Case Study Questions for Class 9 Science Chapter 7 Work, Energy, and Simple Machines (Exploration Book) 2026-27

As an object falls or moves due to gravitational force, its potential energy decreases while its kinetic energy increases, or vice versa. However, the sum of the kinetic energy and the potential energy of the object, called its mechanical energy, remains constant if no other external forces act on it. This principle is called the conservation of mechanical energy.

For example, when a simple pendulum oscillates, its energy continuously changes forms. At the extreme positions, the pendulum bob reaches its maximum height and has only potential energy because it momentarily stops. As it moves towards the lowest point, its potential energy decreases and kinetic energy increases, reaching a maximum at the mean position. Throughout this motion, the total mechanical energy remains constant, although in real life, a pendulum eventually stops due to energy loss from friction and air resistance.

In everyday life, we often need to do work against forces, such as lifting or moving heavy objects. Devices that make these tasks easier by changing the magnitude or direction of the applied force are called simple machines. A lever is a rigid bar that can rotate about a fixed point. It has three main parts: the fulcrum (the fixed point), the load (the force to be overcome), and the effort (the force applied).

The distance from the load to the fulcrum is called the load arm, and the distance from the effort to the fulcrum is called the effort arm. By increasing the effort arm, a small effort can lift a much heavier load, providing a mechanical advantage. Levers are classified into three types depending upon the relative positions of the effort, fulcrum, and load. For instance, a seesaw has the fulcrum in between, while a lemon squeezer has the load in between.