Reviewing Class 9 Science Notes and Exploration Chapter 7 Work Energy and Simple Machines Class 9 Notes regularly helps in retaining important facts.
Class 9 Science Chapter 7 Work Energy and Simple Machines Notes
Class 9 Science Exploration Chapter 7 Notes
Class 9 Science Chapter 7 Notes – Class 9 Work Energy and Simple Machines
→ Work: Work is said to be done when a force applied on an object produces displacement in the direction of the force.
→ If a constant force F causes displacement s, then work done W = F × s. More precisely, W = Fs cosθ, where θ is the angle between force and displacement.
→ Work is maximum when force and displacement are in the same direction (θ = 0°) and zero when perpendicular (θ = 90°).
→ Work depends on the effect of force over a distance, not just force alone.
→ SI Unit of Work: The SI unit of work is joule (J). is named after the scientist James Prescott Joule. He studied how mechanical energy and thermal energy are related, and can be converted from one to the other. This helped develop a unified way to understand energy. One joule is defined as the work done when a force of 1 newton moves an object through a distance of 1 metre in the direction of the force. 1 J = 1 N m.
→ Work is a scalar quantity, meaning it has only magnitude and no direction, even though force and displacement are vectors.
→ Zero Work: Work done is zero in three situations. First, when no force is applied (F = 0), so nothing can cause energy transfer. Second, when there is no displacement (s = 0), even if a large force is applied, such as pushing a wall that does not move.
→ Third, when force is perpendicular to displacement, such as centripetal force in circular motion, where force changes direction but does not contribute to motion in that direction.
→ Positive Work: Work and displacement are in the same direction ( θ between 0° and 90°). In this case, force helps motion and increases the object’s energy.
→ Examples include pushing a cart or wheelchair forward or gravity acting on a falling object. Positive work generally increases kinetic energy.
→ Negative Work: Work is negative when force acts opposite to displacement (θ between 90° and 180°). In this case, force opposes motion and removes energy from the system. Examples include friction slowing down a moving object or a goalkeeper stopping a moving ball. Negative work reduces kinetic energy.
→ Work-Energy Theorem: The work-energy theorem states that the net work done on an object equals the change in its kinetic energy.
Wnet = ∆K = Kfinal – Kinitial. This means work is net final internal directly responsible for changing motion. If
positive work is done, kinetic energy increases; if negative work is done, kinetic energy decreases.
→ Energy: Energy is the capacity to do work. It is the ability of a system or object to cause change, especially to apply force and produce motion. Energy exists in many forms such as mechanical, heat, light, chemical, and electrical. The SI unit of energy is joule, same as work. Energy is also a scalar quantity and cannot be created or destroyed, only transformed.
→ Kinetic Energy (K): Kinetic energy is the energy possessed by a body due to its motion. It depends on mass and velocity. The formula is K =\(\frac{1}{2}\) mv2.
→ This shows that kinetic energy increases with the square of speed, so doubling speed increases kinetic energy four times. Faster and heavier objects have more kinetic energy.
→ Potential Energy (U): Potential energy is stored energy due to position or configuration. Gravitational potential energy is U = mgh, where m is mass, g is acceleration due to gravity, and h is height above reference level. It represents energy stored due to height. Other forms include elastic potential energy stored in stretched or compressed objects like springs.
→ Mechanical Energy: Mechanical energy is the sum of kinetic energy and potential energy of an object. M.E. = K.E. + P.E. It represents total energy due to motion and position. In ideal conditions without non-conservative forces like friction, mechanical energy remains constant.
→ Gravitational Potential Energy: It is the energy possessed by an object due to its position above the surface of the Earth. It is equal to the work done in lifting the object against the force of gravity.
→ Conservation of Mechanical Energy: The law of conservation of mechanical energy states that when only conservative forces (like gravity) act, total mechanical energy remains constant. As an object falls, potential energy decreases while kinetic energy increases by the same amount.
→ However, in real life, friction and air resistance convert some mechanical energy into heat, so total mechanical energy is not strictly conserved.
→ Power (P): Power is the rate of doing work or rate of energy transfer. It is given by P= W/t. A higher power means more work is done in less time. The SI unit is watt (W), where 1 watt = 1 joule per second. Power can also be expressed as P = F υ when force and velocity are in the same direction.
→ Simple Machines: Simple machines are devices that make work easier by changing the magnitude or direction of force. They do not reduce total work but help by allowing effort to be applied more conveniently. Examples include levers, pulleys, and inclined planes. They help by increasing force or changing its direction.
→ Mechanical Advantage: Mechanical advantage (MA) is the ratio of load (output force) to effort (input force). MA = Load / Effort. A machine with MA greater than 1 multiplies force, making it easier to lift or move heavy objects. When Mechanical Advantage is less than 1, the machine does not provide force gain. Instead, it provides a gain in speed or distance. Therefore, more effort is required to move the load, but the load moves faster or over a larger distance.
→ Pulley: A pulley is a grooved wheel with a rope used to lift loads. A fixed pulley changes the direction of force but has MA = 1. A movable pulley reduces effort required and provides MA greater than 1. Combined pulley systems (block and tackle) can greatly increase mechanical advantage.
→ Inclined Plane: An inclined plane is a sloping surface used to raise objects with less force. Mechanical advantage is MA = L/ h, where L is the A box being pushed up the ramp length of slope, and h is height. A longer slope reduces required effort but increases distance traveled. It trades force for distance.
→ Lever: A lever is a rigid bar that rotates around a fixed point called the fulcrum. It works on the principle of moments: effort × effort arm = load × load arm. Mechanical advantage is MA = effort arm/load arm. A longer effort arm reduces required force.
→ Classes of Levers: Class I levers have fulcrum in the middle, such as scissors or seesaws. They can multiply force or speed depending on arrangement. Class II levers have load in the middle, such as a wheelbarrow or bottle opener, and always multiply force. Class III levers have effort in the middle, such as tweezers or broom, and they increase speed but require more effort.
→ Energy Conservation in Machines: Machines do not create energy; they only transform it from one form to another or transfer it from one point to another. In ideal machines, work input equals work output. In real machines, some energy is lost due to friction, heat, and sound, so efficiency is always less than 100 percent.
Efficiency = (output work/input work) × 100%.
The post Work Energy and Simple Machines Class 9 Notes Science Chapter 7 appeared first on Learn CBSE.
📚 NCsolve - Your Global Education Partner 🌍
Empowering Students with AI-Driven Learning Solutions
Welcome to NCsolve — your trusted educational platform designed to support students worldwide. Whether you're preparing for Class 10, Class 11, or Class 12, NCsolve offers a wide range of learning resources powered by AI Education.
Our platform is committed to providing detailed solutions, effective study techniques, and reliable content to help you achieve academic success. With our AI-driven tools, you can now access personalized study guides, practice tests, and interactive learning experiences from anywhere in the world.
🔎 Why Choose NCsolve?
At NCsolve, we believe in smart learning. Our platform offers:
- ✅ AI-powered solutions for faster and accurate learning.
- ✅ Step-by-step NCERT Solutions for all subjects.
- ✅ Access to Sample Papers and Previous Year Questions.
- ✅ Detailed explanations to strengthen your concepts.
- ✅ Regular updates on exams, syllabus changes, and study tips.
- ✅ Support for students worldwide with multi-language content.
🌐 Explore Our Websites:
🔹 ncsolve.blogspot.com
🔹 ncsolve-global.blogspot.com
🔹 edu-ai.blogspot.com
📲 Connect With Us:
👍 Facebook: NCsolve
📧 Email: ncsolve@yopmail.com
😇 WHAT'S YOUR DOUBT DEAR ☕️
🌎 YOU'RE BEST 🏆