Machines - Ch. 7

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Machines - Ch. 7. I. Introduction to Machines (p.180-183) Machines Force Work Mechanical Advantage. A. Machines. Machine device that makes work easier changes the size and/or direction of the exerted force. B. Force. Effort Force (F e ) force applied to the machine

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Machines - Ch. 7

I. Introduction to Machines

(p.180-183)

Machines

Force

Work

A. Machines
• Machine
• device that makes work easier
• changes the size and/or direction of the exerted force
B. Force
• Effort Force (Fe)
• force applied to the machine
• “what you do”
• Resistance Force (Fr)
• force applied by the machine
• “what the machine does”
C. Work
• Work Input (Win)
• work done on a machine

Win = Fe × de

• Work Output (Wout)
• work done by a machine

Wout = Fr × dr

C. Work
• Conservation of Energy
• can never get more work out than you put in
• trade-off between force and distance

Win = Wout

Fe × de = Fr × dr

C. Work
• In an ideal machine...

Win = Wout

• But in the real world…
• some energy is lost as friction

Win > Wout

• number of times a machine increases the effort force
• MA > 1 : force is increased
• MA < 1 : distance is increased
• MA = 1 : only direction is changed

Fr

Fe

MA

• A worker applies an effort force of 20 N to open a window with a resistance force of 500 N. What is the crowbar’s MA?

GIVEN:

Fe = 20 N

Fr = 500 N

MA = ?

WORK:

MA = Fr ÷ Fe

MA = (500 N) ÷ (20 N)

MA = 25

Fr

Fe

MA

• Find the effort force needed to lift a 2000 N rock using a jack with a mechanical advantage of 10.

GIVEN:

Fe = ?

Fr = 2000 N

MA = 10

WORK:

Fe = Fr ÷ MA

Fe = (2000 N) ÷ (10)

Fe = 200 N