1. III. Using Machines (p.180-183) Compound Machines Efficiency Power
Machines - Ch. 7
III. Using Machines
(p )
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

5. B. Efficiency Fe = 500 N Win = (500N)(4.0m) = 2000 J de = 4.0 m
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 ramp’s 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 = J × 100%
2000 J
E = 75%
4.0m
500N
1.0m
1500N

6. 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 P W t F = 450 N P = W ÷ t d = 1.5 m W = F·d t = 3.0 s
A figure skater lifts his partner, who weighs 450 N, 1.0 m in 3.0 s. How much power is required? P W t
GIVEN:
F = 450 N
d = 1.5 m
t = 3.0 s
WORK:
P = W ÷ t
W = F·d
W = (450 N)(1.5 m) = 675 J
P = 675 J ÷ 3.0 s
P = 225 W