# Tag Info

1 vote

### Basic: Newton's Laws of Motion doubt regarding resolution of forces

Think of ice and a sidewalk. Both will hold you up, keeping you from sinking into them. That is, both exert a reaction for on you just as hard as you push into them. This force does not lift you off ...
• 41.6k
Accepted

### Basic: Newton's Laws of Motion doubt regarding resolution of forces

The reason why the second method fails is that physically this is the assertion that an object on the slope cannot roll downwards, rather than the assertion that the object cannot sink into the slope (...
• 158

### Can't one always define $U=-m\ddot xx$?

Can't one always define $U=-m\ddot xx$? One can make such a definition, but one should not do so. A function $U$ defined as $-m\ddot x x$ would not be the correct potential energy for any case beside ...
• 22k
1 vote

### Can't one always define $U=-m\ddot xx$?

Comments to the question (v3): In a nutshell, the Newton's 2nd law$^{1}$ $$m{\bf a}~\approx~{\bf F}\tag{1}$$ ("EOM") equates kinematics ("mass times acceleration") with dynamics (&...
• 207k
1 vote

• 8,443

### What is the absolute Value of Normal force on a block on Wedge?

The first method doesn't work because the forces are not balanced in the vertical direction. A block on a ramp tends to accelerate downwards, so there should be a net vertical downwards force. The ...
• 1
1 vote

### How can you have a potential in a theory without any forces?

In terms of a field theory, the term $V$ in the Lagrangian is not connected with an interaction with a different field (which is missing), but it rules the dynamics and the ground state of the "...
Accepted

### What does Feynman mean here about static and sliding friction?

In the lubrication case, in the real world as two surface slide over each other there are hydraulic effects that cause the coefficient of friction to drop signicantly with velocity before gradually ...
• 5,020

### When is Normal force 0?

I am assuming you are talking about two objects at rest on a flat surface as illustrated below The two arrows signify the normal force that exist between each block and the flat surface they rest on ...
• 5,020

### When is Normal force 0?

Normal force is a reaction to a good old push acting on some body. A simple contact doesn't suffice for a normal force to generate. Someone must push on someone else so that the someone else can push ...

### When is Normal force 0?

So, if a body is just placed on a frictionless horizontal surface, it will exert a normal force on the surface and the surface will exert a normal force on the body, if gravity or any other external ...

### When is Normal force 0?

All objects are at least slightly compressible, no matter how rigid they are. Therefore, they act "spring-like" when pushed on. Thus, when two objects are placed just in contact with each ...
• 8,369

### How does a vehicle's brake affect the friction between the vehicle and ground?

While answering another question, I found a neat way to think about this problem. What you are essentially asking, is how does the force applied by the brakes reach the ground. I think that root ...
• 12.5k
Accepted

### How does force vector affect rotation and translation?

This is what you obtain, if you transfer the forces to the center of mass. Why ? For the center of mass you obtain two equations $$m~a= \sum F$$ $$I~\alpha=\sum \tau$$ You obtain the same results ...
• 12.5k
1 vote

### How does force vector affect rotation and translation?

Situation 3. There is only one vector, placed on the edge of the object, perpendicular to the center. Will it result only in rotation No. It will result in rotation plus translation since there is a ...
• 74.3k

### How does force vector affect rotation and translation?

Situation 3: there is a net vertical force, so there will be vertical motion. And there is a net torque about the center of mass, so there also will be rotation. That's why it's a combination of (1) ...
• 121
Accepted

### Conceptual doubt in existence of normal force in a constrained motion problem

As Farcher noted in the comments, there must be a horizontal force acting on $B$ if it is to accelerate horizontally. So there must be a force acting at least one of the points $P_1$ or $P_2$. There ...
• 50.1k

### Are there any "induced" forces which are repulsive in nature?

Diamagnetic materials are not a good example of a repulsive force. If you place a superconducting body under a magnetic field, the body will remain suspended at some distance from the magnet. A ...
• 10.6k
1 vote
Accepted

### Conceptual doubt related to normal force in a two-block system kept on frictionless surface

Here are a couple of ways of thinking about it. The two blocks are accelerating to the right. Consider the block on the left. For it to accelerate to the right the applied force acting on it to the ...
• 8,828
1 vote
Accepted

### Wedge constrained motion problem

Yes, both of your method is correct. Here is answer to your doubts. In the first method you have solved the question in ground frame of reference, for which the motion of block B is in some direction ...
Accepted

### Are there any "induced" forces which are repulsive in nature?

Yes, diamagnetic materials are repelled from the source of magnetic field (pushed in direction of field decrease). For example, bismuth, copper and even water at normal temperatures are diamagnets (...
• 39.2k
Accepted

### Conceptual doubt related to motion of two blocks on an incline

Contact between them does not necessarily imply a normal force between them. A normal force between them will only appear if they are pressed together - that is, if one exerts a force on the other. ...
• 51.6k

### Conceptual doubt related to motion of two blocks on an incline

If the accelerations of the blocks along the incline are the same then they should maintain contact irrespective of their masses as they are released. So, how can the normal contact force between them ...
• 6,516
1 vote

### Conceptual doubt related to motion of two blocks on an incline

Suppose the two masses were sitting in contact on the table top. They would exert normal forces on each other just strong enough to keep themselves from penetrating into each other. But not strong ...
• 41.6k
Accepted

### Interesting Aerofoil Logical Fallacy

There is no possibility for perpetual motion, but you have to be careful about how you treat the drag. In your diagrams, the air particles all flow with uniform velocity, which suggests you are ...
• 500

### Confusion on resolving the normal force on an inclined plane

I don't understand how different coordinate system can predict different normal forces? They don't. The normal force is defined as the force orthogonal to the surface and this is the same however we ...
• 6,516

### Vertical Circular Motion ($u$ minimum)

If we have mass $m$ on a string of length $R$, on Earth ($g$), then the potential energy vs standard pendulum angle is: $$U(\theta) = mgR(1-\cos\theta) + C$$ where $C$ is a constant. I want to pick $C$...
• 35.9k
Accepted

### Vertical Circular Motion ($u$ minimum)

For the vertical pendulum, the string will not be taut for speeds at the apex $v < v_c$ where $v_c$ is the minimum speed calculated by letting the tension approach zero. Therefore the bob will ...
1 vote
Accepted

### How can a non-derivative interaction involve the derivative of a scalar field?

First, just a small terminology note. People normally say "derivative" instead of "4-derivative" to refer to $\partial_\mu$. People might say "4-gradient" if they want to ...
• 51.4k
1 vote

### How can a non-derivative interaction involve the derivative of a scalar field?

The kinetic term does not count as an interaction. Quadratic terms in the Lagrangian are present in the free theory. Terms with three or more instances of the field are interactions. Hence, in the ...
• 22.3k
1 vote

### How can a non-derivative interaction involve the derivative of a scalar field?

If $U(\phi)$ is not a quadratic function, then the higher-order terms (e.g. a $\phi^4$ term) can be interpreted as field self-interactions that donâ€™t involve derivatives.
• 48.5k

### How exactly does curved space-time describe the force of gravity?

If we ... use a fictional device to cancel all the inertia of the particle, it is obvious that the curve of space-time is towards the star but what is not obvious is what would make the particle begin ...
• 6,516

### With two balls connected to a string find minimum upward velocity that can be given to one of the balls such that the other leaves the ground

I have done upto this, but I don't know how to further solve this second order differential equation.
Perhaps it would help to think of relative displacement and relative velocity. Suppose m and M are on the table. Suppose M is at $x_M = -1 \space m$ and m is at $x_m = 2 \space m$. You can say what ...