NCERT Solutions for Class 9th Science Chapter 7 Work, Energy, and Simple Machines

About this chapter

Work done by a constant force = force × displacement in the direction of the force, W = F × s. Its SI unit is the joule: 1 J = 1 N × 1 m = 1 kg m 2 s –2 . Work is zero when F = 0, when s = 0, or when the force is perpendicular to the displacement. Work is positive when the displacement is along the force and negative when it opposes the force. Work has no direction — only a sign. For a force that varies, the work done is the area under the force–displacement graph . Work–energy theorem: work done on an object (or system) = change in its energy. This holds even when the forces are not constant, and it is the tool that gives every other formula in the chapter. Kinetic energy K = ½mv 2 — the energy of motion. Gravitational potential energy U = mgh near the Earth's surface — the energy of posi

• Chapter opener
• .1 Work Done by a Constant Force
• .2 The Work-Energy Theorem
• .4.1 Kinetic energy
• .4.2 Potential energy
• .4.3 Conservation of mechanical energy
• .6.2 Inclined plane
• .6.3 Lever
• End-of-chapter exercise
• Project work