Understanding Newton's Third Law: The Impact of Forces on Objects in Motion

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Discussion Overview

This discussion revolves around the interpretation and implications of Newton's Third Law of Motion, particularly in the context of forces acting on objects in motion. Participants explore various scenarios, including throwing a ball, launching a rocket, and the dynamics of projectiles, while addressing common misconceptions related to the law.

Discussion Character

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

Main Points Raised

  • One participant expresses confusion about how objects can move upward when equal and opposite forces are at play, specifically questioning the upward motion of a thrown ball and a rocket.
  • Another participant suggests that the initial velocity given to the ball and rocket allows them to move, despite the forces acting on them.
  • A participant clarifies that Newton's Third Law states that equal and opposite forces act on different objects, using examples like push-ups and punching a wall to illustrate this point.
  • Discussion includes the idea that the force exerted by a rocket on exhaust is equal and opposite to the force on the rocket, explaining the rocket's upward acceleration.
  • Participants discuss the necessity of exerting a force greater than 100N to slow a car moving at 100N, emphasizing that the opposing force acts on the road, not the car.
  • Questions arise about projectiles in the air and how opposing forces affect their motion, with some participants noting that air resistance acts on the projectile.
  • A later reply elaborates on the forces acting on the Earth and the projectile when an object is thrown, highlighting the minuscule acceleration of the Earth compared to the object.
  • Some participants humorously reflect on the clarity of Newton's original formulation of the law and the misunderstandings that arise from modern interpretations.

Areas of Agreement / Disagreement

Participants generally agree on the principle that equal and opposite forces act on different objects, but there remains some confusion and debate about the implications of this law in various scenarios, particularly regarding motion and acceleration.

Contextual Notes

Some participants highlight the importance of understanding that forces act on different bodies and that the mass of the objects involved significantly affects their motion. There are unresolved questions about the nuances of force interactions in specific scenarios.

  • #31
malu said:
both the block will move at an acceleration of 1m/sec2, i understand that but i want to know when there was a bang the what push did the bigger block exert on smaller block and why in-spite of that pushing back the block, the smaller block continues to move in the direction of bigger block why ?
When the first block hits the second, the second block pushes back with some force. But a push causes an acceleration in that direction, not necessarily motion in that direction. The first block is already moving to the right. Due to the force from the second block, it slows down. But it keeps moving to the right. (Here I assume that the collision is perfectly inelastic--that the blocks stick together.)

Perhaps you are thinking that just because there's a force acting on something, that something must move in the direction of the force? Not necessarily.
 
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  • #32
Doc Al said:
When the first block hits the second, the second block pushes back with some force. But a push causes an acceleration in that direction, not necessarily motion in that direction. The first block is already moving to the right. Due to the force from the second block, it slows down. But it keeps moving to the right. (Here I assume that the collision is perfectly inelastic--that the blocks stick together.)

Perhaps you are thinking that just because there's a force acting on something, that something must move in the direction of the force? Not necessarily.

how much maximum force with which bigger block can push on samller Block? i mean on what does that depend.
in other terms what can be the maximum push back bigger block can offer?? considering everything a ideal scenario
 
  • #33
malu said:
how much maximum force with which bigger block can push on samller Block? i mean on what does that depend.
in other terms what can be the maximum push back bigger block can offer?? considering everything a ideal scenario
The faster the speeds of the blocks change, the greater the force required. If the blocks are squishy, the time of collision might be relatively large and thus the force relatively small. There's no such thing as an 'ideal scenario'--it depends on the material.

Note that it's not simply the force that the blocks exert on each other that matters--it's also the time over which that force acts. Force X time gives you the impulse--the change in momentum.

I'm still trying to get at your real question. There's something you think doesn't quite make sense. What is it?
 
  • #34
Doc Al said:
The faster the speeds of the blocks change, the greater the force required. If the blocks are squishy, the time of collision might be relatively large and thus the force relatively small. There's no such thing as an 'ideal scenario'--it depends on the material.

Note that it's not simply the force that the blocks exert on each other that matters--it's also the time over which that force acts. Force X time gives you the impulse--the change in momentum.

I'm still trying to get at your real question. There's something you think doesn't quite make sense. What is it?
perhaps what you said is correct, but when i ask myself that question, then a simple query comes to my mind, what happens at that moment when there is an impact (considering material has no affect, let me take for time being),
1. Whether the acceleration and velocity of smaller block goes zero. and then with in that same moment (or in the just next moment )the force of 20 N again takes over and then the both block move together.
2. So my question is if the acceleration and velocity of smaller object goes zero at that moment then is it dependent on the mass of bigger object ? (that means capacity of bigger object to impart deceleration on smaller object).
3. To put it in other terms suppose if 5 kg block is moving at an velocity of 1m/sec and then how much mass is required to make the velocity zero.
 
  • #35
malu said:
perhaps what you said is correct, but when i ask myself that question, then a simple query comes to my mind, what happens at that moment when there is an impact (considering material has no affect, let me take for time being),
1. Whether the acceleration and velocity of smaller block goes zero. and then with in that same moment (or in the just next moment )the force of 20 N again takes over and then the both block move together.
2. So my question is if the acceleration and velocity of smaller object goes zero at that moment then is it dependent on the mass of bigger object ? (that means capacity of bigger object to impart deceleration on smaller object).
Why would you think the velocity of the smaller object goes to zero?
3. To put it in other terms suppose if 5 kg block is moving at an velocity of 1m/sec and then how much mass is required to make the velocity zero.
Treating this as a simple inelastic collision, there is no mass that will make the velocity equal to zero.
 
  • #36
Doc Al said:
Why would you think the velocity of the smaller object goes to zero?

Because i know that when smaller block pushed on bigger block, bigger block wants to stay in its inertial position so the bigger block pushes back...Ya this is my question with what force does the bigger block pushes back ?

if it is 20 N back then is that dependent on the mass of bigger block or not ?

Treating this as a simple inelastic collision, there is no mass that will make the velocity equal to zero.

i mean to say how much bigger block at-restor how much mass at-rest (to which this block collides) is required to make velocity zero ?
 
  • #37
malu said:
i mean to say how much bigger block at-restor how much mass at-rest (to which this block collides) is required to make velocity zero ?
Again I ask: Why do you think the velocity goes to zero?
 
  • #38
Doc Al said:
Again I ask: Why do you think the velocity goes to zero?

i am not sure whether velocity goes to zero or not that's what i am asking ?
but my real question is

Because i know that when smaller block pushed on bigger block, bigger block wants to stay in its inertial position so the bigger block pushes back...Ya this is my question with what force does the bigger block pushes back ?

if it is 20 N back then is that dependent on the mass of bigger block or not ?
 
  • #39
malu said:
Because i know that when smaller block pushed on bigger block, bigger block wants to stay in its inertial position so the bigger block pushes back...Ya this is my question with what force does the bigger block pushes back ?
The details of the momentary force that the blocks exert on each other during the collision are complicated to estimate.

Forget the complication of the applied force of 20 N. Imagine that your first block is moving at some speed, say 1 m/s, when it hits the other block. Neither block 'wants' to change its motion, thus they exert forces on each other. That force may be quite high, but the resulting impulse can be calculated by comparing initial and final velocities of the blocks. (Those velocities can be calculated using conservation of momentum.)
 
  • #40
Actually i was really looking for the scenario of impact so i came through mit lectures http://ocw.mit.edu/courses/physics/8-01-physics-i-classical-mechanics-fall-1999/video-lectures/embed07
in one of the experiments, he is showing the impact load along the time scale i am really trying to understand that, i hope this may help out.
 
  • #41
malu said:
Actually i was really looking for the scenario of impact so i came through mit lectures http://ocw.mit.edu/courses/physics/8-01-physics-i-classical-mechanics-fall-1999/video-lectures/embed07
in one of the experiments, he is showing the impact load along the time scale i am really trying to understand that, i hope this may help out.
Where exactly is the point in the lecture where he discusses impact forces?
 
  • #42
Doc Al said:
Where exactly is the point in the lecture where he discusses impact forces?

at 35.49, the experiment starts and then then after the experiment he shows the impact on time scale have a look its amazing
 
  • #43
malu said:
at 35.49, the experiment starts and then then after the experiment he shows the impact on time scale have a look its amazing
Yes, nice demo. What he's demonstrating there is that the apparent weight goes to zero for an object in free fall. And as it collides, the apparent weight shoots up to a momentarily high value. Good stuff. The exact value of the force depends on the cushioning that he's dropping it on. Note that he drops it on a cushion--if he didn't, the thing would break because the impact forces would be too high.

(You can do a sloppy version of this on your own by jumping on your bathroom scale. Don't jump too hard or you'll break it.)
 
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