SIMPLE MACHINES 9.2

Chapter Nine: Simple Machines

 9.1 Types of Simple Machines  9.2 Mechanical Advantage  9.3 Levers in the Human Body

Chapter 9.2 Learning Goals  Define mechanical advantage in terms of input and output forces.

 Classify levers as first, second, or third class.

 Evaluate the mechanical advantage of different simple machines.

9.2 Mechanical advantage

 Machines multiply forces.

 One person could lift an elephant—quite a heavy load—with a properly designed system of ropes and pulleys!

9.2 Mechanical advantage



Mechanical advantage

is the ratio of output force to input force.

Output force (N) mechanical advantage Input force (N) MA = F o F i

9.2 The Lever

 A

lever

includes a stiff structure (the lever) that rotates around a fixed point called the

fulcrum.

9.2 The Lever

 Levers are useful because you can arrange the fulcrum and the

input arm

and

output arm

to adjust the mechanical advantage of the lever.

9.2 The Lever

 Each class of levers is defined by the location of the input and output forces relative to the fulcrum.

9.2 Gears

 Many machines require that rotating motion be transmitted from one place to another.



Gears

change

force

and

speed

.

9.2 Designing Gear Machines

 The

gear ratio

to input turns.

is the ratio of output turns  You can predict how force and speed are affected when gears turn by knowing the number of teeth for each gear.

Turns of output gear Turns of input gear T T i o = N N i o Number of teeth on input gear Number of teeth on input gear

9.2 Tension

 Ropes and strings carry along their length.

tension

forces

9.2 Ramps

 A

ramp

is a simple machine that allows you to raise a heavy object with less force than you would need to lift it straight up.

 The mechanical advantage of a ramp is the ramp length divided by the height of the ramp.

9.2 Screws

 A

screw

is a rotating ramp.

 You find the mechanical advantage of a screw by dividing its circumference by the

lead .

9.2 Screws

 A

wedge

is like a ramp that can work while in motion (a ramp is always stationary).  A wedge has a side that slopes down to a thin edge.  The mechanical advantage for a wedge is inversely related to the size of the wedge angle.

9.2 Wheel and axle

 A

wheel

rotates around a rod called an

axle.

 The mechanical advantage is the ratio of the radius of the wheel to the radius of the axle.

 The wheel and axle move together to move or lift loads.

Solving Problems  A crowbar is a type of lever that you use to pull a nail out of a piece of wood.

 If the handle of a crowbar is 40 centimeters and the foot is 2 centimeters, what is its mechanical advantage?

Solving Problems 1.

 Looking for: …mechanical advantage of lever 2.

 Given …input arm = 40 cm; output arm = 2 cm 3.

 Relationships: M.A. = Length of input arm Length of output arm 4.

 Solution M.A. = 40 cm ÷ 2 cm = 20