# What is Newton 2nd law: Definition + 142 Threads

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. ### Is it friction or tension acting on the person hanging on a rope?

The first question statement was under the chapter ##Newton's Laws Of Motion (Without Friction)##. Whereas, the second question was under ##Friction##. The free body diagram for the first question is given as: And the free body diagram for the other question is given as: In the first...
2. ### Monkey accelerating up and down a rope

Body M has a mass of 80kg and monkey has a mass of 20kg. No friction between table and the object M. Can someone please explain to me if the acceleration of the system will be greater with monkey accelerating up or down(relative to the rope), and why. Monkey accelerates both up and down at...
3. ### For the friction formula when to use Fn(normal force)×mu and when F×mu

Is there some kind of rule when we are supposed to use Fn×mu compared to F×mu or mg×mu to calculate friction? Because I don't get the same answer when I use Fn×mu and F×mu. In magnitude the forces are the same but they are not same in directions. Why is it so? Here is the photo. Problem goes...
4. ### Why did I get two different results for the mass m2?

I wonder why did I get two different answers for m2 using two different approaches? The correct answer is m1=3kg, m2=7kg. So pleases if someone can take a look. Also can someone suggest how am I supposed to find the force that dynamometer is showing? I was thinking that it equals (m1+m2)*g...
5. ### Why body A has greater acceleration?

Apparently body A has greater acceleration and I don't get why. Can someone please explain. It's stated to take g=10/s^2. Both bodies have same mass. Nothing about rope elasticity is added, and nothing about the pulley. So I assumed they are both massless, and no friction between them. Also...
6. ### Solving for Masses in an Atwood Machine

I've tried to solve this exercise but I haven't used one of the properties of the system (the displacement of the masses) so I don't know if I'm wrong about my procedure. First of all, we (obviously) know that $$P=P$$ And since we can express the power of a force in two different ways, we...
7. ### Why Isn't the Right Side Negative in a Vertical Spring-Mass System Equation?

I have a doubt with the last part. Why isn't the right side negative? Because when the block is released, it moves upwards. thanks image was obtained from here Thanks.
8. ### Acceleration of stacked blocks on an inclined plane

In order to calculate the acceleration, I sum the equations sum fx^B and sum fx^A. F_{r,B} - m_Ag sin(theta) - (m_A+m_B) g sin(theta) = - (m_A+m_B) a then a = -[F_{r,B} - (2 m_A+m_B) g sin(theta)]/(m_A+m_B) =-[muk (m_A+m_B) g cos(theta) - (2 m_A+m_B) g sin(theta)]/(m_A+m_B) but the...
9. ### How many discs will slide off the board as it decelerates?

I don't undertand the equation. It is Newtons's second law of motion, so it decribes a force that acts on a single disc relative to the ground. So when the force is proportional to velocity, shouldn't it be ##-bv##? Because the dics's velocity is ##v## relative to the ground. Relative to the...
10. ### 2 masses connected by spring, one is pulled, how much does the spring stretch?

Hi, I am having trouble with this problem. I'm thinking the solution is this but I'm not sure. Fnet=m1a+m2aFnet=m1a+m2a , m1a=kxm1a=kx, m2a=F−kxm2a=F−kx so x=m1ak=−(m2a−F)kx=m1ak=−(m2a−F)k

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12. ### I Meaning of the terms in the formula of the net external force

The mathematical representation of the net external force on a system(obtained from Newton's second law) is ##\vec F_{net} = \frac {d\vec P}{dt}##, which is the rate of change of linear momentum of the system. If we substitute ##\vec P = m\vec v## into the formula for force and differentiate, we...
13. ### What Is The Maximum Force The Locust Exerted on The Plate?

At the beginning, I just looked for the highest point in the graph, which is approximately 90 \frac m s^2. Then I plugged it in the formula F = ma and got the force equaled to 0.045 N. However, when I looked back, the graph is about the change in acceleration. So really, I'm dealing with...
14. ### Variable mass, uniform body, force -- pulling a massive rope

So i got some equations but i think i am missing something, my main doubt is what is the relation between dx / dt and v(o) [ here] . Workings in attachment
15. ### I What friction causes objects to decelerate?

When I exert enough force that overcome the static friction. The object start moving and surface create kinetic friction on object if I exert harder overcome the maximum of friction it start accelerate. When I release the object will the kinetic friction disappear immediately proportional to...
16. ### I Stuck in understanding Newton's 3 laws

I'm stuck to understand 3 laws of Newton. It doesn't make sense to me. 1. Suppose the case when a person stay in a rest vehicle.When we accelerate the car the person still at rest so the person has to move to the tail of the vehicle cause his intertia of staying rest. If we decrese the...

46. ### Three masses, a string and pulley and a table -- solve for the acceleration

Let T be the tension in the string, a be the acceleration of mass 2m, 2a be the acceleration of mass m T = (m) (2a) ---eq(1) The mass 3m will come down with acceleration a’ = (a+2a)/2 = 3a/2 3mg - 2T = 3m . 3a/2 from equation 1 3mg - 2(2ma) = 3m . 3a/2 thus a = 6/17g thus acceleration of 3m...
47. ### Finding the correct change of momentum in this system,

I tried out the F=Δp/Δt equation and i came up with a change of momentum of the box-bullet fusion (M+m)V-0, but the textbook says otherwise (using a change of momentum of JUST the box itself, excluding the bullet). According to the textbook, the correct change is MV-0, without the added mass of...
48. ### Arguing about the magnetic force vector

I am writing about the nature of force in classical mechanics and what does really imply, in terms of change in motion. I am using as an example a circuit, on which we exert a force. I am trying to justify the following scheme (concretely, ##f_{mag}##): The thing is that I am wondering how...
49. ### 2 objects connected by a spring - Minimum Force required to move the objects

On object 2: There are only 2 horizontal forces - Friction and Tension (of the spring). T = km2g On Object 1: There are 3 horizontal forces and the minimum value for F is when: F - km1g - km2g = 0 F = kg(m1 + m2) However, Solution is: F = kg(m1 + 0.5 m2) Any opinion?
50. ### Object causing another object to move with zero friction

Homework Statement This is the problem from the book "physics for scientists and engineers..." by Serway, Jewett / Chapter 5, problem 98 Initially, the system of objects shown in Figure P5.93 - is held motionless. The pulley and all surfaces and wheels are frictionless. Let the force F be...