Class 9 Science - CBSE
Work Energy and Simple Machines
The Class 9 CBSE Science chapter 'Work, Energy, and Simple Machines' explores the fundamental physical concepts of doing work, the different forms of energy, and how simple tools make our tasks easier. You will learn the scientific definition of work (where force and displacement must be in the same direction), study the laws of conservation of energy involving kinetic and potential energy, and understand how levers, pulleys, and inclined planes function as simple machines. This chapter is a crucial building block for Class 11 Physics and regularly features in CBSE board exam numericals and conceptual questions.
Start Learning FreeKey Concepts
Work
In physics, work is done only when a force applied on an object causes it to displace in the direction of the force. Formula: W = F × s × cos(theta).
Kinetic Energy
The energy possessed by an object due to its motion. Faster moving objects or heavier objects have more kinetic energy.
Potential Energy
The energy stored in an object due to its position, state, or configuration, such as an object raised to a height.
Law of Conservation of Energy
Energy can neither be created nor destroyed; it can only be transformed from one form to another, keeping the total energy constant.
Simple Machines
Mechanical devices like levers, pulleys, and inclined planes that make work easier by multiplying force or changing its direction.
Important Formulas
Board Exam Info
In the CBSE Class 9 Science exam, this chapter typically carries around 6 to 8 marks. Expect a mix of 1-mark objective questions, 2-mark conceptual reasons, and 3-mark numerical problems based on work, kinetic energy, potential energy, and power.
Frequently Asked Questions
Because the force of gravity acts vertically downwards while the displacement is horizontal, making the angle between force and displacement 90 degrees, so W = Fs cos(90°) = 0.
Because the force applied by the coolie is upward against gravity, but the displacement is horizontal, making the angle between force and displacement 90 degrees.
Can an object have momentum when its mechanical energy is zero?
No, because if mechanical energy is zero, kinetic energy is zero (since potential energy cannot be negative relative to the ground), meaning velocity is zero, and therefore momentum is also zero.
How does a simple machine make our work easier if it does not save work?
A simple machine does not reduce the total amount of work needed, but it reduces the effort force required by increasing the distance over which the force is applied.
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