Problem #1
What are the weight in newtons and the mass in kilograms of(a) a 5.0lb bag of sugar,
(b) a 240lb fullback, and
(c) a 1.8ton automobile? (1ton = 2000lb.)
Answer:
(a) The bag of sugar has a weight of 5.0lb. (“Pound” is a unit of force, or weight.) Then its weight in newtons is
5.0 lb = (5.0lb)(4.45N/1lb)= 22 N
Then from W = mg we calculate the mass of the bag,
m = W/g = 22 N/(9.80 m/s2) = 2.3 kg
(b) Similarly, the weight of the fullback in newtons is
240lb = (240lb)(4.45 N/1 lb) = 1070 N and then his (her) mass is
m = W/g = 1070 N/(9.80 m/s2) = 109 kg
(c) The automobile’s weight is given in tons; express it in newtons:
1.8 ton = (1.8ton)(2000lb/1 ton)(4.45N/1 lb) = 1.6 × 104 N.
Then its mass is
m = W/g = 1.6 × 104 N/(9.80 m/s2) = 1.6 × 103 kg
Problem #2
If a man weighs 875N on Earth, what would he weigh on Jupiter, where the free–fall acceleration is 25.9 m/s2?
Answer:
The weight of a mass m on the earth is W = mg where g is the free–fall acceleration on Earth. The mass of the man is:
m = W/g = 875 N/(9.80 m/s2) = 89.3 kg
His weight on Jupiter is found using gJupiter instead of g:
WJupiter = mgJupiter = (89.3 kg)(25.9 m/s2) = 2.31 × 103 N
The man’s weight on Jupiter is 2.31×103 N. (The statement of the problem is a little deceptive; Jupiter has no solid surface! The planet will indeed pull on this man with a force of 2.31×103 N, but there is no “ground” to push back!)
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