Simple Machines
Senior 2 · Physics
Simple Machines
Lesson 2 of 4
22 min
Let's begin
The wheelbarrow on a building site, the crowbar that shifts a boulder, the bottle opener at a party and even your own forearm are all levers. They are the simplest machines of all, and a single balancing rule explains every one of them.
A lever is a rigid bar that turns about a fixed point called the fulcrum (pivot). Levers are sorted into three classes by what lies in the middle:
| Class | In the middle | Examples | MA |
|---|---|---|---|
| First | Fulcrum | Crowbar, seesaw, claw hammer, pliers, scissors | Can be more or less than 1 |
| Second | Load | Wheelbarrow, bottle opener, nutcracker | Always more than 1 |
| Third | Effort | Tweezers, broom, fishing rod, human forearm | Always less than 1, but the load moves further and faster |
For a lever in balance, with friction ignored:
effort × effort arm = load × load arm
Here the effort arm is the distance from the fulcrum to the effort, and the load arm is the distance from the fulcrum to the load. This is the principle of moments. The velocity ratio of a lever is VR = effort arm ÷ load arm.
A crowbar is used to lift a rock that presses down with 1200 N at 0.10 m from the fulcrum. The worker pushes 1.5 m from the fulcrum. Ignoring friction, what effort is needed?
E × 1.5 = 1200 × 0.10, so E = 80 N. VR = 1.5 ÷ 0.10 = 15.
A wheelbarrow carries 400 N of bricks whose weight acts 0.40 m from the wheel axle (the fulcrum). The handles are 1.2 m from the axle.
Open the Balancing Act simulation. Place bricks on both sides of the plank and move them until it balances. Check that mass × distance is the same on each side.
Tip
"1-2-3 → F-L-E": in class 1 the Fulcrum is in the middle, in class 2 the Load, and in class 3 the Effort.
To make a lever easier to use, lengthen the effort arm (a longer handle) or move the load closer to the fulcrum.
Name the class of lever: (a) a bottle opener, (b) a pair of tweezers, (c) a seesaw.
Solution: (a) second class, because the load (the cap) is in the middle. (b) Third class, because the effort (your fingers) is in the middle. (c) First class, because the pivot is in the middle.
The handles of a pair of pliers are 12 cm from the pivot and the wire is cut 3.0 cm from the pivot. Find the VR, and the ideal force on the wire when you squeeze with 20 N.
Solution: VR = 12 ÷ 3.0 = 4. Force on the wire = 20 × 12 ÷ 3.0 = 80 N.
A 30 kg child sits 2.0 m from the pivot of a seesaw. Where must a 40 kg child sit to balance it?
Solution: 30 × 2.0 = 40 × d, so d = 1.5 m on the other side.
A broom is a third-class lever with MA less than 1. Why is it still useful?
Solution: Your hands move a short distance, but the brush end moves a long distance quickly, so it sweeps a wide area with each stroke.
Summary
Sources and credits
The Rwandan examples, worked examples and practice items are written by Ganzaa.
