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ID: 2037880 • Letter: W
Question
WARNING: Do not follow the links if you want to avoid spoilers for the Star Wars Saga.
13) You are planning on hanging the famous Wookie Lisa in a turbolift. The cables you have available can each support 100 N of tension. The painting's mass is 60 kg. In the event of an emergency, the turbolift is designed to brake at a rate of 4g, where g corresponds to Earth gravity. In order to avoid damage to the painting, you want to use enough cables so that no one cable exerts a force greater than 70 N on the painting. On the other hand, Chancellor Palpatine himself, citing the clone army budget, has demanded that you use the minimum number of cables. How many cables do you use?
Answer: Last Answer: 23 Not yet correct, tries 1/25
Hint: How is the amount of force exerted by a cable on the painting related to the amount of force the painting exerts on the cable? Consider the case of an emergency brake and how this affects the apparent weight (not the actual weight) of the painting. Use free body diagrams to help work out the relationship between the forces exerted by the cables, the weight of the painting, and the acceleration of the lift during an emergency stop.
Explanation / Answer
In case of emergency, the total force that acts on the painting downwards is the sum of its weight and the pseudo force, which should be counter balanced by the tensions in the cables
F = mg + ma
= mg + m*4g
= 5mg = 5*60*9.81
F = 2943 N
Since each cable can exert 70 N
the number of cables used will be, = 2943/70 = 42
hence,
minimun number of cables required are 42
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