two double pendulums working in unison, passing our front

door at 300 fps. Clip2 and Clip3 show that gravity is not the only force acting. Muscle

action is needed to swing each pendulum segment. Arm and leg pendulum motion is

described in Am. J. Phys. 67 (4), 304-309 (1999), 72 (3), 305-312 (2004),

77 (1), 36-43 (2009) and 79(5), 470-476 (2011) (when swinging bats and racquets).

How does he do this “Ollie”? A skateboard rider can jump in the air with the board

almost glued to his feet. The physics can be seen more clearly is this

slow motion demonstration with a tennis ball and two pieces of wood.

It is not well known, but the Physics Department hosts a family

of kookaburras. Here is Mum with a juicy worm. And off to get another one.

Amazing aerodynamic control!

LISTEN TO A BELL (recorded at the same volume and ball speed) hit with (a) a golf ball or (b) a tennis ball

Why are the sounds so different? It’s the same fundamental reason that some bats and racquets have bigger sweet spots than others. The amount of vibration depends on the ratio of impact duration to the vibration period. The bell might sound tinny if you use internal speakers. Try it anyway as a test of the bass response of your internal speakers. The fundamental frequency is 975 Hz. Internal speakers will not respond at all to the f < 200Hz racquet sounds included on the tennis page.

Escher Movie clip of me reflected in a 2 inch steel ball (click ball)