Machines - Ch. 7

1
Machines - Ch. 7

• III. Using Machines
• (p.180-183)
• Compound Machines
• Efficiency
• Power

2
A. Compound Machines

• Compound Machine
• combination of 2 or more simple machines

3
A. Compound Machines

• Rube Goldberg Machine

Rube Goldberg walks in his sleep, strolls through
a cactus field in his bare feet, and screams out
an idea for self-operating napkin As you raise
spoon of soup (A) to your mouth it pulls string
(B), thereby jerking ladle (C) which throws
cracker (D) past parrot (E). Parrot jumps after
cracker and perch (F) tilts, upsetting seeds (G)
into pail (H). Extra weight in pail pulls cord
(I), which opens and lights automatic cigar
lighter (J), setting off sky-rocket (K) which
causes sickle (L) to cut string (M) and allow
pendulum with attached napkin to swing back and
forth thereby wiping off your chin. After the
meal, substitute a harmonica for the napkin and
you'll be able to entertain the guests with a
little music.
4
B. Efficiency

• Efficiency
• measure of how completely work input is converted
  to work output

• always less than 100 due to friction

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B. Efficiency

• A worker exerts a force of 500 N to push a 1500 N
  sofa 4.0 m along a ramp that is 1.0 m high. What
  is the ramps efficiency?

GIVEN Fe 500 N de 4.0 m Fr 1500 N dr 1.0
m
WORK Win (500N)(4.0m) 2000 J Wout
(1500N)(1.0m) 1500 J E 1500 J 100
2000 J E 75
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C. Power

• Power
• rate at which work is done
• measured in watts (W)

P power (W) W work (J) t time (s)
7
C. Power

• A figure skater lifts his partner, who weighs 450
  N, 1.0 m in 3.0 s. How much power is required?

GIVEN F 450 N d 1.5 m t 3.0 s
WORK P W t W Fd W (450 N)(1.5 m) 675
J P 675 J 3.0 s P 225 W