Tagged Questions
1
vote
1answer
111 views
Buoyancy Problem - Cubes in water
I have a tank with water (10 m high) , with an ideal seal at the bottom (water can't fall down, but can enter bodies).
I have a system of 6 cubes ( of polystyrene density= 20 Kg/m^3) with dimension ...
1
vote
1answer
67 views
Inclined plane question
An object, mass $m$ is placed on an incline, angle $\theta$. System is at equilibrium. coefficients of static and kinetic frictions are $\mu_s$ and $\mu_k$ respectively. Then:
1) What is the Total ...
0
votes
1answer
50 views
Another Inclined plane question
I did the FBD, and I found too many variables which are not eliminating...Moreover, I believe this question is based on kinetic and static friction. But, $\mu$ here is ambiguously defined...How Do I ...
1
vote
0answers
19 views
Acceleration by spherical particles (micron-scale) by an external force
I am looking for an expression for the velocity of a micron sized (1 - 10 micron diameter) sized particles under accelerating forces.
I have aerosols in mind.
This is what I have in mind
The ...
0
votes
3answers
148 views
Why does a rod rotate?
I'm a physics tutor tutoring High School students. A question confused me a lot.
Question is:
Suppose a mass less rod length $l$ has a particle of mass $m$ attached at its end and the rod is ...
-1
votes
2answers
52 views
Force applied in a body moving at high speed [closed]
Consider a rod of length $l$ and uniform density is moving at high speed. I want to deflect the rod where should I need to apply the minimum force, so that the rod is deflected..?
1
vote
2answers
76 views
Resolution of vectors
What is the fundamental basis of resolution of vector. Suppose we have a vector $\vec{mg}$, now we resolve it into two components, horizontal and vertical. My question is what is the basis for telling ...
2
votes
1answer
126 views
Is there a geometrical way to obtain a relationship between these vectors?
Suppose we have a setup like this. Here $a_1,a_2,b_1,b_2$ are acceleration magnitudes($b_1,b_2$ being relative) and $P,Q,R,S$ are not pulley/blocks but are points on the rope. If I use a geometrical ...
1
vote
4answers
209 views
Bat hitting a ball
When a bat hits a ball, consider two cases:
1) The batsman goes for a defense, and stonewalls it, to reduce its speed.
2) the batsman goes for a shot, e.g. a home-run, etc.
in which case will the ...
0
votes
0answers
52 views
An ideal toilet roll on a flat surface [closed]
Suppose we have an ideal toilet roll on a flat surface with the outer end of the roll fixed to the surface. After we give the roll an initial velocity such that the roll unwinds, what happens to the ...
3
votes
1answer
167 views
Is there any case in classical mechanics where Newton's (strong) third law doesn't hold?
Is there any case in classical (non relativistic) mechanics where the strong form of Newton's third law does not hold (that is, reaction forces are not collinear)? For example, if we consider a system ...
7
votes
3answers
336 views
Force through quantum mechanics
In classical physics force is:
$$F=\frac {dp}{dt}$$
How about quantum mechanics?
In Old Quantum Mechanics momentum is: $p=\hbar \cdot k$ so force will be:
$$F=\hbar \frac {dk}{dt}$$
what does $\frac ...
0
votes
0answers
340 views
How to find the tension? [closed]
How do I find the tension in the string? And how should I find out if the boxes have moved?
1
vote
2answers
104 views
Is it easier to move or rotate an object? [closed]
This is a problem I'm facing on designing a moving bookcase for my home, I just don't have enough physics background to tackle the problem.
Which of the following requires more force/effort?
A ...
0
votes
2answers
664 views
When does not Newton's 3rd law apply?
Is Newton's 3rd law valid in non-inertial frames?
If so, then are there other cases for which Newton's 3rd law is not applicable?
3
votes
1answer
139 views
In $\textbf{f} = -\boldsymbol{\nabla} u$, what is $u$?
I know that force is the negative gradient of the potential:
$$\textbf{f} = -\boldsymbol{\nabla} u$$
where force $\textbf{f}$ is a vector and $u$ is a scalar.
This is a relatively soft question, ...
4
votes
2answers
165 views
Is it normal for physical functions to lack a 2nd derivative?
My question is about the appearance of a non-analytic function in the formula for the resistive force in air or other medium. Considering the 1-dimensional case as covered by Walter Lewin in his 8.01 ...
1
vote
2answers
182 views
What does $\textbf{f} = -\boldsymbol{\nabla} u$ mean in practice and how is it computed?
In classical computer simulations such as molecular dynamics (MD) simulations, one integrates Newton's equations of motion to determine particle trajectories. If we think of Newton's Second Law as ...
0
votes
0answers
84 views
Aerodynamic drag on a cannonball?
I'm trying to build a ballistics simulation where I shoot a cannonball. I want to allow for drag and am trying to work out the math to do so.
I can work the drag out using $F = Cd\times S\times ...
0
votes
1answer
105 views
Say we're driving a bike and suddenly hold the brakes?
It's easy for me to imagine that if we brake the front wheel then there is a chance that I'll flip.
On the other hand if I brake the back wheel, there is no way it'll happen no matter how fast I ...
4
votes
3answers
515 views
Why do we need the quantity momentum?
Why do we need the quantity Momentum in physics when we have the quantities like Force and Energy? Isn't it possible to substitute the usage of Momentum with equivalent of Force and Energy?
6
votes
2answers
596 views
Can a force in an explicitly time dependent classical system be conservative?
If I consider equations of motion derived from the pinciple of least action for an explicilty time dependend Lagrangian
$$\delta S[L[q(\text{t}),q'(\text{t}),{\bf t}]]=0,$$
under what ...
0
votes
2answers
503 views
How to interpret this vertical circular motion problem?
A bucket of water is tied to a rope and swung in a vertical circle.
The distance from the bucket centre to the axis of rotation is
$2.08m$. Calculate the angular velocity (in $rad s^{-1}$) of ...
1
vote
1answer
266 views
What techniques can be used to analyze a rod rotating about the edge of a table?
A uniform rod of length $4x$ is rotating
about the edge $O$ of the table. (The rod does not fall off the table.) The centre of mass $G$ of the rod is distance $x$ away from $O$. The rod is making ...
2
votes
1answer
431 views
Normal force in a compound pendulum (physical pundulum) system?
Consider a compound pendulum pivoted about a fixed horizontal axis, illustrated by the force diagram on the right:
#
Okay, I can't figure out where the normal force on the pendlum should point ...
2
votes
2answers
214 views
What sustains a rigid body's rotation at its constant angular(rotational) speed?
Continuing from the following scenario from my previous question Centripetal force of a rotating rigid body? :
Consider someone pushing a roundabout in a playground. Initially the
roundabout is ...
2
votes
2answers
320 views
What happens when the string slackens during vertical circular motion?
Consider a particle attached to one end of a string of length $l$ moving anti-clockwise in a vertical circle whose centre is $O$. What exactly happens physically when the string becomes slack and ...
3
votes
1answer
320 views
A Question about Virtual Work related to Newton's Third Law
In describing D'Alembert's principle, the lecture note I was provided with states that the total force $\mathbb F_l$ acting on a particle can be taken as,
$$\mathbb F_l=F_l+\sum_mf_{ml}+C_l,$$
...
3
votes
2answers
313 views
Can these figures demonstrating the safety of “Archery Tag” arrows be correct?
There is a new sport called "Archery Tag" that involves shooting opponents with foam-tipped arrows fired out of a real bow. The official Archery Tag web site presents data that claims to show the ...
2
votes
0answers
555 views
Forces and torques about the CENTER OF MASS of a physical pendulum
I'm currently stumped by the following situation. Say we've got a rectangular physical pendulum (think ruler with a hole-punch at one end).
It's trivial to analyze the motion of the pendulum with the ...
2
votes
5answers
302 views
Find drag force on link of rotating chain
Given a closed chain with a total length of 1.2m rotating at 1'800 rpm and a total mass of 0.4kg, what is the drag force pulling on one chain link?
I originally thought that since no link size was ...
1
vote
1answer
230 views
What conditions must be met for a ball to roll perfectly down an incline without slipping?
What conditions must be met for a ball to roll perfectly down an incline without slipping? A mathematically rigorous definition, please.
I honestly don't know where to begin with answering this ...
2
votes
3answers
209 views
Why can't we ascribe a (possibly velocity dependent) potential to a dissipative force?
Sorry if this is a silly question but I cant get my head around it.
2
votes
1answer
384 views
Countersteering a motorcycle
Everyone knows the story about countersteering. For those who don't I will explain it below and after the explanation i will ask my question.
You can watch this short video as a beginning: ...
3
votes
3answers
322 views
About constructing potential energy functions
There are many classical systems with different potential functions. My problem is that I do not understand how one can construct a certain potential function for a certain system. Are there any ...
3
votes
2answers
227 views
The Double Integrator: Matching velocity and position as quickly as possible with only a limited amount of force available
If a body with mass $m$ begins at position $x_0$ with velocity $v_0$ and experiences a force that varies as a function of time $f(t)$ (and we ignore gravity, friction, and everything else that might ...
3
votes
2answers
180 views
Why doesn't phase space contain acceleration/forces?
I'm watching some Physics lectures on the internet by Leonard Susskind:
http://www.youtube.com/watch?v=pyX8kQ-JzHI&feature=BFa&list=PL189C0DCE90CB6D81&lf=plpp_video
In this lecture, and ...
3
votes
3answers
741 views
Define Pressure at A point. Why is it a Scalar?
I have a final exam tomorrow for fluid mechanics and I was just looking over the practice exam questions. They do not provide solutions. But pretty much I have to define pressure at a point and also ...
2
votes
2answers
968 views
Book against a wall and forces
If you take a book with mass of 1kg and push it against the wall. With how much force do you have to push the book so it does not fall?
The problem is I know how to calculate this problem, you say ...
2
votes
1answer
169 views
Additional accelerating force during take off of a rocket?
During the take off of a rocket, the exhaust produces some pressure below the rocket, which gives an additional force. How large is this force in comparison to the force produced directly by the ...
3
votes
2answers
517 views
Classical car collision
I have a very confusing discussion with a friend of mine.
2 cars ($car_a$ and $car_b$) of the same mass $m$ are on a collision course. Both cars travel at $50_\frac{km}{h}$ towards each other.
They ...
4
votes
3answers
2k views
Force as gradient of scalar potential energy
My text book reads
If a particle is acted upon by the forces which are conservative; that is, if the forces are derivable from a scalar potential energy function in manner $ F=-\nabla V $.
I ...
2
votes
2answers
877 views
Normal force: up or down?
The normal force obviously always has direction perpendicular to the surface of contact, but I'm a bit confused about its sense: is it going 'up' or 'down'? I've seen articles on the web that describe ...
1
vote
3answers
163 views
Constrained particles under distance dependent force
This question is from the 1975 Canadian Association of Physicists Exam. No solutions are posted and I am quite lost on how to proceed with it.
A particle is constrained to move along the x-axis of a ...
3
votes
4answers
397 views
What do we consider “Perpetual Motion”
I know this is a bad question to most serious Physics but I have a question about what is considered “Perpetual motion.” The Foucault pendulum in the UN consists of sphere that passes directly over a ...
6
votes
2answers
304 views
Why don't we consider centrifugal force on a mass placed on earth?
Let us say a block of mass is placed on the surface of earth. Then while drawing the forces on that body, we say:
Force F = mg acting towards the center of earth
Normal reaction N offered by the ...
2
votes
7answers
2k views
What are the Constraints on Building a Tower to Space?
When I was a boy I used to daydream about building a tower so tall that the top of it would stick out of the top of Earth's atmosphere project into near space.
There would perhaps be a zero gravity ...
1
vote
1answer
2k views
Find velocity and displacement from force equation
How to find velocity and displacement equations from a given force equation? For instance, it was given the following 1-D equation:
$$F = b_1(v_1-v) - b_2 v$$
$v_1$, $b_1$ and $b_2$ are constants.
...
4
votes
2answers
666 views
Calculate stainless steel pole necking limit
Background
Trying to determine how much weight a post can support without necking when a monitor is attached to an articulated arm: a cantilever problem.
Problem
There are three objects involved in ...
7
votes
4answers
828 views
How far does a trampoline vertically deform based on the mass of the object?
If a baseball is dropped on a trampoline, the point under the object will move a certain distance downward before starting to travel upward again. If a bowling ball is dropped, it will deform further ...
