Newtonian mechanics covers the discussion of the movement of classical bodies under the influence of forces by making use of Newton’s three laws. For more general discussion of energy, momentum conservation etc., use classical-mechanics, for Newton’s description of gravity, use newtonian-gravity.

learn more… | top users | synonyms (3)

17
votes
2answers
3k views

Is there an intuitive reason the brachistochrone and the tautochrone are the same curve?

The brachistochrone problem asks what shape a hill should be so a ball slides down in the least time. The tautochrone problem asks what shape yields an oscillation frequency that is independent of ...
17
votes
5answers
1k views

Conservation of Mathematical Constraints when deriving Energy and Momentum from $F=ma$

Background: Starting from $F = ma$, integrating with respect to time, and using basic calc, one can derive $\int Fdt = m (v_f - v_i)$ Starting from $F = ma$, integrating with respect to distance, ...
16
votes
5answers
2k views

Why are these periods the same: a low earth orbit and oscillations through the center of the earth?

Related: Why does earth have a minimum orbital period? I was learning about GPS satellite orbits and came across that Low Earth Orbits (LEO) have a period of about 88 minutes at an altitude of 160 ...
16
votes
6answers
6k views

Zero velocity, zero acceleration?

In one dimension, the acceleration of a particle can be written as: $$a = \frac{dv}{dt} = \frac{dv}{dx} \frac{dx}{dt} = v \frac{dv}{dx}$$ Does this equation imply that if: $$v = 0$$ Then, ...
16
votes
6answers
27k views

Is Melancholia's orbit impossible?

In the 2011 movie Melancholia, a planet, also called Melancholia, enters the solar system and hits the Earth. I want to leave aside the (also unreasonable) aspect that planet "hides behind the Sun" ...
16
votes
3answers
2k views

Does holding weights allow you to run faster?

I'm not talking about training with weights so that when you run without them your muscles are used to pushing more weight so with less weight it'll be easier, I'm talking about while actually holding ...
16
votes
3answers
6k views

Why are Saturn's rings so thin?

Take a look at this picture (from APOD http://apod.nasa.gov/apod/ap110308.html): I presume that rocks within rings smash each other. Below the picture there is a note which says that Saturn's rings ...
16
votes
5answers
442 views

Does the mass point move?

There is a question regarding basic physical understanding. Assume you have a mass point (or just a ball if you like) that is constrained on a line. You know that at $t=0$ its position is $0$, i.e., ...
15
votes
5answers
10k views

How fast would someone have to run to travel vertically up a wall?

I am currently doing a physics project on the effects of so-called 'super-speed'. I was wondering how fast you would have to run to vertically travel up a wall? That is, to negate the force of ...
15
votes
4answers
5k views

How did Newton discover his third law of motion?

How did Newton discover his third law? Was it his original finding or was it a restatement of someone else's, like the first law coming from Galileo? What initiated the concept of what is now known as ...
15
votes
2answers
4k views

Is kinetic energy a relative quantity? Will it make inconsistent equations when applying it to the conservation of energy equations?

If the velocity is a relative quantity, will it make inconsistent equations when applying it to the conservation of energy equations? For example: In the train moving at $V$ relative to ground, ...
15
votes
10answers
29k views

What do people actually mean by “rolling without slipping”?

I have never understood what's the meaning of the sentence "rolling without slipping". Let me explain. I'll give an example. Yesterday my mechanics professor introduced some concepts of rotational ...
15
votes
3answers
3k views

What happens in a car crash?

My mom and I were in a car accident. We are ok, but I want to know how fast the car that hit us was going. We were stopped at a light. The car that hit us from behind was a big GMC SUV. Our car was a ...
15
votes
3answers
6k views

Is the normal force a conservative force?

Most of the time the normal force doesn't do any work because it's perpendicular to the direction of motion but if it does do work, would it be conservative or non-conservative? For example, consider ...
15
votes
4answers
787 views

Anti-gravity in an infinite lattice of point masses

Another interesting infinite lattice problem I found while watching a physics documentary. Imagine an infinite square lattice of point masses, subject to gravity. The masses involved are all $m$ and ...
15
votes
1answer
2k views

Can I move an oil tanker?

Let's say that I'm standing at the edge of a basin made of concrete. This basin is filled with salt water and a ship floats on the water close to the edge of the basin. The ship is neither anchored ...
15
votes
5answers
63k views

jumping into water

Two questions: Assuming you dive head first or fall straight with your legs first, what is the maximal height you can jump into water from and not get hurt? In other words, an H meter fall into ...
15
votes
1answer
285 views

How fast would a truck have to go to pull a pedestrian onto the road? [closed]

Let's suppose a pedestrian P is walking or standing next to a highway. Suppose a truck T will drive by the said pedestrian at speed V leaving distance L between the two. Assuming L is a reasonably ...
15
votes
2answers
603 views

Why do speed skaters use their right hand to accelerate?

I was watching the speed skating event, and was wondering, why do the athletes use their right hand while accelerating? If they want to increase the moment of inertia, then they should move the hand ...
14
votes
7answers
3k views

What does an undefined formula in physics mean?

I am trying to figure out how undefined formulas in mathematics relates to physics. Take the following formula for terminal velocity. $$V_\text{terminal} = \sqrt{ mg \over{c \rho A}} $$ Say we have ...
14
votes
6answers
4k views

Throwing an object around the Earth. Is it possible?

Some of you might be familiar with the cartoon called Astérix. In one of the episodes, Obelix (the really strong fat guy in the stylish striped pants) enters a spear throwing competition. He beats the ...
14
votes
6answers
4k views

Can you ever exert more downwards force than your weight?

So, because I'm a hardcore person, I risked all this afternoon by going out in the wind, the rain and the cold to construct a willow den. Yes, it seems a menial task, but it was actually quite ...
14
votes
7answers
7k views

Why do we fall down when the bicycle slows down? [duplicate]

My question is: It is easy to balance a bicycle when it is moving at a fairly high velocity, say 7 m/s or 25 km/hr. But when a bicycle slows down, it is hard to keep it upright, and the person riding ...
14
votes
6answers
2k views

Why $F=ma$ and not $F=m\dot{a}$?

What would happen if instead of $F=m*d^2x/dt^2$, we had $F=m*d^3x/dt^3$ or higher? Intuitively, I have always seen a justification for $\sim 1/r^2$ forces as the "forces being divided equally over ...
14
votes
8answers
5k views

Does leaning (banking) help cause turning on a bicycle?

I think it's clear enough that if you turn your bicycle's steering wheel left, while moving, and you don't lean left, the bike will fall over (to the right) as you turn. I figure this is because the ...
14
votes
4answers
4k views

Deriving Newton's Third Law from homogeneity of Space

I am following the first volume of the course of theoretical physics by Landau. So, whatever I say below mainly talks regarding the first 2 chapters of Landau and the approach of deriving Newton's ...
14
votes
5answers
2k views

Newton's Bucket

Newton's Bucket This thought experiment is originally due to Sir Isaac Newton. We have a sphere of water floating freely in an opaque box in intergalactic space, held together by surface tension and ...
14
votes
6answers
5k views

Is the energy conserved in a moving frame of reference?

Consider this situation: When the box is at the bottom of the frictionless incline, it will have a velocity of $v_f$. The person is an inertial frame of reference that moves at a constant ...
14
votes
7answers
2k views

Is rotational motion relative to space?

Let's assume that there is nothing in the universe except Earth. If the Earth rotates on its axis as it does, then would we experience the effects of rotational motion like centrifugal force and ...
14
votes
3answers
7k views

What exactly are Hamiltonian Mechanics (and Lagrangian mechanics)

What exactly are Hamiltonian Mechanics (and Lagrangian mechanics)? I want to self-study QM, and I've heard from most people that Hamiltonian mechanics is a prereq. So I wikipedia'd it and the entry ...
14
votes
2answers
3k views

Are there examples in classical mechanics where D'Alembert's principle fails?

D'Alembert's principle suggests that the work done by the internal forces for a virtual displacement of a mechanical system in harmony with the constraints is zero. This is obviously true for the ...
14
votes
1answer
418 views

Modelling the movement and jumps of a chalk while drawing a dashed line on a blackboard

You probably know that if you try to draw a line using a piece of chalk on a blackboard , under some conditions (for example, $\alpha<\frac{\pi}{2}$ in the picture below) you will have a dashed ...
14
votes
3answers
583 views

Underdetermined forces in a statics problem

I was trying to cook up a good exam problem on Newtonian mechanics, and I came up with one that I didn't fully understand myself. Always fun when you can outsmart yourself. In the figure a cylinder ...
13
votes
4answers
4k views

Why doesn't a tied balloon behave like a pendulum?

It is well known that a tied weight will oscilate under the effect of gravity if left from aside, like a pendulum. However, if we tie a helium balloon to the ground from and left it form the floor ...
13
votes
6answers
3k views

Why is there a $\frac 1 2$ in $\frac 1 2 mv^2$?

For elastic collisions of n particles, we know that momentum in the three orthogonal directions are independently conserved:$$ \frac{d}{dt}\sum\limits_i^n m_iv_{ij} =0,\quad j=1,2,3$$ From this, it ...
13
votes
6answers
9k views

Why I think tension should be twice the force in a tug of war

I'm going to provide my argument for why I think the tension in a rope should be twice the force exerted on either side of it. First, let's consider a different example. Say, there is a person named ...
13
votes
2answers
1k views

An example which contradict to Newton's 3rd law?

Let a,b be two charged particles. $$\vec{r}_a(0)=\vec{0}$$ $$\vec{r}_b(0)=r\hat{j}$$ $$\vec{v}_a(t)=v_a \hat{i}$$ $$\vec{v}_b(t)=v_b\hat{j}$$ In which both $v_a$ and $v_b$ $<<c$. Then ...
13
votes
3answers
3k views

Is there a deep reason why springs combine like capacitors?

I was solving a practice Physics GRE and there was a question about springs connected in series and parallel. I was too lazy to derive the way the spring constants add in each case. But I knew how ...
13
votes
3answers
2k views

Throwing a ball upwards in an accelerating train

If I throw a ball upwards to a certain height in an accelerating train, will it end up in my hand? At the moment I release the ball, it will have a velocity equal to that of the train at that instant. ...
13
votes
3answers
2k views

Galilean invariance of Lagrangian for non-relativistic free point particle?

In QFT, the Lagrangian density is explicitly constructed to be Lorentz-invariant from the beginning. However the Lagrangian $$L = \frac{1}{2} mv^2$$ for a non-relativistic free point particle is ...
13
votes
4answers
1k views

Is the second law of thermodynamics a fundamental law, or does it emerge from other laws?

My question is basically this. Is the second law of thermodynamics a fundamental, basic law of physics, or does it emerge from more fundamental laws? Let's say I was to write a massive computer ...
13
votes
3answers
2k views

What is the physics of a spinning coin?

When we spin a coin on a table, we observe 2 things: It slows down and stops after sometime. It does not stay at just one point on the table but its point of contact with table changes with time. ...
13
votes
1answer
407 views

Where is the “event horizon” on a basketball hoop?

I'm watching a lot of basketball this month. A common event is the ball going part way into the hoop and then coming out again. Announcers sometimes claim that the ball was "halfway through" when it ...
12
votes
17answers
33k views

Recommendations for good Newtonian mechanics and kinematics books

What are some good books for learning the concepts of Kinematics, Newton laws, 2D Motion of Object etc.?
12
votes
5answers
7k views

Why doesn't this magnetic perpetual motion machine work?

I know that this machine does not work, via thermodynamics. I am asking for an analysis in terms of mechanics and magnetism. Anyway, so here is the machine: The magnet (the red ball) pull the ball ...
12
votes
15answers
11k views

Is Newton's third law always correct?

Newton's third law states that every force has an equal and opposite reaction. But this doesn't seem like the case in the following scenario: For example, a person punches a wall and the wall breaks. ...
12
votes
5answers
2k views

What causes the back of a bike to lift when the front brake is applied?

What causes the back of a bike to lift when the front brake is applied? (Like in an endo.) Also, if I were to replicate this effect with a wood block with wheels that crashes against a wall (only the ...
12
votes
6answers
2k views

If the solar system is a non-inertial frame, why can Newton's Laws predict motion?

Since there is no object in the universe that doesn't move, and the solar system likely accelerates through space, how did Newton's Laws work so well? Didn't he assume that the sun is the ...
12
votes
9answers
11k views

How do you explain spinning tops to a nine year old?

Why don't spinning tops fall over? (The young scientist version) My nine year old son asked me this very question when playing with his "Battle Strikers" set. Having studied Physics myself, I am very ...
12
votes
5answers
30k views

Can the coefficient of static friction be less than that of kinetic friction?

I was recently wondering what would happen if the force sliding two surfaces against each other were somehow weaker than kinetic friction but stronger than static friction. Since the sliding force is ...