# Tagged Questions

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.

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### How much more “work” (or energy) is required for short people to hike uphill?

Let's assume Short Person is 5 feet tall and the Normal Person is 5 feet and 8 inches tall. Let's assume the uphill hike has steps with 1 foot height. How much more work or energy does this require ...
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### Resolving horizontal and vertical components on an inclined plane

Here is a very simple mechanics question related to inclined planes: We assume that the body is at equilibrium or at constant speed. To work out the force 'R', I resolve the 100N force due to ...
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### Free Fall with Air Resistance

Can somebody help me understanding why when free falling the force on the vertical axis is F= -mg-kv, where k is the constant of air resistance and v the velocity? Suppose the vertical axis is ...
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### If “For every action there is an equal and opposite reaction”, why does a gun not injure the shooter's hand?

If "For every action there is an equal and opposite reaction", why does a gun not injure the shooter's hand? Would not the "equal and opposite reaction" of firing a bullet be the kickback the shooter ...
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### How is angular momentum conserved

from a classical perspective, what is it about angular momentum fundamentally that means it has to be conserved? Surely if I have a rod about a fixed axis and a moving particle hits the end it will ...
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### Terminal Velocity of 100lb Hail [closed]

In the Apocalypse there is meant to be hail weighing 100 pounds or 45 kg. What would the terminal velocity be, assuming the hailstones are perfect spheres of solid ice? Also, is there a way to guess ...
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### How is normal force distributed along the surface of contact?

Help me settle this argument. A mass $m$ is placed on a thin diving board. The base of the diving board has mass $M>>m$. Does the board tip over? I drew the following FBD and concluded there ...
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### If Earth and Jupiter are orbiting around Sun in same orbit, will they have same time period?

Let us suppose that Earth and Jupiter are orbiting around the Sun in same orbit, If we talk about at any particular point. (Which is not possible but for the sake of question) I was wondering that ...
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### Does circular motion cause centripetal force OR does centripetal force cause circular motion?

Does circular motion cause centripetal force, or does centripetal force cause circular motion, or are they both occurring hand in hand together instantaneously? One more question: If I project a body ...
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### A precise definition of macroscopic and microscopic objects

What are the formal definitions of "macroscopic object" and "microscopic object". How can one differentiate between them? I mean, is there any fixed condition by which we can distinguish between them?
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### Is not force required to start each and every motion,uniform or non-uniform however they might be?

A body which is at rest will remain at rest; and a body which is in motion will continue motion in a straight line as long as no unbalanced force acts on it. This is Newton's 1st law of motion,...
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### Bouncing ball time problem

I'm studying a problem and I encountered a strange problem: When a ball bounces how much time does the ball spend while touching the floor? To be more clear I suppose that when a ball bounces ...
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### What is the third law action pair of buoyancy?

Consider a block floating in a fluid. The force due to gravity balances the buoyant force exerted on the block. But there must be a third-law action pair to the buoyant force. If the block exerts a ...
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### Why doesn't the potential energy of any object equal 0

Consider a particle on the ground. This particle is raised by a force of magnitude $mg$ to a height $h$ above the ground. At this point, the work done on the particle by the force is $mgh$, which is ...
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### Proof that the ground pushes you up [duplicate]

It has been said that when you try to jump you are exerting your force on the ground and the ground pushes you up. I was wondering if this was an over-simplification or not. If so, what's the full ...
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### Intuitive understanding of centripetal vs. centrifugal force

I am having trouble understanding how centripetal force works intuitively. This is my claim. When I have a mass strapped on a string and spin it around, I feel the mass pulling my hand. So, I want ...
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### Block and inclined plane (INPhO Problem)

The figure shows two blocks on an inclined plane of mass 1kg each.The coefficient of static as well as kinetic friction is $0.6$ and angle of inclination is $30^\circ$ . Find the acceleration of the ...
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This page has an interesting video of beads 'syphoning' out of a glass beaker: http://blog.zennioptical.com/weekly-optical-illusion-crazy-beads/ The host has a few explanations for the effect, but ...
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### Action Reaction when I pushing a trolley? [duplicate]

I tried to explain how those force work but I can hardly figure it out. I exerted a force on the trolley and there will be a force on trolley on me as well. This is the newton's third law. But why the ...
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### How is the singularity in Newtonian gravity resolved?

In Who's afraid of a Black Hole? at time ~20:38, Michio Kaku makes a claim that $1/r$ when $r=0$ equals $\infty$ and when physicists see the $\infty$ they see a monstrosity. Coincidentally, Newton's ...
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### Simple Projectile Motion Question

A volcano erupts 50m below the sea level. A rock leaves the crater at 20 m/s at an angle 30 deg with the vertical line. The rock has a mass of 15kg. IGNORE WATER RESISTANCE. It gets out of the water, ...
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### Work done by spring over distance

I'm working through a problem involving energy conservation. Unfortunately, I cannot calculate the work done by a spring. ...
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### Newton's Second Law the real one. Is my theory correct?

In Newton's Principa, $\sum \vec{F}_{ext} = \frac{d\vec{\rho}}{dt}$ If the momentum vector is in mutildimensions, wouldn't a more general equation be $\sum \vec{F}_{ext} = \vec{\nabla}{\vec{p}}$ I ...
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### Work done by internal forces of a rigid body

I am reading Goldstein's Classical Mechanics book, and I came across that: In a rigid body the internal forces do no work Is this statement based on the assumption that the internal forces are ...
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### Is drag force in the direction of particle motion or opposite to motion?

Suppose water is flowing in horizontal direction (positive $x$-direction) and a particle immersed in that water is also moving in the same direction. In this case, is the drag force $F_D$ in the ...
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### Doubt on series springs [closed]

Here is a description of the motion of two springs in series. The premise is that the force on the two springs is the same. This is derived from the following reasoning: when I pull the mass with a ...
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### Is it correct to say Newtonian mechanics is a subset of Quantum mechanics?

I grew up in a three dimensional (3D) reality described quite well by Newtonian mechanics as opposed to the reality described by Quantum mechanics. That is I could go to bed at night without worrying ...
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### how is a pendulum clock's time and the time period of the pendulum in it related? [closed]

I'm working out how much time a pendulum clock will gain or loose due to change of the length of the pendulum due to temperature. so far I've got, new time period, $$T_2=T_1(1+\frac12\alpha\Delta T)$$ ...
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### Optimal curve for a marble drop [closed]

I'm doing a marble roller coaster project for my physics class at school. The first part of our roller coaster involves a marble falling downwards into a curve that will drop then go up 12 inches. ...
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### Why is it easier to pull a lawn roller rather than pushing it?

Does it have something to do with friction or weight distribution in pulling is favorable or anything else ?
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### How to calculate escape velocity and orbital velocity for a cubular planet?

Given a planet that is a cube, how would one calculate the escape velocity and the orbital velocity? For a circular planet with an even distribution of mass (aka center of mass=center of volume) the ...
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### Can the force acting on a body remain constant if the body is accelerating?

Since $F=ma$, the force should change if acceleration changes shouldn't it? And since acceleration is the rate of change of velocity, can velocity be constant if acceleration changes?
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### Why does mechanical energy have to equal zero to find escape velocity?

A object orbiting the earth has total mechanical energy equal to \begin{align*} E^{mech} = \frac{1}{2} m v^2 - \frac{GMm}{r} \end{align*} with $M$ the mass of the earth and $r$ the distance. My course ...
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### How can i prove that $\frac{d\vec{p}}{dt} = \vec{F_{net}}$? [duplicate]

I could prove that $d\vec{p}/dt = m\vec{a}$, but how can I prove that $\vec{F_{net}}=d\vec{p}/dt$? Is force just defined by this expression? What is the rigorous definition of a general force? In my ...
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### Quantum violation of Newton's Third Law? [closed]

From this site: http://www.learning-mind.com/5-thought-provoking-quantum-experiments-showing-that-reality-is-an-illusion/ I gained the knowledge that a group of scientists, upon measuring a tiny ...
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### In general relativity, how do we think of Newton's third law for gravity?

In GR everything become geometric, gravity becomes curvature in spacetime. How do we think of Newton's third law in the context of GR? What corresponds to action and what to reaction?
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### Newton's Third law of motion: A hammer hits a wall vs. hits a tire

When we hit the wall by a hammer, Newton’s third law says that hammer applies force on the wall (action) and wall applies force on hammer (reaction) equal in magnitude but opposite in direction ok ...
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### Period of a simple pendulum accounting for friction

The period of a simple pendulum is $$T=2\pi\sqrt{\ell/g},$$ but no where in there do I see that it accounts for friction. Does it somehow account for friction, and if not, how could you do that?
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### Would a planet in a binary star system follow Kepler's Laws?

I'm thinking it would not. I am thinking Kepler's Laws were a simplification that were possible with a simple two body (sun, planet) but would not apply for something more complex (sun1, sun2, planet)...
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### Does the Earth revolve around the Sun?

I am aware of this Phys.SE question: Why do we say that the earth moves around the sun? but I don't think this is a duplicate. In a binary star system, where the masses of the 2 stars are not so ...
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### Two boys pull a rope in the two edges with the force 50 N each. Will the rope rip if it can handle only a pull of 80 N? [closed]

I think we should make the sum the two forces 50+50=100 N and say it can't handle? Can you explain me using the 3rd Newton Law?
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### Why is there conservation of kinetic energy in elastic collision and not in inelastic collision?

Why is there conservation of kinetic energy in elastic collision and not in inelastic collision? What would be the difference that lead to conservation of kinetic energy in elastic collision and not ...
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### Is linear momentum conserved in an isolated system always?

Consider a isolated system of particles. If no external force acts on this system, the momentum of the system should remain constant. If the isolated system of particles represents living creature....
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### Question relating to energy transfer in between two particles

How would I go about calculating the maximum kinetic energy transferred from one particle when it forms an elastic collision with another particle. For example, if I had two billiards one with mass m ...
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### Distinction between torque and force

Distinction between Torque and Force. Consider a case when a cylinder is rolling down a rough inclined plane. While analysing its motion, we consider the force applied by the friction and the ...
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### If an asteroid were threatening the Earth, could I deviate it just by jumping on it?

An impact by a 10 kilometres asteroid on the Earth has historically caused an extinction-level event due to catastrophic damage to the biosphere. There is also the threat from comets coming into ...