# Tag Info

1 vote

### Why does a plastic bag rip when it is raised to a higher position when being held but not when it is stationary?

Gravitational pull acting on the strength and integrity of the bag, combined with a short, quick pull as the bag is lifted, straining the integrity further still.
1 vote

### Static friction when rolling with constant speed

Different contact points So, I am not entirely sure what other answers you have gotten, but I will say that in my own case, when I was trying to learn about friction on wheels, one of the most ...
• 36.9k

### Static friction when rolling with constant speed

The torque causes a force $\tau = rF$ on the contact point of the wheel No. The torque isn't creating any forces on the wheel. On a frictionless surface, the wheel would rotate but no linear ...
• 38.4k

### Static friction when rolling with constant speed

1 There are static frictional forces between the tyre and the ground: static frictional force on tyre due to ground, and static frictional force on ground due to tyre. The first of these two static ...
• 93.4k
1 vote

### Why does a plastic bag rip when it is raised to a higher position when being held but not when it is stationary?

It is because of the bag's inertia. It takes force to accelerate any mass, like a bag, and if you start moving it up then that force is added on top of the force the bag already exerts on the handles ...

### Why does a plastic bag rip when it is raised to a higher position when being held but not when it is stationary?

(An attempt to explain this with non-technical language). There're two forces acting on the bag - the force from your hand, and the Earth's gravitational force (the weight). When you lift the bag, you ...
• 19.7k

### Why does a plastic bag rip when it is raised to a higher position when being held but not when it is stationary?

You move the bag upwards with an acceleration. If you were to pull the bag up with infinitesimal acceleration, it would not rip then, because the force required for that would approach zero. But since ...
• 119

### Why does a plastic bag rip when it is raised to a higher position when being held but not when it is stationary?

This is like holding a weight $mg$ with a string which can break with a tension of $1.1 mg$. When you are not moving the bag, or moving it at a uniform speed, the string's tension is constant at $mg$,...
• 9,638

### Possibility of losing contact when a block is sliding on a moving plane

I don't see how it could lose contact in that idealized situation because to lose contact the triangular plane would have to be moving to the left away from the block. But the block is the thing that ...
• 8,448

### Solving for deceptively simple two-block pulley system

We can analyse the forces acting on each pulley, which must add up to $0$ assuming the ropes and pulleys have no mass. It is obvious that there’s no physical solution to this problem without ...
• 471

### Solving for deceptively simple two-block pulley system

Let's define coordinates: $m$ has $x_1$, $2m$ has $x_2$ and the block above $m$ has $x_0$. Overall length of the string will therefore be expressed: $$l = 2x_2 + x_1 - x_0$$ Differentiating this ...
• 180
Accepted

### Direction of undetermined contact force & undetermined Normal force

In a situation where two corners are in contact, the contact force (acting on the blue part below) can be in any direction (and magnitude) and thus can be decomposed into two components. In a ...
• 37.4k

### How does friction applied to a rolling sphere in 3 dimensional space

The plane is described with u , v parameter and the trajectory on the plane with s parameter . Thus the position vector to the contact point is $$\vec R=[x(u(s),v(s)), y (u(s),v(s)),z (u(s),v(s))]^T$$...
• 11.5k

### How to mathmematically direction of friction of spool?

We use the concepts of Impulse and Angular impulse. If the force is applied below the center of gravity in forward direction then, frictional force will act in backward direction because the contact ...

### How to mathmematically direction of friction of spool?

With friction, the bottom doesn't slide. It matches the speed of the table, which is $0$. The spool rolls. Suppose the spool was sitting on ice, so F was the only force. How would the spool accelerate?...
• 36.3k

### Why can multiple eggs support more weight per egg when there are more of them?

Because the pressure changes and the weight is distributed to each egg.What causes fracture or any other plastic deformation is pressure not force. Pressure $P$ is related to force $F$ by this formula:...
Accepted

### Why can multiple eggs support more weight per egg when there are more of them?

With a fracture process the fracturing is usually caused by a local concentration of stress, that is there is some small region where a lot of force is concentrated in a small area and the resulting ...
• 351k

### How can the energy of a system increase even if net work done on it is zero?

This will result in increased kinetic energy of the blocks, thus result in an increase in the total energy of the system. is a correct statement but must include an extra word, $\dots$ increase in the ...
• 93.4k
1 vote
Accepted

### How can the energy of a system increase even if net work done on it is zero?

In $W=\vec F \cdot \vec s$ the $\vec F$ is a force acting on the system and $\vec s$ is the displacement of the material of the system where the force is acting. So in your example both forces are ...
• 94.4k

### How can the energy of a system increase even if net work done on it is zero?

Either the pair of blocks are one thing, in which case the one thing's kinetic energy is the kinetic energy associated with the translation of the center of mass of the pair of blocks, which doesn't ...
• 12.7k

### Would one be pushing as much as their weight with this machine?

His arms move a longer distance than he is lifted. Therefore the force he needs to push with is lower than his weight. There is a mechanical advantage. Much like to lift the rock in this picture, the ...
• 6,301

### Would one be pushing as much as their weight with this machine?

I was looking at the following machine in the video below, and was wondering, would one be pushing as much as their weight? His pushing force is less than his weight due to the mechanical advantage ...
• 68.8k

### Is the weight of something being dropped the same as the force of something being static?

The answer is very simple. When a body is dropped from a certain height, the quantity of motion is involved. Another name for quantity of motion is linear momentum. The change in linear momentum is ...
• 91

### Forces on spring, Newtons third law

Consider the system which contains the mass, the ideal spring and the Earth (to which the spring is attached in some way). There are three Newton third law pairs of forces and for each pair the ...
• 93.4k

### Forces on spring, Newtons third law

Consider the system which contains the mass, the ideal spring and the Earth (to which the spring is attached in some way). There are three Newton third law pairs of forces and for each pair the ...
• 93.4k

### Forces on spring, Newtons third law

Let $m_s$ be the mass of the spring, $m_b$ the mass of the block and $g$ the gravitational acceleration. The ceiling is pulled down by a force equal to $(m_s + m_b)g$. It doesn't matter to the ...
• 194
1 vote

### Forces on spring, Newtons third law

There is a force on the block due to gravity and a force on the block due to the spring. These are not newton's third law force pairs, but they do happen to cancel out since the block is moving at a ...
• 17.6k

### Is the weight of something being dropped the same as the force of something being static?

A true causation for your demonstration relies in the concept of Impulse. Discarding the conventional answer, if you want to go far beyond and evaluate the truth see https://www.researchgate.net/...

### Plank overhanging off a building — where does the normal force go?

Treat the plank as two parts. Part A, resting on the ground. And part B hanging. The condition for balance will be that part A with its mass (its mass should be calculated proportional to the length) ...
1 vote

### What happens when my initial applied force is greater than my frictional force?

If you're applying a force against the ground, the faster you go the harder its going to be to apply a force. For example, if you're on a skateboard and you can swing your leg at 20 mph to push off ...
• 7,362

### What happens when my initial applied force is greater than my frictional force?

Even more profound, if your initial force is exactly the same as your friction force, you'll keep moving if you were already moving. Of course, nothing beats a practical example. Here is strongman ...
• 45.6k
1 vote

### What happens when my initial applied force is greater than my frictional force?

Friction forces are continuously applied as long as the moving body is in contact with a surface. The friction force is, at least approximately, dependent on the velocity of the body that is moving. ...
• 1,990

### Is the weight of something being dropped the same as the force of something being static?

After falling from height $h$ and impacting elastically with the spring (of constant k) the weight W and top part of spring travel down together by a common deflection $\delta$. Equating work done to ...
• 955

### Is the weight of something being dropped the same as the force of something being static?

Weight is the force that the earth does on the block. It doesn't change if it is falling or not. But the force that the block does on your feet is a different matter. If it is just there in a static ...
• 15.8k

### Is the weight of something being dropped the same as the force of something being static?

Your foot brings the falling block rapidly to rest. Suppose the block has been dropped from a height of 0.90 m above the top of your foot. When it reaches the top of your foot it will be falling at 4....
• 34.8k

### Is the weight of something being dropped the same as the force of something being static?

When the dropped block hits your foot, it has a certain amount of momentum, p. A force F(t) must be provided over a time interval to bring it to rest, where $\int F(t)$dt = p. The smaller the time ...
• 2,668
1 vote
Accepted

### When a person jumps from boat does friction force between person feet and boat surface moves the boat backward or the force applied by the person?

The force applied by the person on the boat(F)can I Say it friction force which moves the boat in backwards direction and in reaction force acts on person feet can also be called friction force?? Yes ...
• 68.8k

### How can we prove that tension on both sides of string will be equal?

The string is massless and inextensible; so if there is a net force on any part of the string it will cause its acceleration. Since m approaches zero, acceleration for even a small unbalanced force ...
• 119

### How can we prove that tension on both sides of string will be equal?

Let me put it one more way: If the tensions at two points of the string were different, the string piece between these two points would experience a net force from the tension difference. There are no ...
• 2,816
Accepted

### How can we prove that tension on both sides of string will be equal?

$\dots$ the tension on both sides of mass $m$ will be equal $\dots$ if there is no friction between the bead and the string. If there are no frictional forces then the fact that the string is massless ...
• 93.4k

### How can we prove that tension on both sides of string will be equal?

The phrase "inextensible string" and the fact it's tied down at a point, is what determines that the tension at every point along the string is the same: "Inextensible" doesn't ...
• 2,211

### How can we prove that tension on both sides of string will be equal?

"A massless inextensible string wearing a bead of mass m" From this line I assume there is only one string used throughout, and is passed through the hollow centre of the bead. As long as ...

### A Conceptual Doubt in a Question on Work-Kinetic-Energy Theorem

In this answer I want to clarify the nature of the Work-Energy theorem. (The issue of the normal force exerted by the supporting surface has already been addressed in the answer by contributor Er ...
• 19.8k

### A Conceptual Doubt in a Question on Work-Kinetic-Energy Theorem

The normal force corresponds to the defining constraint of a "solid surface" that motion can only exist along the surface and can't be through the surface. According to this definition, the ...
• 715
Accepted

### A Conceptual Doubt in a Question on Work-Kinetic-Energy Theorem

You could indeed use the work-energy theorem to argue that the energy gained by the block is equal to the work done on it by the net external force acting on it, which is its weight $m\vec g$ plus the ...
• 47.4k

### A Conceptual Doubt in a Question on Work-Kinetic-Energy Theorem

Indeed, since the net force is not calculated this question does not use the work energy theorem. The work energy theorem speaks only of the “net work”, which is the work done by the net force. You ...
• 94.4k
Accepted

### Why a person when jumps out of boat applies a force at the corner of boat?

This is true not only for the boat, but when jumping from a hard surface (e.g., over a trench) or when descending steps: this way the distance of a jump is shorter one pushes not only in vertical, ...
• 57.5k
Accepted

### What force push ball outward in rotating tube in inertial frame?

The apparent problem arises from the pre-Newtonian prejudice that the motion of a body should be along the direction of the total force exerted on the body in all cases. That is not the case as it ...
• 69.9k