Inertia and Friction: What a Drag!

Simulating friction and drag in an animation is all about the timing. Think about some simple practical situations you may encounter in everyday life, such as rolling a chair across different floor surfaces or the difference between running on land and running in a swimming pool. Not only does friction increase the force needed for movement, it also slows down the speed at which the motion is made.

This project is a simple exercise in creating motion drag—a principle that can be easily applied to your animations and adjusted for different types of surfaces and conditions. We will look at variations in floor texture and their effects on a rolling chair with the same momentum and starting speed.

The office chair is pushed into the frame from the left on a smooth floor finish, and the energy of the momentum is absorbed by the wheels on the chair and is affected by the weight of the chair. The chair eventually comes to rest in a smooth motion. If it were to be pushed with the same amount of thrust on hard, slick ice, it would most likely go right through the frame without slowing down, because it would then be sliding on the surface instead of rolling on its wheels.

Note: You can review this project file, ChairDrag.aep,on the [

Import the single-layer Photoshop image of the office chair (ChairwShadow-Small.psd) into After Effects as a composition, and place it into an existing comp scaled to 640 x 480 NTSC with a white background. Position the chair layer outside the Comp 1 window frame, and set the Position Stopwatch at Frame 0. Drag the image across the Comp window and stop before reaching the other side—at approximately the 01:15 mark. This will be our stopping point for the chair's motion path (Figure 4.17). Select the Easy Ease In Keyframe Assistant, and set the Keyframe Velocity to 85%.

Figure 4.17 The chair will glide on a smooth surface almost all the way across the frame and come to a rest in a smooth motion.

Apply a motion blur for improved visual realism by selecting the Directional Blur filter (Effect > Blur & Sharpen > Directional Blur) and adding a 15-pixel blur at a 90° angle. Set the Blur Length to 15, and click the Stopwatch at the beginning of the Timeline. It's important now to find just the right placement for the next marker in order to end the motion blur before the chair slows down too much. Locate the motion path dots, and look for the area of the path where the dots get closer together (Figure 4.18). Run the RAM Preview to test the motion and blur effects, and make adjustments as necessary.

Contrary to the chair's motion on the smooth surface, a course surface will cause more friction and expel the energy from the initial thrust much more quickly. How gritty or bumpy the surface is (hard-packed dirt, sandy concrete, carpet, etc.) will determine how you will need to adjust the chair's animation.

Duplicate the chair layer in the Timeline window (F/Ctrl+D), and hide the original chair layer. Activate the gritty surface layer, placed beneath it. Move the Indicator in the Timeline to about Frame 7, and drag the chair back to the far-left side of the frame so it's barely touching the edge.

Move the old end marker to Frame 20. Set the Easy Ease In Keyframe Velocity to 100%. Drag the chair back to just right of the last marker, about 30 pixels (Figure 4.19). This will give the chair the appearance that it is being thrust into the frame at the same rate as the smooth floor example but is quickly brought to a stop due to the friction caused by the gritty floor surface.

Figure 4.18

Adding a Directional Blur to the faster portion of the motion path will greatly aid in the animation's realism.

Figure 4.18

Adding a Directional Blur to the faster portion of the motion path will greatly aid in the animation's realism.

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Figure 4.19

A gritty floor surface causes more friction and stops the chair abruptly.

Figure 4.19

A gritty floor surface causes more friction and stops the chair abruptly.

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Apply the Directional Blur filter to the chair layer as directed in the smooth floor example, setting the Frame 0 Blur Length at 15 pixels and the end of the blur just short of the Frame 20 mark to 0. This will provide the motion blur to the chair in motion.

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