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Conversion Factors are used to change from one unit to another
arrange the conversion factor into a fraction – you choose the numerator / denominator
arrangement so that the final unit is the desired one
1kg = 2.2 lbs
How many kilograms in 150 lbs?
150 lbs x 1 kg
= 68.2 kg
2.2 lbs
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How many lbs is 75 kg?
75 kg x 2.2 lbs = 165 lbs
1 kg
(Notice we used the same conversion factor as above just inverted)
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we can also use multiple conversion factor together to make changes in units
How many seconds in 12 hours?
12 hours x 60 min x 60 sec = 43,200 sec
1 hour
1 min
Change 50 mi/hr into meters/second
given: 1 mile = 1609 meters
50 mi x 1609 meters x 1 hr x 1 min = 22.3 meters or m/s
1 hr
1 mi
60 min
60 sec
sec
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Change 80 km/hr to meters/second
80 km x 1000 meters x 1 hr x 1 min
hr
1km
60 min
60 sec
Determining an objects or persons weight in newtons
Suppose a student weighs 140 lbs. what is this weight in Newtons?
First convert lbs to kilograms
140 lbs x 1 kg = 63.6 kg
2.2 lbs
Weight in newtons is the force of gravity times an objects mass
weight = 63.6 kg x 9.8 m/s2 = 623 N
weight is a special type of force - the downward force of gravity on a person or object
Fg= mg
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What is the weight in Newtons of a 116 lb person?
116 lbs x 1 kg = 52.7 kg
2.2 lbs
weight = 52.7 kg x 9.8 m/s2 = 516 N
weight is a special type of force - the downward force of gravity on a person or object
F = mg
Two students move a 200 N desk 10 m in height from the 1st to the 2nd floor of a school.
WHICH PATH REQUIRES MORE WORK?
10 m
10 m
20 m
When doing work against gravity only the vertical distance matters.
THE PATH TAKEN DOES NOT MATTER!!!!!!,
THE AMOUNT OF WORK DONE IS THE SAME REGARDLESS OF THE PATH.
In both cases above 2000 joules of work is done
W=Fd
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How much work does a 135 lb student do when climbing a flight of stairs that are
8 meters in height and 30 meters in length? If the stairs are climbed in 4 seconds
calculate the power in Watts.
The length does not matter only the height is important for our calculation.
First convert lbs to kg
135 lbs x 1 kg = 61.4 kg
2.2 lbs
then determine weight in Newtons
weight (force) = mg
F = 61.4 kg x 9.8 m/s2 = 610 N
Next use the height of the stairway (8m) to calculate the work done
W = Fd
W = 610 N (x) 8 m = 4880 J
Now calculate power in watts
P=W
t
P = 4880 J = 1220 watts
4 sec
Finally convert watts to HP
1220 watts x
1 HP
=
746 watts
1.6 HP