Suppose a car travels 108 km at a speed of 30.0 m/s, and uses 2.0 gal of gasoline. Only 30% of the gasoline goes into useful work by the force that keeps the car moving at constant speed despite friction. (See Table 7.1 for the energy content of gasoline.) (a) What is the magnitude of the force exerted to keep the car moving at constant speed? (b) If the required force is directly proportional to speed, how many gallons will be used to drive 108 km at a speed of 28.0 m/s?

Step 1</p>

From the friction force in the engine of the car keeps the car moving at 30% of gasoline. We have to find the force that keeps the car speed at constant.

Step 2</p>

A)

Given we know that the heat content of gasoline is

We have to find the work at 30% of the gasoline usage.

To find the magnitude of the force.

Where W= work, F= magnitude of the Force.

670N is the magnitude of the force at which the car is moving.