 August 06, 2020, 07:55:18 am

### AuthorTopic: Forces in an elevator  (Read 242 times) Tweet Share

0 Members and 1 Guest are viewing this topic.

• • Posts: 10
• Respect: 0 ##### Forces in an elevator
« on: May 07, 2019, 08:21:20 pm »
0
Hey guys,

Got one question that is confusing me. For part b, the solution says that the force is 18.9 N downwards, but I don't understand how to calculate that?
From my understanding, the sum of all forces on A would be 10.5N upwards. I'm assuming that the sum of all forces on A would just be its weight force plus normal force, and that the normal force A experiences would be the force that B exerts on A. Letting upwards be positive, I would then assume 10.5 = -29.4+x, where x is the normal force that A experiences, or the magnitude of the force that A exerts on B. However, this gives a final answer 39.9 N downwards, could someone please explain this?
2018: Methods 45
2019: Physics 47 | Specialist 41 | Further 49 | English 42 | Religion & Society 41
ATAR: 99.55

#### fun_jirachi

• MOTM: AUG 18
• HSC Moderator
•     • • Posts: 664
• All doom and Gloom.
• Respect: +381 ##### Re: Forces in an elevator
« Reply #1 on: May 07, 2019, 09:22:05 pm »
+2
Hi there For part b), the force due to gravity exerted on B by A is equal to (mass of A) x g ie. 3x9.8=29.4 N downwards. Now, when the elevator accelerates downwards, think of it as inducing a sort of reaction force equal to the mass of the object by the acceleration from F=ma, but in the opposite direction. (though this is oversimplified and not technically how it works). This gives you a force upwards of (mass of A) x acceleration, which is 3x3.5=10.5 N upwards. Add these up and you have the correct answer of 18.9 N down. You have the wrong answer because you're not resolving the vectors correctly ie. you've factored in the 'reaction' force, just in the wrong direction.

Hope this helps Failing everything, but I'm still Flareon up.

HSC 2018: Modern History  | 2U Maths 
HSC 2019: Physics  | Chemistry  | English Advanced  | Maths Extension 1  | Maths Extension 2 
ATAR: 99.05

UCAT: 3310 - Verbal Reasoning  | Decision Making  | Quantitative Reasoning  | Abstract Reasoning 

Subject Acceleration (2018)
UCAT Question Compilation/FAQ (2020)

•  oh my god, this whole time I thought it was accelerating upwards Thanks!