Trouble with Newton's Third Law of Motion

Click For Summary
SUMMARY

The discussion centers on the confusion surrounding Newton's Third Law of Motion, specifically the principle that for every action, there is an equal and opposite reaction. Participants clarified that action and reaction forces act on different bodies, preventing them from canceling each other out. A physical example involving two blocks during a collision illustrated that while forces exerted on each block are equal and opposite, they do not negate each other's effects on their respective motions. The conversation concluded with an understanding that resolving forces requires considering each object individually to grasp the conservation of momentum.

PREREQUISITES
  • Understanding of Newton's Laws of Motion
  • Basic knowledge of momentum and its conservation
  • Familiarity with force interactions in physics
  • Concept of action-reaction pairs in mechanics
NEXT STEPS
  • Study the "horse cart paradox" for deeper insights into action-reaction forces
  • Explore momentum conservation in elastic and inelastic collisions
  • Learn about the implications of Newton's Third Law in various physical systems
  • Investigate real-world applications of action-reaction principles in engineering
USEFUL FOR

Students of physics, educators explaining Newton's laws, and anyone interested in understanding fundamental mechanics and force interactions.

Daniel Wqw
Messages
12
Reaction score
1
Please forgive me for my naiveté but I've got to resolve a conceptual problem I'm having.

If every action has an equal and opposite reaction then why don't all the forces in the universe cancel each other out.

For example if i am in space and I hit a ball with force X then the ball reacts back with force X which would mean the forces cancel each other meaning there would be no resultant force and therefore no movement. I know this is nonsense but I'm just giving you an idea of the conceptual difficulty I'm having.

If anyone could resolve this I would be very appreciative. Thanks!
 
Science news on Phys.org
Daniel Wqw said:
Please forgive me for my naiveté but I've got to resolve a conceptual problem I'm having.

If every action has an equal and opposite reaction then why don't all the forces in the universe cancel each other out.

For example if i am in space and I hit a ball with force X then the ball reacts back with force X which would mean the forces cancel each other meaning there would be no resultant force and therefore no movement. I know this is nonsense but I'm just giving you an idea of the conceptual difficulty I'm having.

If anyone could resolve this I would be very appreciative. Thanks!

Hi Daniel.

Action and reaction forces acts on two different bodies so they never cancel out.
 
  • Like
Likes   Reactions: Philethan
The two forces are not applied to the same body. When kicking a ball you act on the ball with force F and the ball acts on your foot with force -F.
 
Consider this scenario. You slap your friend. Your friend slaps you back with the same force. Equal and opposite. If they were to cancel each other out, you wouldn't feel a thing. Try it :-P
 
  • Like
Likes   Reactions: NotASmurf and A.T.
siddharth23 said:
Consider this scenario. You slap your friend. Your friend slaps you back with the same force. Equal and opposite. If they were to cancel each other out, you wouldn't feel a thing.
Yeah, the right way to cancel forces is to have two friends slap you simultaneously from opposite sides. Report back if you felt anything.
 
I'm sorry but I still really don't understand. Could somebody give a physical example please?
 
Daniel Wqw said:
I'm sorry but I still really don't understand. Could somebody give a physical example please?
A block of mass M1 is sliding on a frictionless surface with constant velocity V. It collides with a stationary block of mass M2.
During the collision (duration t), block M1 exerts force F1 on block M2. At the same time, block M2 exerts force F2 on block M1. $$\vec{F_1}=-\vec{F_2}$$
As a result of the two forces, each acting on a different block during the collision, block M1 will have lost momentum and block M2 will have gained momentum. Total momentum is conserved.
 
Daniel Wqw said:
Could somebody give a physical example please?
Search the forum or the www for "horse cart paradox".
 
A.T. said:
Search the forum or the www for "horse cart paradox".
YES! I got it. You only resolve one objects forces, not two objects. If you resolve two objects forces the forces do cancel but if you resolve each objects forces individually then you get the right result.
Ok my fault coming from an incorrect assumption you can resolve two objects forces together when the whole principle of resolving forces rests on the fact that they're all acting on one object! At least I think this explains my error.
I actually realized this from thinking about the horse cart paradox which I found.
What everyone says actually makes sense now and I didn't see the implications of the fact the opposite forces are acting on different objects!

Thanks everyone for your help except for siddharth23's advice which, if I didn't know better, would create more problems than I'm trying to solve :)
 
  • Like
Likes   Reactions: Chestermiller
  • #10
Daniel Wqw said:
Thanks everyone for your help except for siddharth23's advice which, if I didn't know better, would create more problems than I'm trying to solve :)
Haha. I wouldn't expect you to do it. Just imagine it. Anyways, as long as you got it..
 
  • #11
Daniel Wqw said:
If every action has an equal and opposite reaction then why don't all the forces in the universe cancel each other out.
If you consider the force on the two bodies together by looking at the acceleration of the centre of mass of the two interacting bodies, the forces do indeed result in a net zero force (on the two bodies as a whole). That leads to the principle that momentum is always conserved in any interaction. If you take the universe as a whole (which is a bit hard to define given present knowledge) the net force on the universe due to all interactions would be zero. So in that sense all the forces in the universe should sum to zero and, in that sense, cancel each other out.

AM
 
  • #12
example if i am in space and I hit a ball with force X then the ball reacts back with force X which would mean the forces cancel each other meaning there would be no resultant force and therefore no movement.

The forces don't cancel because they act on different objects. One acts on the ball and the other acts on you. You go in the opposite direction to the ball.
 
  • #13
the way i see it is, if there wasn't an equal but opposite reaction, the ball would simply pass through the object you used to hit it with. the equal but opposite reaction means that the ball won't move relative to the object used to hit it with during the contact of the two. hope this helps a little
 

Similar threads

Graduate The violation of Newton's third Law
  • · Replies 28 ·
Replies
28
Views
4K
High School Trouble understanding Newton's Third Law in Pulleys
  • · Replies 7 ·
Replies
7
Views
7K
High School Why Does a Ball Bounce Back from a Wall But Not from Sand?
  • · Replies 3 ·
Replies
3
Views
3K
High School Trouble with Newton's Third Law and motion
  • · Replies 27 ·
Replies
27
Views
3K
Undergrad Center of mass, Newton's third law, and rocket thrust
  • · Replies 8 ·
Replies
8
Views
3K
High School Violation of Newton's third law
  • · Replies 7 ·
Replies
7
Views
3K
Graduate Exceptions to the Laws of Physics: Newton's Third Law
  • · Replies 10 ·
Replies
10
Views
7K
Undergrad An atomic explanation of Newton's Third Law of Motion
  • · Replies 20 ·
Replies
20
Views
4K
Graduate Elastic collisions and Newton's third law
  • · Replies 2 ·
Replies
2
Views
3K
Undergrad Newton's Third Law: Action & Reaction Along the Line Connecting Two Objects
  • · Replies 5 ·
Replies
5
Views
2K