Class 9 Science Exploration Solutions English Medium

NCERT Solutions Class 9 Science Exploration English Medium

Work, Energy, and Simple Machines

Topic: Conservation of Mechanical Energy

Chapter 7. Work, Energy, and Simple Machines

This section explains the conservation of mechanical energy, one of the most important principles in physics. As an object moves under the influence of gravity, its potential energy continuously changes into kinetic energy and vice versa. Although these two forms of energy keep changing, their sum remains constant when no external force such as friction acts on the object. This principle helps explain the motion of freely falling objects, pendulums, roller coasters and many other natural phenomena. 

Conservation of Mechanical Energy

Mechanical energy is the total energy possessed by an object due to its motion and position. It is equal to the sum of kinetic energy and potential energy.

Mechanical Energy

Mechanical Energy = Kinetic Energy + Potential Energy

ME = KE + PE

Law of Conservation of Mechanical Energy

When an object moves only under the influence of gravity and no external force such as friction acts on it, the total mechanical energy of the object remains constant. During motion, potential energy changes into kinetic energy and kinetic energy changes back into potential energy, but their total always remains the same. 

Mechanical Energy During Free Fall

Consider an object released from a height. As it falls towards the Earth, its height decreases while its speed increases.

Energy Transformation During Free Fall

• At the Highest Point – The object possesses maximum potential energy and zero kinetic energy.
• During the Fall – Potential energy continuously converts into kinetic energy.
• Near the Ground – Almost all the potential energy changes into kinetic energy.
• Total Mechanical Energy – Remains constant throughout the motion if friction is neglected.

Why is Mechanical Energy Conserved?

Gravity does not destroy energy. It only converts one form of mechanical energy into another. Therefore, the total mechanical energy remains unchanged as long as no external force removes energy from the system.

Real-Life Applications

• Roller Coaster – Potential energy at the top changes into kinetic energy while moving downward.
• Pendulum – Energy continuously changes between kinetic and potential forms during oscillation.
• Hydroelectric Power Plant – Water stored at a height possesses potential energy which changes into kinetic energy and finally electrical energy.
• Waterfalls – Falling water converts gravitational potential energy into kinetic energy.
• Bungee Jumping – Energy changes repeatedly between kinetic and elastic potential energy.

Mechanical Energy Flow

Highest Point → Potential Energy → Kinetic Energy → Lowest Point

During the upward journey, the reverse process takes place.

Important Facts

• Mechanical energy is the sum of kinetic energy and potential energy.
• Mechanical energy is conserved only when external forces like friction are absent or negligible.
• Potential energy decreases as kinetic energy increases during free fall.
• Kinetic energy decreases while potential energy increases during upward motion.
• The SI unit of mechanical energy is the joule (J).

ATP Education Concept Builder

Students often think that potential energy disappears during free fall. In reality, it is simply converted into kinetic energy. The total mechanical energy remains constant throughout the motion, demonstrating the law of conservation of energy. This is one of the most important concepts for CBSE competency-based and numerical questions.

ATP Education Exam Booster

• Remember the formula: Mechanical Energy = KE + PE.
• Understand energy transformation during free fall.
• Learn the difference between kinetic energy, potential energy and mechanical energy.
• Practise identifying energy changes in roller coasters, pendulums and waterfalls.
• Remember that friction causes a part of the mechanical energy to convert into heat and sound energy.
• CBSE frequently asks conceptual questions based on the conservation of mechanical energy.

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