More fun with Newton\'s 3rd Law/and elevators Two blocks, A and B, are in an ele
ID: 1280086 • Letter: M
Question
More fun with Newton's 3rd Law/and elevators Two blocks, A and B, are in an elevator as shown. The mass of block A is twice the mass of block B. The elevator is moving downward at a constant speed. In the specs provided below, draw and label separate force diagrams for the blocks. Rank the forces on the blocks according to magnitude, from largest to smallest. Explain your reasoning and how you applied Newton's second or third law. Is the magnitude of the net force on block A greater than, less than, or equal to that on block B? Explain your choice.Explanation / Answer
i) Block A has a normal force upwards (this is the force due to block B) and gravity downwards. There are no other forces present.
Block B has a normal force upwards (due to the ground), gravity downwards and the force that block A puts on it downwards.
ii) First thing is first. We know the mass of block A is larger than the mass of block B. So the weight of A is larger than the weight of B. Since the normal force on A has to cancel the weight of A, the normal force equals the weight of A. But the normal force on A is due to block B (it's how hard block B presses against A). This means, by Newton's 3rd law, that the force that block A presses against block B has to be equal to the normal force on A, and so is equal to the weight of A. Finally, the normal force on B has to balance out both the weight of B and the force that A puts on B, which means that it has to be bigger than both. So, it's bigger than the weight of A (since it is bigger than the force that A puts on B, which is equal to the weight of A, as explained before). Putting this all together:
Normal on B > Weight of A = Normal on A = Force of A on B > Weight of B
iii) They are both zero. This is because both blocks are moving at a constant speed. This means the magnitude of both net forces are the same.
Hope this helps.
Related Questions
drjack9650@gmail.com
Navigate
Integrity-first tutoring: explanations and feedback only — we do not complete graded work. Learn more.