Gravity and Collision: The Bounce and Hang Time

"It's not a fear of falling—it's the sudden stop at the end that scares me!" We've heard that from many characters, and it reflects the fact that the movement of different materials is affected by more than just their mass and the atmosphere. If you drop something heavy, solid, and inflexible, it will either break or it will make a dent in whatever it lands on or collides with. Soft, semi-flexible objects, such as tomatoes, could explode from the pressure, depending on the amount of gravitational force applied (how high was it dropped from?).

In between these extremes, gravity causes a force on impact that makes a flexible object, such a basketball, repel with nearly equal pressure in the opposite direction. This is known as a bounce, and when the downward force is continued in a similar method repeatedly (dribbling the ball, for example), the motion is continued for a sustained period of time—at least until someone's arm gets really tired!

So what happens when you bounce the ball really hard and watch it until it bounces again? There is an effect at the top of the bounce cycle called hang time, where the upward energy that thrusts the ball into the air ceases and gravity takes over again. 76 When there is no continued downward thrust of the ball, then the energy gets absorbed

H with each bounce, and the ball is repelled less with every bounce until it comes to a

S complete stop.

™ Let's defy the natural laws of physics for a moment and create a 1-second ani-

< mation loop so we can study the bounce effect and the hang time at the top of the

3 bounce:

u 1. Create a new project in After Effects and import the file Basketball_Small.psd from the DVD as a composition. Resize the composition to 640 x 480 NTSC.

2. Make the rulers visible, and drag down two guidelines to set the bottom and top g bounce points. Drag the ball up to the top guideline, and click the Position Stopwatch in the Timeline window.

3. Move the Indicator on the Timeline to 00:15 (Frame 15), and drag the ball straight down to the bottom guideline.

4. Go to the 1-second mark and drag the ball back up to the top (Figure 4.9).

Set the work area slider to the 1-second mark, and run a RAM preview of the loop. At this point, it looks more like a game of Pong than it does a bouncing ball. To simulate the hang time at the highest point of the bounce, we'll slow down the animation at both ends of the loop.

5. Select the first marker in the Timeline and apply the Easy Ease Out Keyframe Assistant. Select the last marker at the end of the animation and apply Easy Ease In. Adjust the speed in the Keyframe Velocity dialog box (Figure 4.10). Increase the percentage to slow down the delay, and lower it to increase the delay.

6. To add visual realism to the bounce, we need to add motion blur, increasing in effects through the quickest part of the animation. Apply the Directional Blur filter to the ball layer (Effect > Blur & Sharpen > Directional Blur). Move the

Indicator on the Timeline to the Frame 10 position, and press the E key to reveal the Directional Blur Effect settings. Click the Stopwatch to create a marker, and set the Blur Length to 25 pixels, leaving the Direction in the default position of 0°. 7. Move the Indicator back to the Frame 2 position and set the Blur Length to 0. Move the Indicator to the Frame 18 position and set the Blur Length to 0 there also (Figure 4.11).

Run a RAM Preview to see the timing of the hang time and motion blur effect, and make any necessary adjustments.

Figure 4.9

Set the animation to loop in a 1-second bounce.

Figure 4.9

Set the animation to loop in a 1-second bounce.

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Applying an Ease In/Out to both ends of the animation loop will create the illusion of hang time.

Figure 4.10

Applying an Ease In/Out to both ends of the animation loop will create the illusion of hang time.

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Figure 4.11 Adding motion blur through the fastest part of the animation for visual realism

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