Which of the following shows how mechanical energy has both stored energy and energy in motion? (2 points)
a
The energy from food can make us move.
b
The energy from an outlet can toast the bread.
c
The energy from the sun can help plants grow.
d
The energy of water blocked by a dam can turn big turbines.
Answers
In a previous part of Lesson 1, it was said that work is done upon an object whenever a force acts upon it to cause it to be displaced. Work involves a force acting upon an object to cause a displacement. In all instances in which work is done, there is an object that supplies the force in order to do the work. If a World Civilization book is lifted to the top shelf of a student locker, then the student supplies the force to do the work on the book. If a plow is displaced across a field, then some form of farm equipment (usually a tractor or a horse) supplies the force to do the work on the plow. If a pitcher winds up and accelerates a baseball towards home plate, then the pitcher supplies the force to do the work on the baseball. If a roller coaster car is displaced from ground level to the top of the first drop of a roller coaster ride, then a chain driven by a motor supplies the force to do the work on the car. If a barbell is displaced from ground level to a height above a weightlifter's head, then the weightlifter is supplying a force to do work on the barbell. In all instances, an object that possesses some form of energy supplies the force to do the work. In the instances described here, the objects doing the work (a student, a tractor, a pitcher, a motor/chain) possess chemical potential energy stored in food or fuel that is transformed into work. In the process of doing work, the object that is doing the work exchanges energy with the object upon which the work is done. When the work is done upon the object, that object gains energy. The energy acquired by the objects upon which work is done is known as mechanical energy.
Mechanical energy is the energy that is possessed by an object due to its motion or due to its position. Mechanical energy can be either kinetic energy (energy of motion) or potential energy (stored energy of position). Objects have mechanical energy if they are in motion and/or if they are at some position relative to a zero potential energy position (for example, a brick held at a vertical position above the ground or zero height position). A moving car possesses mechanical energy due to its motion (kinetic energy). A moving baseball possesses mechanical energy due to both its high speed (kinetic energy) and its vertical position above the ground (gravitational potential energy). A World Civilization book at rest on the top shelf of a locker possesses mechanical energy due to its vertical position above the ground (gravitational potential energy). A barbell lifted high above a weightlifter's head possesses mechanical energy due to its vertical position above the ground (gravitational potential energy). A drawn bow possesses mechanical energy due to its stretched position (elastic potential energy)
The energy of water blocked by a dam can turn big turbines (option d) is the correct response for the given question.
This can be explained in the following way:
- According to physical science, any object's mechanical energy is caused by its motion, orientation, or position. Kinetic energy is the energy held as a result of an object's motion, whereas potential energy is the energy stored as a result of an object's position.
- In this scenario, the stored potential energy of water is represented by the energy of water obstructed by a dam.
- The turbines are turned by a powerful force of water stored in the dam, demonstrating the kinetic energy of motion.
- As a result, mechanical energy is defined as the total of potential and kinetic energy in this example. Hydroelectricity is generated using this approach.