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What are kinetic energy and potential energy?

Class 9 ScienceNCERT chapter: Work and Energyइसे हिंदी में पढ़ें

Exam-ready answer

Kinetic energy is the energy possessed by a body due to its motion, given by KE = ½ mv². Potential energy is the energy possessed by a body due to its position or configuration, given by PE = mgh for a body at height h. Both are measured in joules (J).

Explanation

Kinetic energy is the energy an object has because it is moving. A moving car, a flying bird, or a rolling ball all have kinetic energy. The faster an object moves and the greater its mass, the more kinetic energy it has. The formula is KE = ½ mv², where m is mass in kg and v is speed in m/s. The unit is joule (J).

Potential energy is stored energy due to an object's position or shape. For example, water stored in a dam at a height has gravitational potential energy. The formula for gravitational potential energy is PE = mgh, where m is mass, g is acceleration due to gravity (9.8 m/s²), and h is height above the ground. A stretched rubber band or a compressed spring also has potential energy due to its configuration.

In the NCERT textbook, the example of a moving car and a stretched bow illustrates these energies. Kinetic energy can be converted into potential energy and vice versa. For instance, when a ball is thrown upwards, its kinetic energy decreases as it rises and potential energy increases, and at the highest point, all kinetic energy becomes potential energy.

Points to remember

  1. Kinetic energy (KE) = ½ mv², where m is mass in kg and v is speed in m/s. Unit: joule (J).
  2. Potential energy (PE) = mgh for gravitational potential energy, where m is mass, g = 9.8 m/s², h is height. Unit: joule (J).
  3. Kinetic energy is due to motion; potential energy is due to position or configuration.
  4. Both are scalar quantities and have the same unit, joule (J).
  5. Energy can be transformed from one form to another; the total energy remains conserved.

Exam tip

Always write the formula with its SI unit, e.g., KE = ½ mv² (unit: J), and use the correct symbols (m for mass, v for speed, h for height).

Questions students also ask

What is the SI unit of energy?
The SI unit of energy is the joule (J). One joule is the energy possessed by a body when a force of one newton moves it through a distance of one metre.
Can an object have both kinetic and potential energy?
Yes, an object can have both at the same time. For example, a flying bird has kinetic energy due to its motion and potential energy due to its height above the ground.
What is the difference between kinetic and potential energy?
Kinetic energy is due to motion, while potential energy is due to position or configuration. Kinetic energy depends on speed and mass; potential energy depends on height (or configuration) and mass.

Chapter labels help you find the topic in your textbook. Check your own edition: chapter names and order can change. These are Know.it explanations, not official NCERT solutions.