Figure 11 The flow pattern of a smoke trail is analyzed by computers to determine the fluid friction forces (air resistance) acting on the vehicle. Engineers use these test data to optimize a vehicle's shape for maximum fuel efficiency.

Efficiency is usually expressed as a percentage. For example, if the efficiency of a machine is 75 percent, then you know that 75 percent of the work input becomes work output. If a machine requires 10.0 J of work input to operate, then the work output is 75% of 10.0 J.

$Workoutput=\genfrac{}{}{0.1ex}{}{Workinput\times Efficiency}{100\%}Workoutput=\genfrac{}{}{0.1ex}{}{10.0J\times 75\%}{100\%}=7.5J$

Reducing friction increases the efficiency of a machine. Automobiles, for example, are designed so their wheels roll on bearings that contain many small steel rollers. Recall that rolling friction is less than sliding friction. Thus, the roller bearings reduce the friction of the rotating wheels. To further reduce the rolling friction, the roller bearings are also lubricated with grease.

As shown in Figure 11, the shapes of many cars are designed to minimize air resistance. The lower the air resistance, the more easily a car passes through the air. At highway speeds, more than 50% of the output work of the engine is used to overcome air resistance. Streamlining the car's body reduces the amount of work the engine must do to move the car at any speed.

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