What is newtons 3rd law: Definition and 18 Discussions

Newton's laws of motion are three basic laws of classical mechanics that describe the relationship between the motion of an object and the forces acting on it. These laws can be paraphrased as follows:

A body remains at rest, or in motion at a constant speed in a straight line, unless acted upon by a force.
When a body is acted upon by a force, the time rate of change of its momentum equals the force.
If two bodies exert forces on each other, these forces have the same magnitude but opposite directions.The three laws of motion were first stated by Isaac Newton in his Philosophiæ Naturalis Principia Mathematica (Mathematical Principles of Natural Philosophy), originally published in 1687. Newton used them to investigate and explain the motion of many physical objects and systems, which laid the foundation for classical mechanics. In the time since Newton, the conceptual content of classical physics has been reformulated in alternative ways, involving different mathematical approaches that have yielded insights which were obscured in the original, Newtonian formulation. Limitations to Newton's laws have also been discovered; new theories are necessary when objects move at very high speeds (special relativity), are very massive (general relativity), or are very small (quantum mechanics).

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  1. Lotto

    What force is needed to break a rope?

    I would say that it will not break in both cases, since tension force is 10 N, no matter if I pull it against a tree or a person. But I am not sure. How is it?
  2. Hamiltonian

    Force between charged Ring and Rod, disproved Newtons 3rd law?

    This is the initial setup of the problem: The electric field due to the ring is: $$E = \int\frac{k(dq)}{(\sqrt{R^2 + x^2})^2}\frac{x}{\sqrt{R^2 + x^2}} = \frac{kqx}{(R^2 + x^2)^{3/2}}$$ the force on the rod due to this Electric field produced by the ring is: Consider a differential element...
  3. S

    Pendulum's Tension using Force reasoning and Newtons 3rd Law.

    Hi, I have a conceptual question. I was doing some problems on pendulums and found something that confused me. I attached a drawing. I used to always solve these problems by using some trigonometry and trying to find the Tension. i.e. ## m*g = F (of the Tension) * cos(theta) ## so ## \frac...
  4. bland

    Rice University gets Newtons 3rd Law wrong

    I couldn't remember which number was which in Newton's laws of motion, and so I clicked the first link I found, but to my amazement it seems that while Rice University got the number right, they got the facts of how it works wrong. Ironically it seems they have made the same mistake as that...
  5. A

    Forces of gravitation of moon and earth + Newtons 3rd law

    In relation to two objects with a gravitational field on them (i.e. a planet and its moon), would there be two pairs of partner forces (the force of gravity exerted from each object and the resulting partner force from each force of gravity)? For example with the Earth and moon: 1. Force of...
  6. K

    Newtons 3rd law - how can this rope move?

    Hi I'm having trouble understanding this situation. Appologies if I posted in the wrong place, this is pretty simple but it's not really a homework question. A horse is attached to a large stone by means of a rope (modelled as light and inextensible). Let the mass of the horse be m and the...
  7. P

    Newtons 3rd law with a truck hitting a person

    I having a little trouble grasping the 3rd law because of this idea in my head about a situation where a large truck going very very fast hits a person standing still. 1)Lets say the truck is going fast enough where the person's body will slightly rupture due to the impact. How can that tiny...
  8. P

    Newtons 3rd Law (Ropes, Pulley)

    Homework Statement A 2.5 kg mass is attached to a 3.5 kg mass by a light string that passes over a friction-less pulley. The masses are released. a) What is the magnitude of the acceleration of the masses? b) What is the tension in the string? Homework Equations (I think) Ft - mg = ma The...
  9. 0

    How much does Newtons 3rd Law account for Lift in %?

    I would like to know how much of a % does Newtons 3rd law (action / reaction) account for lift of a chopper.. i would also like to know this for Bernoulli's principle.. Can you give a source to back it up? I can't find an answer any where
  10. Z

    Does Newtons 3rd law corespond with pressure?

    If I were to take a standard sharp led pencil and push it on my desk, the tip of pencil will feel the same force as the force exerted on the desk, but what about the pressure? Since pressure is defined as: P = F / A The surface area of the tip of the sharp pencil is far smaller than...
  11. S

    Can you me with Newtons 3rd Law

    how do you find the Tension force,acceleration,magnitude. Its those questions where you have pullys,boxes pushing each other... I am really confused for the 3rd law
  12. K

    Kinetic/ Static friction- Newtons 3rd Law

    Homework Statement The coefficient of static friction is 0.60 between the two blocks in figure. The coefficient of kinetic friction between the lower block and the floor is 0.20. Force causes both blocks to cross a distance of 5.0 m, starting from rest. What is the least amount of time in...
  13. C

    Understanding Newton's 3rd Law: Finding Opposing Forces Between Two Boxes

    I understand the law and can apply it to many situations except for one. You are pulling a box weighing X kg attached by a connection which isn't loose or anything to another box weighing Y kg and you have to find out the strength of the opposing forces inbetween the boxes. Why is there...
  14. E

    Showing forces between 2 current loops are in accord with Newtons 3rd law

    Homework Statement show that the forces between two current loops obey Newtons 3rd law. Homework Equations \textbf{F}_{12}=\frac{\mu I_{1}I_{2}}{4 \pi s^{2}} d\textbf{r}_{1}\times(d\textbf{r}_{2}\times \textbf{s}) s is the distance between the elements of current The Attempt at a Solution...
  15. C

    Solving Newtons 3rd Law: Motorboat Position in 10s

    I have to apply the 3rd law i think to this, just not sure how to. A 2000 kg motorboat's propeller pushes on the water toward the west with a 2000 N force. The wind blowing toward the south pushes the motorboat with a 500 N force. Determine the location of the motorboat 10 seconds from the...
  16. A

    Friction and Forces: Solving Physics Problems with Newton's Laws

    I am needing help on a few problems that I have to do for class. If anyone could help on any of them that would be great. 1)Bob, who has a mass of 75 kg, can throw a 500 g rock with a speed of 30 m/s. The distance through which his hand moves as he accelerates the rock forward from rest until...
  17. F

    What is the tension in rope 2 if a sled dog pulls 2 sleds with different masses?

    Hi I need help on these questions, please try to be specific. A sled dog pulls 2 sleds, A and B mu=0.10 If tension in rope 1 is 150 N What is the tension in rope 2? ----------------------------------- Mass 1 on a table (mu=0) is connected by string through a hole in the table to a...