Free body diagram - Weight and normal reaction

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Discussion Overview

The discussion revolves around the concepts of weight and normal reaction forces in the context of a stationary ball on the floor. Participants explore the definitions and relationships between these forces, particularly in relation to Newton's laws of motion and the distinctions between different types of forces.

Discussion Character

  • Conceptual clarification
  • Debate/contested
  • Technical explanation

Main Points Raised

  • Some participants assert that the normal reaction force acts upwards in response to the weight of the ball acting downwards, but question the nature of this reaction.
  • Others clarify that the normal force exerted by the floor on the ball is a reaction to the force exerted by the ball on the floor, which is not the same as the weight of the ball.
  • A participant raises a question about the nature of the force when the ball is moving horizontally, suggesting that the vertical acceleration is what determines the relationship between the normal force and weight.
  • Another participant emphasizes the distinction between the normal force as a contact force and weight as a gravitational force, noting that the normal force balances the weight when the ball is stationary.
  • Clarifications are made regarding the terminology of "reaction" forces, distinguishing between constraint forces and action/reaction pairs as per Newton's third law.

Areas of Agreement / Disagreement

Participants express some agreement on the definitions of weight and normal force but exhibit disagreement on the implications of these forces in different scenarios, particularly regarding the action-reaction pairs and the nature of forces when the ball is in motion.

Contextual Notes

There are nuances in the definitions and interpretations of forces that remain unresolved, particularly concerning the terminology used and the implications of motion on force interactions.

Who May Find This Useful

This discussion may be useful for students and enthusiasts of physics seeking to understand the relationships between different types of forces, particularly in mechanics and Newtonian physics.

Jadaav
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Let's consider a stationary ball lying on the floor.

The weight of the ball acts downwards and the normal reaction acts upwards. They are of the same magnitude but not of the same type.

So normal reaction is the reaction to what, if not weight ?
 
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The "normal reaction" force of the floor on the ball is the reaction force to the normal force of the ball on the floor.

The force of the ball on the floor is not the weight of the ball. However, for a ball that is stationary and supported only by the floor, it will be equal to the weight of the ball.
 
Okay.

So if the ball is stationary, the force of the ball on the floor is equal to the weight of it but isn't that of weight; if I understood what you said. Then, what is the force ?

What if the ball is moving horizontally with a speed, u m/s on the floor ?
 
Jadaav said:
Let's consider a stationary ball lying on the floor.

The weight of the ball acts downwards and the normal reaction acts upwards. They are of the same magnitude but not of the same type.

So normal reaction is the reaction to what, if not weight ?
There are two meanings of the term "reaction" force:

(1) Since the ball does not fall through the floor, we can deduce that the floor must be exerting an upward normal force. This is often called a constraint or a "reaction" force. This term is used in engineering, not in physics! In physics it's usually just called the normal force.

(2) Per Newton's 3rd law, any two objects exert equal and opposite forces on each other. These are often called "action/reaction" pairs. That's something of an old-fashioned term; better to call those force pairs just "3rd law pairs".

Do not mix up the two meanings, or you'll just get confused.

If you're asking what's the 3rd law pair to the upward normal force exerted on the ball by the ground, then that's the downward normal force exerted on the ground by the ball.

The 3rd law pair to the weight (the gravitational pull of the Earth on the ball) is the gravitational pull of the ball on the earth.
 
Jadaav said:
So if the ball is stationary, the force of the ball on the floor is equal to the weight of it but isn't that of weight; if I understood what you said. Then, what is the force ?

What if the ball is moving horizontally with a speed, u m/s on the floor ?
What matters is whether the ball is vertically accelerating or not. Since it's just rolling along the horizontal floor, we know that the floor must be exerting an upward force enough to balance the weight of the ball.

Weight is a gravitational force; the normal force is a contact force (largely electromagnetic).
 
Thanks a bunch !

I was actually confused about the action-reaction pair and the normal force. You cleared them :)
 

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