Motion | Class 9 Science Notes

7.6 Simple Machines

Simple machines are devices that help us perform tasks more easily by changing the magnitude or direction of the force applied. Although they do not reduce the total work done, they make tasks more convenient by allowing us to apply smaller forces over larger distances or change force directions.

The force applied to a machine is called the effort, and the force to be overcome is called the load. Mechanical advantage (MA) is defined as the ratio of load to effort:

MA = load / effort

Three common simple machines are studied: pulleys, inclined planes, and levers.

A pulley is a wheel with a groove for a rope. A fixed pulley changes the direction of the applied force, making it easier to pull down rather than lift up. Its mechanical advantage is 1 since effort equals load. Movable pulleys or pulley systems can have MA greater than 1, allowing lifting heavier loads with smaller effort.

An inclined plane is a sloped surface that allows raising a heavy object to a height by pushing it along the slope. The effort needed is less than lifting vertically but must be applied over a longer distance. The mechanical advantage of an inclined plane is the ratio of its length to its height (MA = L/h).

A lever is a rigid bar that rotates about a fulcrum. By applying effort at one end, a larger load can be lifted at the other end. The mechanical advantage is the ratio of effort arm length to load arm length (MA = effort arm / load arm). Levers are classified into three types based on the relative positions of fulcrum, load, and effort.

Activities demonstrate these principles, such as using a spring balance to measure forces on an inclined plane, and balancing coins on a scale acting as a lever.

📊 Diagram: Fig. 7.23: Pulley; Fig. 7.24: Pulling up a load (a) directly, and (b) using a pulley; Fig. 7.25: A system of pulleys; Fig. 7.26: A box being (a) lifted vertically up, and (b) pushed up the ramp; Fig. 7.27: Measuring the force required to pull up a cart along an inclined plank of (a) smaller length, and (b) larger length; Fig. 7.28: A load being lifted up (a) vertically, (b) along an inclined plane, and (c) along an inclined plane of larger length; Fig. 7.31: Lifting a heavier object with lighter object; Fig. 7.32: A lever used to lift a heavy rock; Fig. 7.33: Balancing cups hung on a scale

🧪 Activity: Activity 7.3: Experiment with a cart and spring balance on an inclined plane to measure force required; Activity 7.4: Investigate lifting a heavy object with a lever; Activity 7.5: Experiment with a beam balance to understand lever principle.

🔗 Connection: This section concludes the chapter by linking work, energy, and machines, setting foundation for further studies in mechanics and energy.