Motion | Class 9 Science Notes
By ConceptScroll Team · Published on 17 July 2026 · 4 min read
Motion – this guide gives you a concise, exam-ready overview of Motion from Class 9 Science, written by ConceptScroll editors and reviewed against the latest NCERT textbook.
7.4 Mechanical Energy
Mechanical energy is the energy possessed by an object due to its motion or position. It is the sum of kinetic energy and potential energy.
Kinetic energy is the energy an object has because of its motion. All moving objects, such as a rolling ball or moving bicycle, possess kinetic energy. The kinetic energy (K) of an object of mass m moving with velocity v is given by:
K = (1/2) m v²
This formula is derived using the work-energy theorem and Newton's laws. When a force acts on an object causing it to accelerate from an initial velocity u to a final velocity v over displacement s, the work done equals the change in kinetic energy.
Potential energy is the energy stored in an object due to its position or configuration. For example, a stretched spring or a raised object has potential energy. The gravitational potential energy (U) of an object of mass m raised to height h near Earth's surface is:
U = m g h
where g is acceleration due to gravity.
Mechanical energy is conserved in the absence of external forces like friction. For example, a ball dropped from height h converts potential energy to kinetic energy as it falls, keeping the total mechanical energy constant.
Examples and activities in this section illustrate these concepts, including calculations of kinetic energy for a cricket ball, a jet aircraft landing, and the conversion between kinetic and potential energy in pendulum motion.
📊 Diagram: Fig. 7.11: Calculating change in kinetic energy using the work-energy theorem; Fig. 7.12: A jet aircraft landing on an aircraft carrier; Fig. 7.13: (a) A slingshot, (b) Shooting an arrow using a bow; Fig. 7.14: Spring (a) in its original shape, (b) in compressed shape, and (c) moving back to its original shape; Fig. 7.16: Earth ball system
🧪 Activity: Activity 7.1: Investigate depressions created by a ball dropped from different heights on sand to understand potential energy.
🔗 Connection: This section prepares for understanding conservation of mechanical energy and power in the following sections.
Frequently asked questions
The numerical ratio of displacement to distance for moving object is:
equals to 1 or less than 1
A particle is moving in a circular path of radius r. The displacement after half circle would be:
2r
Which factors determine the energy required to raise a flag from the ground to the top of a tall flagpole using a pulley? Does raising the flag slowly or quickly change the amount of work done? If the speed at which the flag is raised is doubled, how does the power requirement change? Explain your answers.
The energy required to raise a flag depends on the mass of the flag, the height of the flagpole, and the gravitational acceleration (E = mgh). The speed at which the flag is raised does not change the amount of work done because work depends on force and displacement, not on time. However, power is the rate of doing work, so if the speed is doubled, the power requirement also doubles.
A man of mass 60 kg rides a scooter of mass 100 kg. He accelerates the scooter to a velocity ν. The next day, his son with a mass of 40 kg joins him as a passenger. If the scooter reaches the same speed on both days in the same time interval, what is the ratio of the fuel of the tank used on the two days? Assume that the energy transfer to the scooter happens entirely due to fuel, and no other losses occur due to air resistance and friction.
Let the velocity be ν.
Day 1 total mass = 60 + 100 = 160 kg Kinetic energy on Day 1 = (1/2) × 160 × ν² = 80ν²
Day 2 total mass = 60 + 40 + 100 = 200 kg Kinetic energy on Day 2 = (1/2) × 200 × ν² = 100ν²
Ratio of fuel used = Energy on Day 2 / Energy on Day 1 = 100ν² / 80ν² = 5/4 = 1.25
So, the fuel used on Day 2 is 1.25 times that on Day 1.
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