BOUNCING

Long objects can bounce backward if they point backward when

they land. Sliding friction can bring an object to rest but it can’t

reverse the direction of motion. Static friction is responsible.

What happens to a spring when it bounces?

Watch in slow motion.

It’s nothing like the bounce of a ball.

What happens when a water-filled balloon bounces?

Movie1 (1Mb) Movie2 (5Mb) Both 300 fps

This popular demonstration involves only one collision, not two separate

collisions as is usually assumed (ie basketball bounces first and then

collides with the tennis ball). But there is a “bottom” collision and then

a “top” collision before the balls separate. Watch in slow motion.

If one part of a ball or hoop stops rotating while the rest keeps

rotating, what then happens to the ball or the hoop? Watch in

slow motion. Bounce1 Bounce2 Bounce3

Rods of equal length click when they collide & bounce end-on.

Unequal rods ping (ie vibrate) when they collide.

Movie1 Movie2 Movie3

SUPERBALLS

A spinning superball reverses its spin and

bounces backwards. Many times !

Superball bounce, filmed at 1000 frames/sec. The bottom of the

ball squashes in the vertical direction, and it stretches in the horizontal

direction, although it is not easy to see in this film.

Stress patterns observed in a transparent superball, viewed between two

crossed polaroid sheets. Top half of ball is blue, bottom is clear.

Movie1 = vertical finger force, Movie2 = oblique finger force

FOOTBALLS

Will this spinning football bounce to the left or the right?

It depends which way it is pointing when it lands.

Bounce1 Bounce2

The previous bounces were filmed with a backspin ball. If the ball is incident with topspin then the ball bounces forward with topspin but the result depends strongly on the inclination of the ball when it lands. The bounce itself is 15 ms in duration (5 frames when filmed at 300 fps, as shown below) except when the ball rolls then the bounce duration is about 60 ms (20 frames).