Work, Energy, and Simple Machines
NCERT Class 9 Science • Chapter 7
Quick Answer
This Class 9 physics chapter defines work done by a force, kinetic and potential energy, the law of conservation of energy, and power. It also introduces simple machines such as levers, pulleys, and inclined planes, explaining mechanical advantage and how machines make work easier without creating energy.
↓ Free PDF, NCERT solutions, summary, key terms, FAQs below.
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Key Terms
- Work
- Work is done when a force moves an object in the direction of the force. It equals force multiplied by displacement and is measured in joules.
- Kinetic Energy
- The energy an object possesses due to its motion, equal to one half of mass times velocity squared.
- Potential Energy
- The stored energy an object has due to its position or configuration, such as a raised object having gravitational potential energy.
- Conservation of Energy
- The principle that energy cannot be created or destroyed but only changed from one form to another, keeping the total constant.
- Power
- The rate at which work is done or energy is transferred, calculated as work divided by time and measured in watts.
- Simple Machine
- A basic device such as a lever, pulley, or inclined plane that makes work easier by changing the size or direction of a force.
- Mechanical Advantage
- The ratio of the output force a machine provides to the input force applied, showing how much the machine multiplies effort.
Frequently Asked Questions
When is work said to be done in physics?▾
In physics, work is done only when a force acts on an object and moves it in the direction of the force. If there is no displacement, or the motion is perpendicular to the force, no work is done.
What is the difference between kinetic and potential energy?▾
Kinetic energy is the energy of a moving object and depends on its mass and speed. Potential energy is stored energy due to position or shape, such as a stretched spring or a raised weight.
What is the law of conservation of energy?▾
The law of conservation of energy states that energy can neither be created nor destroyed; it can only change from one form to another. The total amount of energy in an isolated system stays constant.
How do simple machines make work easier?▾
Simple machines such as levers, pulleys, and inclined planes make work easier by changing the direction or magnitude of the applied force. They do not reduce the total work but let us use a smaller effort over a longer distance.
What is mechanical advantage?▾
Mechanical advantage is how many times a machine multiplies the input force, found by dividing the output force by the input force. A higher mechanical advantage means less effort is needed to overcome a load.
What is the difference between work and power?▾
Work measures the total energy transferred when a force moves an object, while power measures how quickly that work is done. Power equals work divided by the time taken.
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