Understanding the 3rd Law of Motion: Solving a Common Homework Problem

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SUMMARY

The discussion centers on the application of Newton's 3rd Law of Motion in a practical scenario involving a box tied to a rope. The key conclusion is that the movement occurs due to the difference in mass and frictional forces; the person pulling the rope has greater mass and can exert more force, allowing the box to move. The equations F12 = -F21 and F = ma illustrate that while forces are equal and opposite, the acceleration of the box is greater due to its lower mass. Additionally, the concept of friction is crucial, as the person must dig in their feet to generate enough resistance to overcome the box's friction with the ground.

PREREQUISITES
  • Understanding of Newton's Laws of Motion
  • Basic knowledge of force, mass, and acceleration (F = ma)
  • Familiarity with friction and its role in motion
  • Concept of equal and opposite reactions (F12 = -F21)
NEXT STEPS
  • Study the implications of Newton's 3rd Law in various physical scenarios
  • Explore the relationship between mass, force, and acceleration in detail
  • Investigate the effects of friction on motion in different environments
  • Learn about motion on frictionless surfaces and its theoretical implications
USEFUL FOR

Students studying physics, educators teaching Newtonian mechanics, and anyone interested in understanding the principles of motion and force interactions.

harp AP 2010
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Homework Statement


If I pull forward on a rope tied to a box, and the box pulls back equally, how do we move at all?

Homework Equations


I think I'm over thinking this, but if my actions have equal and opposite reactions how does anything happen?

The Attempt at a Solution

 
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well, you move because of your mass, since it is greater than the box the box moves..

3rd law says F12 = -F21

and F= ma

so you see that the more mass you have the more force you exert, overwhelming the opposite force of the box

edit: hmm..i think I'm wrong i think it's because of the acceleration, since a = F / m , the more mass you have the less you accelerate, so the box accelerates more and it moves..but I'm not an expert on this..
 
You manage to move the box by digging in your feet with more resistance than the box can muster from its friction with the ground. Imagine doing this on a frictionless surface (ice rink, say). You would just close the distance between you and the box unless you turn your skates round to obtain purchase on the ice.
 

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