# What is Newton's law: Definition and 351 Discussions

In classical mechanics, Newton's laws of motion are three laws that describe the relationship between the motion of an object and the forces acting on it. The first law states that an object either remains at rest or continues to move at a constant velocity, unless it is acted upon by an external force. The second law states that the rate of change of momentum of an object is directly proportional to the force applied, or, for an object with constant mass, that the net force on an object is equal to the mass of that object multiplied by the acceleration. The third law states that when one object exerts a force on a second object, that second object exerts a force that is equal in magnitude and opposite in direction on the first object.
The three laws of motion were first compiled by Isaac Newton in his Philosophiæ Naturalis Principia Mathematica (Mathematical Principles of Natural Philosophy), first published in 1687. Newton used them to explain and investigate the motion of many physical objects and systems, which laid the foundation for Newtonian mechanics.

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1. ### B Inertial Mass vs Gravitational Mass

Why does the text saying that the Newton's framework doesn't require the two masses to be equal? If using f = ma give us inertial mass then how is f = Gm1m2/r² a different things? Isn't the law defined as the force is directionly proportional to the product of the masses and we calculated the...
2. ### I Mapertius's Principle and 2nd Newton's law

Hi, I have been working on deriving the second Newton's law from the Mapertius principle applied to a perfectly elastic collision from a free fall acceleration problem. These are my calculations, but I keep getting a coefficient of 2 in my final answer for some reason. Could someone explain to...

Hi Physics Forums, I'm wondering why Newton's Law of Viscosity sometimes has a negative sign in front of the viscosity and sometimes it doesn't? Thanks for your help!
4. ### Newton's law problem: Pushing 2 stacked blocks on a horizontal table

Would anyone be able to help with this Newtons law problem Block A rests on top of block B as shown in (Figure 1). The table is frictionless but there is friction (a horizontal force) between blocks A and B. Block B has mass 6.00 kg and block A has mass 2.00 kg. If the horizontal pull applied to...
5. ### Find the work from Newton's law of gravitation

What I did was just sub in the numbers and convert km to m. So (6.67E-10)(1000)(5.98E24)/(1,000,000+6.37E6)^2 * (1,000,000+6.37E6) So it's just (6.67E-10)(1000)(5.98E24)/(1,000,000+6.37E6). I thought this was a straightforward problem, but it seems that it is not. What am I missing?
6. ### I Newton's Gravity & Lorentz Contraction: Is Modification Needed?

Newton's gravity depends on the euclidean distance between two masses. Two comoving frames will have different values of length between masses so the forces will be different in two frames. Is it enough to prove that the gravity rule has to be modified?
7. ### I Is there an integral version of Newton's law of gravity?

$$F=G\frac{m_1 m_2}{r^2}$$ is presumably for point masses. If the masses weren't a point masses, then wouldn't you need a version of the formula that sums up the gravity for each infinitesimal portion of the masses? And for my money, "summing up" in physics is integrals, right? So would it be...
8. ### I Gas molecular attraction in the sense of Newton's law

Hello What experiment or observation shows that for atoms, molecules or masses of gas, there is attraction in the sense of Newton's law (universal attraction)? Bernadette
9. ### What is the Proof for Newton's Law of Cooling Formula Using Quadratic Equation?

I have went about this problem many different ways but cannot seem to come up with the answer. I am essentially trying to prove the formula provided in the ss of the problem.Could someone help me and tell me if I am approaching this wrong?
10. ### Newton's law question involving force and acceleration

I have no idea what to do but the answer is V=V0+ bt +(ct^3)/3
11. ### Newton's law problem: Helicopter lifting a box with a rope

Forces: Box--> W(weight) and T(tension) Rope-->T1(reaction of T) and T2(because of the helicopter) So first i calculated Weight: W=mg=400*10=4000N In order to find the acceleration i should use Newton's 2nd law so: (Box) : T - W = ma T - 4000=400a The problem is with the rope...
12. ### Mixing Newton's law with Adiabatic Process

I have attached an image showing the perimeters of the problem. I have included what I think is the solution, could someone please take a look and tell me if I am on the correct path, in the solution I am taking Joules as a common term to attempt to solve the question. The gas I have used is N...
13. ### MHB Limit of Newton's Law of Cooling....2

Given u(t) = (u_0 - T)e^(kt) + T, find the limit of u(t) as t tends to 0 from the right side. The answer is u_0. How is the answer found? Seeking a hint or two. Can this Law of Cooling be graphed? If so, what does the graph look like?
14. ### MHB Limit of Newton's Law of Cooling....1

Given u(t) = (u_0 - T)e^(kt) + T, find the limit of u(t) as t tends to positive infinity. The answer is T. How is the answer found? Seeking a hint or two.
15. ### Newton's law of gravitation -- Limitations and Improvements

Hi, I was reading Wikipedia article on Newton's Law of Gravitation and I couldn't make sense of the following section. Source: https://en.wikipedia.org/wiki/Newton's_law_of_universal_gravitation#Limitations How could the improved law of gravitation be written? Like as shown below? Thank you...
16. ### Pulley and Two Masses -- Newton's Law of Motion

From this question, I do not understand why there are three forces exerted at Point C (2 of it being the tension by weight A and the other is the tension by weight B) I understand that there is tension by the two weights but why is there 2 forces exerted by weight A at point C? From the...
17. ### (I.E. Irodov problem 1.72) Dynamics problem involving a pulley

. First thing that I notice is that the other part of string (to which body 2 is connected) is fixed. Therefore, I concluded that the body 2 is stationary with respect to the pulley 2 (let's agree that pulley to which body 1 is connected is pulley 1 and in same way pulley 2 is defined)...
18. ### Is Newton's Law of Gravity Still Accurate Despite Einstein's Discoveries?

I´ ve seen that Newtons law of gravity is considered and used very accurate to predict planet and satellites motion ( eclipses, etc..), but if space time is actually warped as Einstein told, how come Newtons law be accurate when it only depends on the mass and distance of to planets or stars...
19. ### Physics problems with Newton's law of gravitation

I do not know how to do this because in order to find the force of gravity it seemed to me i need to know the mass of the planet and vice versa, and for the 1st exercice i do not know whether the planet fills the astronaut's field of view. If it does in understand that i need to use trigonometry...
20. ### Further Applications of Newton's Law : Friction, Drag, and Elasticity

θ = 4 μs = 1 Fnet = Wpararell + fs m.a = 1/2.m.g.sinθ + μs.1/2.m.g.cosθ (divide by m) a = (g.sinθ+μs.g.cosθ)/2 a = ((9.8)sin4 + (1)(9.8)cos4)/2 a = 5.23 m/s^2 hello guys, I'm having trouble with this problem. Can anybody help me correct my attempt and explain it to me?? thanks
21. ### Second derivative of Newton's law

In one of my textbooks about quantum mechanics, they mention a vehicle moving in a straight line along the x axis. With Newtons first law they take the second derivative from a which is d^2x/dt^2 and that should be equal to -∂V/∂x. What exactly does -∂V indicate? The complete equation...
22. ### How Does Viscous Force Relate to Velocity Gradient?

How does viscous force depends upon velocity gradient?
23. ### Newton's law of cooling - formula for constant k

Hi, recently I got interested with practical applications of Newton's law of cooling. Its main disadvantage is that one has to measure temperature at some point of time (other than ##t=0##) to obtain solution for any other ##t##. However I've found an online calculator...
24. ### Newton's Law of Cooling - "k" constant

Hi, I have been exploring Newtons Law of Cooling. What relationship should i expect to see from the cooling constant "k". I have plotted the "k" values for each equation that i have developed (based off of different volumes of water in the same sized beaker). So, when the "k" vs volume graph is...
25. ### How Is Acceleration Calculated in a Space Shuttle Launch?

Sorry there, relatively new to this topic. Anyways, from my understanding of the formula, the force is equal to mass times acceleration. So, is it safe to assume that the total force is simply all the forces from the engines added up? (1.7MN^3+23MN^2), while the mass is (2.0^106 kg)^ 9.8m/s^2...
26. ### Calculating Net Force and Friction: Understanding Newton's Laws of Motion

I am very new to physics so I am still learning a lot. Here is my attempt: Find the net force acting on the block : Fnet= Fg+FN so I have to find FN before I can complete the answer. FN = Fg (mg) FN = (0.72kg)(9.8m/s^2) = 7.056 or 7.06 N Fnet = 9.8+7.06 = 16.86 N or 16.9 N To find the...
27. ### How Does Newton's Law of Cooling Affect Heat Loss Calculations?

Homework Statement A metal ball of mass 1kg is heated by means of a 20W heater in a room at 20°C. The temperature of the ball becomes steady at 50°C. (a) Find the rate of loss of heat to the surrounding when the ball is at 50°C. fa) Assuming Newton's law of cooling, calculate the rate of loss...
28. ### Newton's law of conservation as it applies to the big bang

I had a thought the other day and I am looking for someone to tell me why it does not work. In consideration of energy be neither created nor destroyed, to me this would say there is not enough energy to continue expanding the universe. I also make the conjecture the gravity never stop...
29. ### Help with Newton's Law of Cooling

Hi - I have been trying to figure out how long an object takes to cool and I am just banging my head on the desk. Info: Metal object is placed into an oven set to 302F (150C). The object is 3' long and 2.75" diameter, and is solid metal (steel). The object is "baked" in the oven for 30 minutes...
30. ### Heat Transfer -- Newton's law of cooling

Hi, I have been trying to set up a funtcion of the temperature over time and I am doubting my results. In the system i have: Fig 1. The system is at rest. For simplicty I've considerd the ambient temperature to be same as T1. By knowing: Fig 2. yeilds: Fig 3. In my case B&C is at a...
31. ### Newton's law and double stars moving in circles

Homework Statement The two components of a double star are observed to move in circles of radii ##r_1## and ##r_2##. What is the ratio of their masses? (Hint: Write down their accelerations in terms of the angular velocity of rotation ##\omega##) Homework Equations ##m\ddot{\vec{x}}=...
32. ### Vector addition and Newton's law

I know that n-body problem can be complicated, but that's for the dynamics. What about a static case: e.g. if I have the distances of several bodies A, B and C etc. and their distance to a reference mass m, can I just use the vector addition of the Newton's gravitational force to add up all of...
33. ### Torque lost using Newton's law of viscosity

Homework Statement Two circular plates, as indicated in the figure, slide one over the other. Calculate the torque lost in this situation. The rotation w of the internal radius of the plates R1, the outer radius of the plates R2, the absolute viscosity of the lubricant between the plates (η)...
34. ### How does Newton's Law change with rotation between frames?

If we have two frames related by ##x' = Rx## where ##R## is a rotation matrix and ##t'=t## Newton's law doesn't remain the same, for $$m \frac{d^2 x'}{dt'^2} = m \frac{d^2 Rx}{dt^2} = mRa$$ whereas it will be just ##ma## in the other frame. How do we solve this?
35. ### B Newton's Law of Gravitation: Balancing Centrifugal Force & Internal Pressure

How is the centrifugal force in an orbiting planet or star, balanced if according to Einstein there is not such gravity force and is only space time curvature. Also, how come in order to determine internal pressure of a planet or star, the force acting near the center is calculated using the...
36. ### Newton's Law: block and incline

Homework Statement Homework Equations Fs= μs*N ΣF=ma The Attempt at a Solution [/B]
37. ### Applying Newton's Laws to Friction: Solving Homework Problems

Homework Statement Homework Equations F = ma Fr = uK * N The Attempt at a Solution My answer is 7 but it is not one of the choices Thanks in advance[/B]
38. L

### Newton's Law Problem (Did i do this correctly?)

Was wondering if i got the answer correctly or got it by coincidence (answer is 3.0 m according to answer key) 1. Homework Statement ≈Homework Equations F = ma The Attempt at a Solution Ff = μk*N N = mgcos(20)=18.41 = W W = fd W = ΔKE = 0-2 = -2 -2/f =d f = just the x components since y...
39. ### B Newton's law under Lorentz transformation

According to this pdf http://www.springer.com/cda/content/document/cda_downloaddocument/9783319011066-c2.pdf?SGWID=0-0-45-1429331-p175291974 Newton's second law is not invariant under Lorentz transformations. To find out the part that says so, use CTRL+F and type "Newton"; it's the first result...
40. ### Newton's law of equal/opposite reaction, and bullet impact...

When fired, a slug's acceleration down a barrel produces an equal backward force. I get that. What I DON'T get, is why there is no rebounding force counter to a slug's impact by the material being struck? The material/molecules simply get impacted and pushed via the force generated. Why is there...
41. S

### Newton's law of gravitation, find the mass and radius of the sphere

Homework Statement Two fully equal sphere's of lead are placed next to each other so that the gravitational force between sums up to 10N. Calculate mass and radius of the two sphere's. F=10N , ρlead=11300kg/m2 Homework Equations F=gm, F=GMm/r2 , V=4πr3/3 , ρ=m/VThe Attempt at a...
42. Q

### Calculating Tension in Ropes for a Suspended Circus Performer

Homework Statement After a mishap, a 76.0-kg circus performer clings to a trapeze, which is being pulled to the side by another circus artist, as shown here. Calculate the tension in the two ropes if the person is momentarily motionless. Include a free body diagram in your solution Homework...
43. ### How did Newton explain his laws of motion?

Hey guy! I used google but it didn't help so can you guy pleas tell me how Mr.Newton explained his law please. Thank!
44. ### A Newton's law question which is too hard for me

Homework Statement Question 1. A triangular prism of mass M is placed one side on a frictionless horizontal plane as shown in Fig. 1. The other two sides are inclined with respect to the plane at angles a1 and a2 respectively. Two blocks of masses m1 and m2, connected by an...
45. ### Newton's Law of Cooling When the Temperature of the Air Isn't Given

Homework Statement You come across a dead body at 3 PM. Its temperature is 83.6 F. 30 minutes later its temperature is 78.6 F. How long was the guy dead before you found him? Homework Equations ##\frac{dT}{dt}=-k(T-Tair)## T(0) = 83.6 F T(0.5) = 78.6 F The Attempt at a Solution This is a...
46. ### Need help on Newton's Law Question and Determining speed

<Mentor's note: moved from a technical forum, therefore no template> My teacher made this question: Iceman is sliding down a 10.0 x 10^(-8) nm ice that is inclined at 20.0 degrees with respect to the horizontal. Iceman is initially at rest. The coefficient of kinetic friction between iceman's...
47. ### Total Time for Newton's Law of Gravitation

Homework Statement Find the total time, t, that an object takes to reach the surface of the Earth from a distance, D, using the Law of Gravitation: $$F_{g} = \frac{GMm}{x^2}$$ R is radius of Earth D is distance from surface R+D is total distance from center of masses ****** One Dimension...
48. ### Newton's Law of Cooling and Resonance Phenomena

Can we find the natural frequency of water's molecules by using of Newton's law of cooling and resonance phenomena?
49. ### Newton's law related question....

Homework Statement [/B] A man weighing 980 N slides down a rope that can support a weight of only 755 N. a) How is this possible? b) What is the least acceleration he can have without breaking the rope? c) What will his minimum speed be after sliding down 8.0 m? Homework Equations Not...
50. ### Simple Newton's Law Situations

#1 A bird strikes the windshield of an airplane and makes a big mess. Which of the following statements is correct? A) The force that the airplane hits on the bird is larger than the force that the bird hits on the windshield. B) The force that the bird hits on the windshield is larger than the...