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# Chapter 9 - Simple Machines - PowerPoint PPT Presentation

Chapter 9 - Simple Machines. 9.2 Mechanical Advantage pp. 210-216. 9.2 Mechanical advantage. Mechanical advantage is the ratio of output force to input force. 9.2 Mechanical advantage. Machines multiply forces.

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### Chapter 9 - Simple Machines

pp. 210-216

• Mechanical advantageis the ratio of output force to input force.

• Machines multiply forces.

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

Output force (N)

MA = Fo

Fi

mechanical

Input force (N)

• A lever includes a stiff structure (the lever) that rotates around a fixed point called the fulcrum.

• Levers are useful because you can arrange the fulcrum and the input arm and output arm to adjust the mechanical advantage of the lever.

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

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

• Gears change force and speed.

• The gear ratio is the ratio of output turns to input turns.

• You can predict how force and speed are affected when gears turn by knowing the number of teeth for each gear.

Number of teeth

on input gear

To=Ni

Ti No

Turns of output gear

Turns of input gear

Number of teeth

on output gear

• Ropes and strings carry tensionforces along their length.

• If the rope is not moving, its tension is equal to the force pulling on each end.

• 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.

• A screwis a rotating ramp.

• You find the mechanical advantage of a screw by dividing its circumference by the lead.

• A wedgeis 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.

• 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.

• 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?

• Looking for:

• Given

• …input arm = 40 cm; output arm = 2 cm

• Relationships:

• M.A. = Length of input arm

Length of output arm

• Solution

• M.A. = 40 cm ÷ 2 cm = 20

• Complete “Your Turn” problems on p. 215

• Complete Skill Sheet