Chapter 9, Problem 47A

When a driver applies brakes to keep a car going downhill at constant speed and constant kinetic energy, the potential energy of the car decreases. Where does this energy go? Where does most of it go in a hybrid vehicle?

To determine

The effect on the energy in the carand a hybrid vehicle while going downhill.

Answer to Problem 47A

The energy in the car converted into heat and the energy of a hybrid vehicle is stored into batteries.

Explanation of Solution

Introduction:

The energy possessed by an object at rest is known as potential energy. The relation for gravitational potential energy is

  $PE=mgh$

Here, $m$ is the mass, $g$ is the gravitational acceleration, and $h$ is the height.

The moving object’s kinetic energy is equivalent to the work needed to bring the object to its actual speed from the rest condition.

The expression for the kinetic energy is,

  $KE=\frac{1}{2}m{v}^2$

Here, $m$ is the mass and $v$ is the speed.

“As per the work-energy theorem, whenever the work is done the associated energy changes”.

When the person applies the brake to slow-down, the friction force lying in between the tyre and the road executes the work in slowing down the car, this work done is the product of the frictional force and the distance moved by the car after the application of brakes.

The work is the product of frictional force supplied by the brakes and distance travelled, when after the brake is applied.

The maximum friction provided by the brake remains same no matter the car moves slow or fast.

Because of the friction, Energy is transferred into the tire as well as into the floor.

The automobile brakes convert the kinetic energy into heat.

The hybrid vehicles convert the braking energy of the vehicle to the energy stored in the batteries.

Hence, the energy in the car converted into heat and the energy of a hybrid vehicle is stored into batteries.

Conclusion:

Thus, the energy in the car converted into heat and the energy of a hybrid vehicle is stored into batteries.