Elementary questions relating to Newton's laws of motion

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Discussion Overview

The discussion revolves around the implications of Newton's laws of motion, particularly in the context of a cannonball striking a wall. Participants explore the nature of forces involved in this interaction, including the equal and opposite forces described by Newton's third law, the conditions under which the wall breaks, and the potential for the cannonball to rebound.

Discussion Character

  • Exploratory, Technical explanation, Conceptual clarification, Debate/contested

Main Points Raised

  • Some participants question whether the wall exerts an equal and opposite force on the cannonball when it breaks.
  • There is a suggestion that the force exerted by the cannonball on the wall might only be the amount necessary to break it, rather than the total force it could exert.
  • One participant hypothesizes about the possibility of the wall breaking while still being able to cause the cannonball to rebound, indicating a need for sufficient strength in the wall.
  • Another participant introduces the concept of impulse, suggesting that the duration of the force applied by the cannonball affects the momentum transferred to the wall.
  • It is noted that once the wall breaks, the situation becomes a work problem involving force and displacement, implying that the wall does exert a reaction force but may not have enough impulse to stop the cannonball.

Areas of Agreement / Disagreement

Participants express varying views on the nature of the forces involved and the conditions under which the wall breaks. No consensus is reached, and multiple competing perspectives remain throughout the discussion.

Contextual Notes

Participants do not fully resolve the implications of impulse and momentum in this scenario, leaving assumptions about the strength of the wall and the specifics of the forces involved open to interpretation.

chipotleaway
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i) If a wall breaks when it gets hit by a cannonball, did the wall exert an equal and opposite force on the cannonball?

ii) Would the force exerted by the cannonball on the wall only be the amount that was needed to break it?

iii) Hypothetically, is it possible for the wall to break but also cause the cannonball to rebound?
 
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chipotleaway said:
i) If a wall breaks when it gets hit by a cannonball, did the wall exert an equal and opposite force on the cannonball?

ii) Would the force exerted by the cannonball on the wall only be the amount that was needed to break it?

iii) Hypothetically, is it possible for the wall to break but also cause the cannonball to rebound?
Good questions. What do you think?
 
Doc Al said:
Good questions. What do you think?

i) I think it does and

ii) I also think that the reaction force from the wall would be equal to the force needed to break it. Like if you were to drop a bowling ball on a table, the table wouldn't feel the entire weight of the ball before it broke (assuming the strength is less than the weight of the ball)

iii) Hmm...the wall would have to have sufficient strength to exert a reaction force big enough to change the momentum of the cannonball...I'll try to work out the details later, heh.
 
To answer your question, I would think about impulse. The longer you exert a force, the greater the momentum. So to break the wall, the cannonball need to exert some amount of force for a period of time. Then, once the wall break and rocks flies in every directions, it's a work problem (force times displacement). Hence, the wall does exert the same force back but not enough impulse to stop the cannonball.
 

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