What is Newton's second law: Definition and 264 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. O

    Newton's Second Law Problem

    Homework Statement The minimum safe distance between vehicles on a highway is the distance a vehicle can travel in 2.0s at a constant speed. assume that a 1.2x10^3 kg car is traveling 72km/h [S] when the truck ahead crashes into a northbound truck and stops suddenly. a) if the car is at the...
  2. V

    Newton's second law clarinet problem

    A clarinetist, setting out for a performance, grabs his 3.41-kg clarinet case (including the clarinet) from the top of the piano and holds it in the air with an upward force of 31.8 N. Find the case\'s vertical acceleration. Indicate an upward acceleration as positive and a downward one as...
  3. F

    What is the Average Force During a Collision?

    Homework Statement As part of a safety test, an eighteen wheeler is being shot down a track by a rocket into a solid concrete wall. The truck is fully loaded, for a total weight of 95,000lbs. If the truck hits the wall at 80mph and causes a deformation of .1m in the wall before coming to a...
  4. S

    Newton's second law is so intuitively obvious

    Hi, question here. Newton's second law is so intuitively obvious to anyone not even hearing or reading his law in words that it makes me wonder: how was it such a great revelation that many years ago? Pardon my ignorance, but I'm very curious.
  5. andyrk

    What is the Constant of Proportionality in Newton's Second Law of Motion?

    "The rate of change of linear momentum of a body with time is directly proportional to the net force acting on it." =>F\proptodp/dt Then how do we suddenly come to: F=dp/dt? We took the proportionality constant as 1 but why? How to determine that the constant of proportionality is 1?
  6. C

    Problem involving Newton's second law

    Homework Statement see here for a situation of the problem http://i40.tinypic.com/6fbblk.jpg Homework Equations F=ma The Attempt at a Solution Ok well I've solved this problem but one thing about the solution process troubles me. Namely when trying to find the force in the bar...
  7. J

    Newton's second Law of Acceleration, Inertia and Momentum, Freefall

    Okay, we have one-thousand pounds; at sea level, and a feather. We elevate them to a thousand meters and support them on a platform. All of which is in a vacuum. We remove the platform, they both hit the earth; the limiting surface; at the same time. Newton's second law suggests that this is due...
  8. C

    A problem involving Newton's second law.

    Homework Statement see this link http://i45.tinypic.com/10gwhw2.jpgHomework Equations F=kx, F=ma The Attempt at a Solution ok first I find the total length of the pulley system, 2Sa+Sb=L, differentiating with respect to time i get, 2Va + VB=0 and thus 2Aa+Ab =0 So Aa =-Ab/2 next I first...
  9. C

    How do I solve for the angle in Newton's Second Law problem with two unknowns?

    Homework Statement See the this link for problem statement, http://i48.tinypic.com/vphsm1.jpg Case 1 I've solved, however I'm unsure about how good my solution is for case 2. Homework Equations F=ma Fu = N * ukThe Attempt at a Solution Ok first I sum the forces in the y direction...
  10. I

    Speed of Block after Newton's Second Law

    Homework Statement http://postimage.org/image/6exf0w475/ B) I have a question on this part. Would I just use the "v = v0 at" equation here because the question states "after it starts moving?" Do i need to include static/kinetic friction into this equation somehow? Homework Equations part...
  11. Q

    Is this correct? Newton's second law related.

    On an object with a mass of 3 kg a single force F acts perpendicular to the speed of the object.The object moves in a circle with a radius of 2 meters and the period is 3 seconds.a) Find the acceleration. I have the formula T=2*pi*r/v ,I find v here and then in the formula a=v^2/2 I replace v...
  12. B

    Newton's Second Law force equations help

    I'm just uncertain (haha), what IS Newton's Second law exactly? F=ma, or F=dp/dt, aren't these defining equations for F itself? what is a force? what would you mean a "force" acts on a object, would it be equally probable to define say, d^3Q/dt^3 = m dx^3/dt^3? does Newton's second law have...
  13. S

    Rolling Objects, Friction, and Newton's Second Law

    Homework Statement In rotational dynamics, a typical problem would be along the lines of a ring with mass m and radius r rolling down a hill with angle θ to the horizontal. Find the acceleration of the ring. Homework Equations Ʃτ=I\alpha = r x (friction) ƩF = ma = mgsinθ - (friction)...
  14. E

    Newton's second law - jumping from a height

    Homework Statement When jumping from a height on to a hard surface, it is advisable to bend one's knees on landing. How does bending the knees affect the time one takes to come to rest? (1 mark) With reference to Newton's second law, explain why it is a good idea to bend one's knees. (2...
  15. E

    Understanding Impulsive Occurrences in Newton's Second Law

    when occurs an impulsive in the Newton's second law?
  16. B

    Problem with Newton's second law

    Homework Statement In the following figure, the boy has a mass of 40kg, and the platform he is sitting on has a mass of 15kg. If the force of the board on the boy is 180N; a) Find the acceleration of the boy b) Find the reading of the scale...
  17. T

    Newton's second law 3 pulley problem.

    There is no friction and the strings are massless. Find Tension and acceleration. 3. The Attempt at a Solution 1. +T-m1g = m1a 2. m2g-2t= m2a 3. +T - m3g =m3a 4. 2a2 = a1+a3 g(-m1+m2-m3) + 2a2 = (m1 + m2 + m3)a + a1 + a2 -29.4 + 2a2 =11a +a1+ a2
  18. F

    Newton's Second Law surface+horizontal w/ friction

    This was a return of my test and I have filled in the forces already. I got a few problems wrong and I would like to see what the correct answers are. Homework Statement 1. Write an expression and determine the value of N1, the normal force acting on M2 2. Write an expression for and...
  19. M

    Question regarding setting up Newton's second law equations

    Homework Statement So the question revolves around two blocks, one of mass A and one of mass B. The Mass A block is on a smooth horizontal surface, connected by a thin chord that passes over a pulley to the second block, of Mass B. The question asks to draw a free body diagram of both objects...
  20. S

    Newton's Second Law Problems

    [b]1. A loaded penguin sled weighing 70 N rests on a plane inclined at 20° to the horizontal. Between the sled and the plane the coefficient of static friction is 0.25, and the coefficient of kinetic friction is 0.16. (a) What is the minimum magnitude of the force F, parallel to the plane, that...
  21. C

    Questions about the downward gravitational force and Newton's second law

    Say an object is dropped out of a plane at height h above the ground and flying at a constant speed v0... Ignoring air resistance, the only force on the object is the gravitational force, but it will still move in the direction that the plane was moving, right? So if Fg is the only force...
  22. C

    Dropping a bundle from a plane: Using Newton's Second Law

    Homework Statement A plane, which is flying horizontally at a constant speed v0 and at a height h above the sea, must drop a bundle of supplies to a castaway on a small raft a) Write down Newton's second law for the bundle as it falls from the plane, assuming you can neglect air resistance...
  23. M

    Newton's Second Law ODE Question

    A car of mass 1200 kg is started from rest and pulled on the level ground by an engine. The resistance of the motion is Kv, where v(m/s) is the velocity of the car at time t(s). The power of the engine is constant and equal to 80000 watts. a) How does P, the power of the engine connect to F...
  24. S

    Newton's Second Law Equation acceleration Question

    Homework Statement A certain force gives object m1 an acceleration of 12.0 m/s2. The same force gives object m2 an acceleration of 3.30 m/s2. What acceleration would the force give to an object whose mass is (a) the difference between m1 and m2 and (b) the sum of m1 and m2. Homework...
  25. N

    Is there any way that Newton's second law can be derived from the

    Is there any way that Newton's second law can be derived from the first
  26. C

    Is it a good idea to get a tattoo of mathematical equations?

    So this is slightly off topic but still physics related. I want to get a tattoo of Newton's second law in a few forms on my arm and this is what I've settled on: \sum{\vec{F}}=\frac{∂\vec{p}}{∂t}=-\nablaφ The only thing I am a bit worried about, is although I aesthetically like the...
  27. M

    Newton's Second Law assumptions

    Hi people Newton's Second Law a=F/m assumes that either a body is pointlike or that force is transmitted instantaneously across it. Special relativity which applies to Newton's First Law and which was developed from the assumption of a finite velocity of energy transmission, has enjoyed...
  28. G

    Newton's Second Law finding value of force problem

    Homework Statement If a soccer ball of mass 0.5 kg is kicked with a speed of 14 m/s and hits a rock and is in contact with it for 0.18 seconds, with what force will it hit?Homework Equations Fnet = ma The Attempt at a Solution I have no idea what to do to get the acceleration for the above...
  29. J

    Friction and Newton's second law

    Homework Statement It's attached. Homework Equations F = m*a F_f = -μ_k*F_N // [Force of friction] = μ_k*[Normal force] The Attempt at a Solution So, I think I managed to solve it -- at least, I got what the book got, but I don't understand part of what I did to get the answer...
  30. P

    Newton's Second Law Concerning Net Force

    Homework Statement • If we add a third block, leaving the 10 N force the same, what happens to f3, the net force on the 3 kg block? A. Increase B. Decrease C. Stay the Same D. Not Enough Info. Homework Equations F net = m * a The Attempt at a Solution Ignore the...
  31. P

    Conceptual Question - Newton's Second Law

    Homework Statement An astronaut is taking a space walk near the shuttle when her safety tether breaks. What should the astronaut do to get back to the shuttle? Attempt to "swim" toward the shuttle. Take slow steps toward the shuttle. Take a tool from her tool belt and throw it away...
  32. J

    Need Help With Newton's Second Law (Net Force) Problem

    Homework Statement http://imageshack.us/photo/my-images/534/jillo.jpg/ Okay, so I was given a diagram that looks like the one I linked to above. This is pretty basic, using Newton's second law to find- A) the weight of the "lamp" B) the tension in the chain The strut is exerting a...
  33. E

    Newton's second law of motion questions

    physics help please... a person with a mas of 75.0 kg is wearing roller blades and stands at rest on top of a long hill. the hill makes an angle of 15.0 above horizontal. assume the coefficient of friction to be .10. a. determine the acceleration of a the person as they roll down the...
  34. G

    What is the Nature of Force in Newton's Second Law?

    Homework Statement Thanks for looking. My understanding of Newton's Second Law, F=MA, is: If I give some mass a force, it will accelerate at some rate. Imagine an infinite, frictionless surface. If I give a box of 1kg a push of 6 Newtons, it will accelerate at 6 m/sec2. So, at t=0, v=0. at...
  35. R

    Why do some people say that Newton's Second law is the real Law of Motion?

    People say that Newton's Second law is the real Law of Motion because both the First and the third law can be proved from the second. If this is true why did Newton state them as separate laws if they are just special cases of the 2nd law? My teacher taught us this. These are the proofs she...
  36. T

    Can you prove Newton's Second Law from this experiment?

    As far as I know, you can't prove laws, but anyway... We got to do an experiment at school. A weight is suspended vertically from a string which is connected to a trolley which is placed horizontally on a flat table. There's a pulley at the edge of the table to reduce friction. Then we let...
  37. P

    Newton's second law if mass changes

    Are there any cases in Newtonian physics where it is valid to apply Newton's second law in the form ƩF = m dv/dt + v dm/dt, in which dm/dt is non-zero? It is my belief that there are no such cases. For example, if one applies momentum conservation to a rocket in a field-free region, we obtain...
  38. T

    Maximum height using Newton's second law

    a small object with mass m=10gr is thrown vertically upwards from the surface of Earth with a velocity v=10m/s i need to find the max height(Hmax) (when v=0) and the time(tmax) when the object reaches the max height by using Newton's second law only. (ignore the effect of air and air...
  39. Q

    Rotational Form of Newton's Second Law - Help

    Homework Statement A turntable must spin at 33.3RPM (3.49 rev/s) to play an old fashioned vinyl record. How much torque must the motor deliver if the turntable is to reach its final angular speed in 2 revolutions, starting from rest? The turntable is a uniform disk of diameter .305m and mass...
  40. L

    Newton's second law F=ma and torque do not agree?

    Homework Statement Suppose a sphere is rolling on the ground and there is friction. Friction is the only net force acting on it, and by Newton's law, it will accelerate the object in the direction of the force (which is backwards). But the object is actually accelerating forward because...
  41. M

    Newton's Second Law: Acceleration Proportional to Force

    Newton second law: the net force applied to a body produces a proportional acceleration. Lets fly a rocket. Rocket has some kind of propulsion system that is producing force, so the rocket should accelerate to the infinity. However when the speed is closing to the speed of light, the...
  42. D

    Equations with Newton's Second Law of Motion

    Hi, I really need some help on these homework problems. Sorry I couldn't paste the pictures in but please help me! 1) When a 58 gram ball is served, it accelerates from rest to a speed of 45 m/s. The impact with the racket gives the ball a constant acceleration over a distance of 44 cm. What...
  43. N

    What is the Net Force on a System with Multiple Objects and Inclined Planes?

    Homework Statement Find the Net force on the system see thumbnail for given. Normal force action on object 1 - 150N Normal force action on object 2 - 346N Force of friction action on object #1 - 15N Force of friction action on object #2 - 34.6N M1 moves to the left Homework Equations F = ma...
  44. P

    Newton's Second Law of Motion: Fnet=ma

    Homework Statement A 90 kg parachutist in free fall has an acceleration of 6.8 m/s^2. Homework Equations What is the frictional force provided by air resistance when she is accelerating at this rate? The Attempt at a Solution I tried this, assuming down is positive. I used the...
  45. W

    Newton's second law and tension

    Newton's second law and tension! Homework Statement A helicopter is lifting two crates simultaneously. One crate with a mass of 139 kg is attached to the helicopter by cable A. The second crate with a mass of 88 kg is hanging below the first crate and attached to the first crate by cable B...
  46. S

    Newton's Second Law Lab with Pulley's and Blocks

    I did a lab earlier this week on Newton's second law with a pulley and carts. M2 was the block that traveled horizontally and M1 was the hanging mass. The mass of the cart that was moving horizontally was constant all experiment with about 0.2 kg. The mass of the hanging gradually increased...
  47. M

    A Problem Involving Newton's Second Law

    Once her chute opens, a sky-diver of mass m is acted upon by a downward force Fg due to gravity, and an upward force Fr due to air resistance. If Fg = mg, where g is gravitational acceleration, and Fr is proportional to velocity v, use Newton’s Second Law of Motion to write acceleration a as a...
  48. H

    Computational Newton's second law problem

    Homework Statement This is actually a very elementar Halliday's computational problem: One person pushes a block with a force given by the equation F = 200e^{-0.15t} (given in Newtons and seconds) and the block has, with the surface, a constant friction force of 80N. Find when the block...
  49. J

    Newton's second law and ice skating

    Hello no answer is given to this problem so could somebody please tell me if the answer is correct? thanks so much! Two ice skaters, Paul and Tom, are each holding on to opposite ends of the same rope. Each pulls the other toward him. The magnitude of Paul's acceleration is 1.25 times...
  50. G

    Newton's second law and Springs

    Spring Force is F = -kx So, because the force is not constant, the acceleration is also not constant.. How then can we apply Newton's second law: F = mass x acceleration to springs?
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