Simple Machines
ICSE Class 6 · Physics · Chapter 4
What this chapter covers
- Simple machine: basic idea
- Mechanical advantage
- Types of simple machines
- Orders (classes) of levers
- Principle of lever
Key points
- A machine is a device that helps us do work more easily; it does not create energy.
- A simple machine changes the size or the direction of the applied force.
- Effort is the force applied; load is what the machine moves; fulcrum is the fixed point a lever turns about.
- Mechanical advantage = Load ÷ Effort, and it has no unit.
- The six simple machines are the lever, wheel and axle, pulley, inclined plane, wedge and screw.
- First-order lever: fulcrum in the middle. Second-order: load in the middle. Third-order: effort in the middle.
- A second-order lever always has a mechanical advantage more than 1; a third-order lever always less than 1.
- Principle of lever: Load × Load arm = Effort × Effort arm.
- Mechanical advantage of a lever = Effort arm ÷ Load arm.
Questions answered
What is a simple machine? Name the six simple machines.
A simple machine is a machine with very few parts that changes the size or the direction of the force we apply. The six are the lever, the wheel and axle, the pulley, the inclined plane, the wedge and the screw.
Name the simple machine in each: (a) a doorknob, (b) an axe, (c) a ramp, (d) a bottle cap.
(a) Wheel and axle, (b) wedge, (c) inclined plane, (d) screw.
Define mechanical advantage.
Mechanical advantage is the number of times a machine multiplies the effort. It equals load divided by effort.
A machine lifts a load of 800 N with an effort of 200 N. Find its mechanical advantage.
800 ÷ 200 = 4.
In which order of lever is the load in the middle? Give two examples.
In a lever of the second order. Examples: a wheelbarrow and a nutcracker.
Why does a pair of tongs not save effort, and why is it still useful?
Tongs are a third-order lever, so the load arm is longer than the effort arm and the mechanical advantage is less than 1. They are still useful because they keep your fingers away from hot food and let you pick things up quickly.
State the principle of lever.
A lever balances when the load multiplied by the load arm equals the effort multiplied by the effort arm.
A load of 400 N sits 1 m from the fulcrum of a lever. The effort is applied 4 m from the fulcrum. What effort is needed?
400 × 1 = Effort × 4, so Effort = 400 ÷ 4 = 100 N.