# All Questions

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I know that force is $F=−∇U$, but isn't it for the forces that's been done by the system itself? Like when i pull up a ball with external force pointing upwards, shouldn't a ball's potential energy ...
36 views

### Angular acceleration :( [closed]

If i calculate the angular acceleration of a rigid body in rotational motion choosing 2 different points does it give the same answer?
32 views

### If friction and normal force are both the components of contact force, shouldn't the friction be repulsive?

Is friction due to intermolecular attractive forces or repulsive ones. Since both friction and normal force are the components of contact forces, shouldn't friction be repulsive.
14 views

### Why do you feel the difference in the force required when a muscle contracts and relaxes even if you apply the same force to an object?

Think about a person lifting or lowering a 50kg object vertically. Lift and lower at constant speed. At this time, the force exchanged between the person and the object is 490 N, and the person is ...
21 views

### Painter on scalffold - Kleppner and Kolenkow Problem 2.12 [closed]

I am currently trying to teach myself mechanics by reading Kleppner and Kolenkow's Introduction to Mechanics, and I've hit a bit of a snag on Problem 2.12. A painter of mass M stands on a scaffold of ...
3k views

### Can there be acceleration without work?

Since $W=Fs$, $F=\frac{W}{s}$. When you substitute this in the formula for acceleration, $a=\frac{F}{m}$, you will get that $a=\frac{W}{ms}$. Then, when work equals zero, acceleration will be zero.
70 views

### Conservation of momentum vs newton's second law

I saw this question from a book: An empty sled is sliding on frictionless ice when Susan drops vertically from a tree down onto the sled. When she lands, does the sled speed up, slow down, or keep the ...
• 3
1 vote
88 views

### Is Energy stored Force?

I am currently studying the relationships between energy work and force. I am trying to conceptualize them into one concept of causality. When a force is applied work is done, and energy is ...
142 views

### Follow up question on a block on an accelerating inclined plane

A follow-up on this post: How can an accelerating inclined plane prevent a block on it from sliding? Farcher's answer states that "If F is larger than in the no relative movement condition then ...
1 vote
39 views

### Hard problem of pulleys [closed]

A string passes over a frictionless, massless pulley attached to the ceiling A mass $m_1$ hangs from one end of this string, and a second massless, frictionless pulley hangs from the other end. A ...
85 views

### Why do cars move slightly backwards just before coming to rest?

You've probably noticed this before: you're in a car or other wheeled vehicle, the brakes are applied, the car's velocity reaches zero, and then the car actually moves backward a tiny amount before it ...
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1 vote
22 views

### Classical mechanics problem of elevators [closed]

A passenger elevator consists of an elevator cage of 900 kg (empty) and a counterweight of 990 kg connected by a cable running over a pair of pulleys (Figure 6.9). Neglect the masses of the cable and ...
1 vote
28 views

### Proof of principle of transmissibility of force

The principle of transmissibility states The point of application of a force to a rigid body can be moved anywhere along its line of action without changing the external reaction forces. My question ...
49 views

### Is Newton's laws formulated using laboratory time?

The second Newton's law can be written as (in SI units) $$\frac{d}{dt}\vec p = \vec F.$$ Newton was considered Galilean transformations and the existence of a "absolute" time. Now suppose ...
1 vote
25 views

### Average Tension in the string of a pendulum: Determining radial component of weight

Self-studying Classical Mechanics right now and was working through an example until I got to a point where I felt certain questions need be addressed. I will list the problem statement, the example ...
1 vote
23 views

### Average Tension in pendulum string: Understanding the radial $F = ma$ equation

Self-studying Classical Mechanics right now and was working through an example until I got to a point where I felt certain questions need be addressed. I will list the problem statement, the example ...
1 vote
28 views

### Resolution of force vectors

So, I have had this query for like the longest time, ever since I first studied this topic. So, take a force vector $F$ that is making an angle of say, 30 degrees with the horizontal axis. Now, I want ...
1 vote
31 views

### Use of Instantaneous axis of rotation (IAOR) in a question of mechanics give different and wrong answer

The above situation can be solved easily when we go in frame of point C then point C will come at rest and mass "m" will do circular motion about point C with 3v velocity and then using ...
1 vote
61 views

### Law of inertia in rotation

Consider an object rotating about its own axis. If the law of inertia is applicable, the object should keep rotating with the same angular velocity if no external torque is provided, but how is the ...
1 vote
29 views

### Can a bicycle whose front wheel cannot turn left/right stay upright?

Most sensible explanations of why a bicycle stays upright involve the turning left/right of the front wheel. If a bicycle's front wheel cannot turn left/right, can it stay upright while being pedalled?...
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45 views

61 views

### What is point of application of force? [closed]

My textbook said that point of application is the exact location at which a force is applied to a body. But I know that there are some forces that their points of application outside the body (like ...
70 views

### Inconsistency regarding the spatial trajectory of a particle subject to a given force field [closed]

I have recently been trying to formalize a mathematical method (for my own recreational purposes) to work out the shape of the path that a particle traces out in space if we are given the resultant ...
21 views

### Why do we consider the whole process, taking a single moment? Universality of the vector equation

I have considered a simple model for describing the elastic force vector of a spring. First, I chose a reference frame in an arbitrary way, then I drew the necessary vectors, we get a ratio that ...
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39 views

### Can We Have 0 Normal Between Any Two Bodies Just Touching Each Other

I am a bit confused... Here if we see x-axis forces they are getting detached from each other. So To Resist them The Normal Should Be Zero. I am not sure if I am applying the right logic or not but ...
56 views

### How To Find Direction of Normal Contact Force?

I am a little bit confused. while doing questions I can see direction of normal forces are different. In Some Cases, It is perpendicular to the contact surface and towards the object on which normal ...
52 views

### Illogical conclusion about a mass on a spring [closed]

If you place a weight on a spring and it is an equilibrium, then you have this equation: $mg = kx$ you would solve for k and get: $k = mg/x$ but, if we used conservation of energy, assuming that when ...
81 views

### Why is a ball that is moving in one direction and spinning in the opposite slowed down if the force of friction is tangential?

If a ball has linear and angular velocity such that there is a slip between the surface of the ball and ground, why does linear velocity of the center of the ball decrease if the force of friction is ...
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124 views

### $F=ma$. $p = mv$. What's $mx$?

So I know that $$F=ma$$ ($F$ is force, $m$ is mass, and $a$ is acceleration). I also know that $$p=m\int a dt=mv$$ (Where $p$ is momentum and $v$ is velocity). So I was wondering if the following ...
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I'd like to derive a conservation principle for the mechanical energy, as sum of kinetic and potential energy, for a multi-body point mass isolated system using the $2^{nd}$ and $3^{rd}$ Newton's ...