- #1
hussianm
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Do weight and normal contact force always act in equal and opposite directions?
Do weight and normal contact force always have equal and opposite effects?
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Homework Help Overview
The discussion revolves around the relationship between weight and normal contact force, particularly whether they always act in equal and opposite directions. Participants explore concepts related to Newton's laws of motion in the context of different surfaces and scenarios.
Discussion Character
- Conceptual clarification, Assumption checking
Approaches and Questions Raised
- Participants question the conditions under which weight and normal force are equal and opposite, particularly in different orientations of surfaces. Some explore the implications of Newton's third law and the necessity of contact forces in relation to weight.
Discussion Status
The discussion includes various interpretations of the relationship between weight and normal force. Some participants provide examples to illustrate their points, while others seek clarification on specific scenarios, indicating a productive exchange of ideas without a definitive consensus.
Contextual Notes
There is an emphasis on understanding the conditions under which normal forces arise, particularly in relation to objects on inclined surfaces versus horizontal surfaces. Participants are also considering the implications of Newton's laws in these contexts.
- #2
berkeman
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hussianm said:
Welcome to the PF.
What do you think, and why?
- #3
hussianm
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Well I saw this question: Newton's third law tells us that... One of the options was that weight and normal contact force are always equal and opposite and the correct answer was forces always arise in pairs. The other option were wrong :pberkeman said:Welcome to the PF.
What do you think, and why?
The correct option makes perfect sense but I wanted to know why the other statement is not correct.. Is it because there is not necessarily a contact force when there is weight acting on a body?
- #4
HallsofIvy
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If an object is sitting on a horizontal surface, like a table or a floor, then its weight is a force downward and normal force upward. Since the object is not moving, those two must be the same (and in opposite directions).
If on the other hand, an object is sitting on a slope, on a slanting surface, gravity is downward, while the normal force is NOT upward. It is "normal" to the surface. In order that the object not move down the slope, there must be a friction force, tangent to the slope. The sum of those two vectors must be equal (and opposite) to the weight force.
If on the other hand, an object is sitting on a slope, on a slanting surface, gravity is downward, while the normal force is NOT upward. It is "normal" to the surface. In order that the object not move down the slope, there must be a friction force, tangent to the slope. The sum of those two vectors must be equal (and opposite) to the weight force.
- #5
hussianm
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oh! i see. i understand now. thank you for your helpHallsofIvy said:
- #6
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HallsofIvy's response was an excellent example of Newton's first law. Newton's third law tells you that if the object exerts a normal force N on the table, then the table must exert an equal normal force N on the object, in the opposite direction. This is the force pair of the normal force. It is important not to confuse his first law with his third.
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