Why weight on an incline is different?

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    Incline Weight
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Discussion Overview

The discussion revolves around the concept of weight and how it is perceived differently when an object is placed on an incline versus a horizontal plane. Participants explore the implications of gravitational force components in these scenarios, particularly focusing on the calculations of weight and normal force.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant presents a scenario involving a box and a block, questioning why the weight appears different on an incline compared to a horizontal plane.
  • Another participant clarifies that weight is defined as mg, but the force along the incline is represented as mgsin(θ) due to the angle of the incline.
  • A participant expresses confusion over the terminology used in a test question, debating whether weight should be considered mg or mgcos(θ) in the context of normal force.
  • It is noted that the term "weight" typically refers to the gravitational force acting downwards, while forces along the incline require different considerations.
  • A participant explains that the components of gravitational force are m g cos(θ) for the normal force and m g sin(θ) for the force parallel to the incline, emphasizing that the total gravitational force remains mg.
  • Another participant suggests that the teacher's phrasing of questions may lead to misunderstandings regarding the definitions of weight and force.
  • One participant mentions that there are errors in the initial post, indicating that some claims may need further clarification or correction.

Areas of Agreement / Disagreement

Participants express differing views on the definitions and implications of weight in various contexts, particularly regarding the distinction between weight and force along an incline. There is no consensus on how to interpret the teacher's question or the terminology used.

Contextual Notes

Some participants highlight potential misunderstandings in the definitions of weight and force, as well as the implications of using different angles in calculations. There is also mention of errors in earlier posts that may affect the clarity of the discussion.

Xeneize
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Hey guys, first post here...
So, I'm having an issue comprehending this topic:
Let's say you have a box which weighs 3 N, and you place a 7 kg block on top of it. So, since the block is laying on a horizontal plane, the weight is mg, or 6*10=60 N. Total weight=60+3=63

Let's say you have an inclined box 45 degrees to the ground (held by a stick of negligible mass, or whatever... just pretend that it is inclined) that weighs 3 N and you place a 6 kg block on top of it, and the friction causes it to remain at rest. My teacher said that the weight here is mgsin45, because Normal=mgsin45 at this case. So, (6)(10)(2...)=120ish N. Total weight=120+3=123.

What I don't understand, is why isn't the weight in BOTH cases mg? Because, if you put both systems each in a separate balance, the weight will be 63 N for both, amirite? The masses certainly don't change, you have the SAME box and the SAME block, except they are at different angles.

Por favor clarify :D
 
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grams are a unit of weight. Edit (opps a unit of MASS, not weight)

(mass)(gravitational acceleration) is measured in Newtons which is a unit of force. F=ma

The force along the plane is mgsin45 since the plane is at a 45 degree angle to the direction of force.

So the force in both cases is mg, unless your measuring the force only along the plane.
 
Last edited:
I guess I understand that, but if a question asks, what is the weight, then it is mg for both cases right? I just have a point of contention on a test which I want to argue, and I put mg but my teacher said it was mgcostheta

EDIT: My bad on the first post, my teacher says that it is mgcostheta (the y component of F which also equals the Normal force.), not mgsintheta.
 
Oh yes, if the question asks only "What is the weight" it is mg. That is almost like a trick question. However if the question is asking what the force is along the plane then it is not simply mg.

The word weight is usually only used for something pressing straight down due to being in a gravitational force. Like measuring something on a scale, a rocket on a platform, or standing on a scale.
 
Hopefully my teacher will understand our logic :P Thanks a bunch
 
The component of gravitational force normal to the plane is m g cos(θ), the component of gravitational force parallel to the plane is m g sin(θ), the vector sum of these forces is m g. If the block and the box are not sliding (no vertical component of acceleration), then the total force is m g, regardless of the angle.
 
Xeneize said:
Hopefully my teacher will understand our logic :P Thanks a bunch

I would approach it as a simple misunderstanding, and that you thought it might be a trick question. Let he/she come to the conclusion that your brighter then the average, don't go in holding the attitude that you are.
 
If you put a scale on the incline then the weight shown would indeed be different. However, the scale would not be showing the weight of the box, but just a component of that weight.

So...yeah, I think your teacher needs to reword his questions in the future. Perhaps he
may argue "that's not what I meant", but there should be no question as to what he meant.

By the way, there are a lot of errors in your first post...
 

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