# All Questions

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### Objects Rolling Downhill - Ratio of Rotational and Translational Kinetic Energies [on hold]

Let's say I roll a uniform disk down a ramp (without slipping), and the disk rolls 10 meters down the ramp which is angled at 30 degrees to the horizontal. At the bottom of the ramp, what is the ratio ...
39 views

### Kinetic Energy Comparison Marble Question

A uniform marble of mass M rolls down a symmetric bowl, starting from rest at the top of the left side. The top of each side is a distance h above the bottom of the bowl. The bowl is rough enough to ...
252 views

### Rotational KE vs Translational KE [closed]

Q. Horizontal electric field E = mg/q exists as shown in the figure and a mass m is attached at the end of a light rod. if mass m is released from the position as shown in the figure. find the angular ...
32 views

### Rolling on a track in vertical plane

A sphere of mass m and radius r is released from rest at point A on a track in vertical plane. The track is rough enough to support rolling between A and B and from B onwards it is smooth. My ...
6k views

### Does it take energy to move something in a circle?

Work can be calculated by $W = Fd\cos\theta$ where $\theta$ is the angle between the force $F$ and the displacement $d$ Let's say there's a ball and a (physically ideal - no friction etc.) robotic ...
196 views

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### Motion of a rotating object in space

What will happen if connect an iron rod with a motor's axis and rotate it in space and after a while disconnect the motor's axis from the rod. Will after this the rod show infinity motion and if not ...
186 views

### Is angular work equivalent to linear work?

When considering the definition of angular work, I assumed that it should reduce to the definition of linear work in order to tie in quite nicely with conservation of energy. However, further ...
213 views

### Inclusion/Exclusion of linear kinetic energy in a rope-pulley system

I find it difficult why in the first problem the linear kinetic energy is not included in the calculation, but in the second it is included, yet the questions are virtually the same.
98 views

### Which ball spins the longest?

I just thought of a mind-boggling question about angular velocity and friction. Here is the picture: The first figure shows a ball spinning around the y-axis, and the seconds shows another ball that ...
3k views

### Rotational kinetic energy when moment of inertia is changed

The following quote comes from: https://physics.stackexchange.com/a/93531/155365 Consider an ice skater that has just gone into a spin with arms stretched out. If it helps, the skater is wearing ...
893 views

### If the angular momentum is conserved in a system whose moment of inertia is increased, it's kinetic energy decreases [duplicate]

The workout that $\Delta K.E.$ is less than zero is attached. But where does this energy go. It is transformed back to rotational KE if moment of inertia decreases. How would you account for the ...
28 views

### What is the energy of a body that rotates in an unfixed radius?

I have a body $m$ that is in a track that limits it's movment only to the radial axis. He is connected to a spring and that track is rotating around the same axis as shown here : I wonder what is ...
1k views

### Curved Slope faster than linear?

So I saw this gif the other day, and was wondering, is this real or fake? And supposing there is no energy dissipated by the friction, why does such thing occur?
866 views

### What gives energy for precessional motion of gyroscope?

Let us suppose we have an ideal friction-less gyroscope and it is suddenly given an angular momentum, and left as in the figure. It will start precessional motion. Since there is no friction, the ...
53 views

### Rotational velocity of tethered shape after falling

My solution to the above question involves equating the potential energy to the to the kinetic energy at the point at which the wire tightens as: $$\frac{1}{2}mv^2 = mgh$$ However, I am having ...
464 views

### Using conservation of kinetic energy to find angular momentum after a collision

I'm self-studying so sadly I don't have a professor or classmate to ask this to. I'm mostly learning through Khan Academy which is where I came across this question: A ball of mass $M$ is rolling ...
236 views

### Unexplained energy loss of 50% when calculating potential vs. kinetic energy

I did a lab where we did this experiment with four different solids. It's a conservation of energy lab, so we calculated the potential energy at the beginning of the incline ($mgh$) and the kinetic ...
3k views

### Why does a rolling sphere stop?

I've just started studying Rotational Mechanics and one thing I'm confused about is that if friction is equal to zero when a body is rolling purely.. then why does a rolling body e.g. sphere.. stop ...
76 views

### The height traveled by the mass center of a thrown rod

I'm learning Newtonian mechanics and want to understand the following example and the methods I can use for it. Suppose I throw a rod in the air with length $L$. The time I'm letting go of the ...
100 views

### Is there any danger of gravity being used up? [duplicate]

Posit a way to harness gravity for energy and widespread use of this new technology world-wide. Could it be damaging to Earth, over a few hundred years, slowing or speeding up rotation or the orbit ...
66 views

### Galaxy equillibrium

I know this is a very dumb question, but, to maintain a galaxy as a bounded system, the total energy and the total angular momentum of every particle component of the galaxy (suppose made of gas and ...
530 views

### Rotation of rigid body after inelastic collision with smaller mass

A slender rod of length $l$, with no mass, is able to rotate around an axis through its center, with no friction. There are two points of mass $M$ and negligible radius attached to each end of the rod....
5k views

### What is rotational potential energy?

I don't understand the concept of rotational potential energy. During linear motion, when a force is applied, the work it does gets converted to kinetic energy and there is no change in the ...
69 views

### Spheres collide with merry-go-round [closed]

Four spheres, with uniform densities $\rho_1, \rho_2, \rho_3, \rho_4$ and radii $r_1, r_2, r_3, r_4$, respectively, roll without slipping with constant velocities $v_1, v_2, v_3, v_4$ along tracks ...
364 views

### Energy method to solving equations of motion? Why does this method work and what is it called?

Given the stated system in the photo we are suppose to prove simple harmonic motion when given an initial displacement $x$ I first considered the total energy of the system which we will call $H$ H=...
129 views

### A sphere rolling down a plank - an error in calculating its energy

Consider the following situation: A sphere with radius $r$ rolls down a plank which forms an angle $\theta$ with the horizontal axis. The starting hight of the centre of mass is $h$. We assume that ...
75 views

### Why does this consider the center of mass instead of the end? (RIGID BODY MOTION) [closed]

A uniform rod of length L and mass M is free to rotate on a frictionless pin passing through one end. The rod is released from rest in the horizontal position. What is its angular speed when it ...
132 views

### Uranus, the Physical model to change its rotation axle

Uranus rotates pretty wierd, it's 90-degrees tilted; Why is Uranus's axis of rotation tilted? The best answer for this is; that at a distant point in its past, Uranus was struck by a very ...
1k views

### Earth's Kinetic energy change

Earth's rotational speed varies. I have checked the data an found following Peaks; Year 1998, 23.May, the Earth rotated in 86400.0023738 seconds. At 9.July the rotation time of the Earth was 86400....
1k views

### Conservation of Energy vs Conservation of Momentum in Rotational Dynamics

It is clear to me why angular momentum is always conserved, and how in some cases energy is not necessarily conserved within the system (in those cases where bodies deform, or friction is involved). ...
99 views

### Spinning rubber ball with equatorial ridge

I have this rubber ball with something like a very slight equatorial ridge (sort of like Saturn's moon Iapetus) which I often spin around on my desk. I keep noticing that no matter the inclination of ...
1k views

### Ball rolling on half-pipe

It is well-known that a ball rolling down a half-pipe where the side it starts on has enough friction for the ball to roll without slipping and on the side other to be frictionless, that the ball will ...
3k views

### Finding the angular velocity of a rod hit at a distance from its pivot [closed]

A 1m long, 2kg stick is nailed to the wall with a single nail, allowing it to pivot and freely rotate at the end. A 1kg ball, with speed 3m/s makes contact with the stick at some distance x (unknown) ...
133 views

### Determine the value of $g$ with rolling ball

At first, I thought the value of $g$ ($9.8m/s^2$) could be determined simply by placing a ball at the top of a ramp at a known height. The ball was released with no initial velocity, and the final ...
421 views

### An unwinding cable problem [closed]

Suppose a light cable is wrapped around a solid cylinder with mass $M$ and radius $R$. The cylinder rotates with no friction about a stationary horizontal axis. The free end of the cable is attached ...
366 views

### Friction and work from torque

I would like to understand where is the error in the third case, for that I gave 2 easier cases where I'm able to find the energy from heating is equal to the energy lost by torque. Case 1/ Purple ...
327 views

### What prevents this rotational perpetuum mobile from working? [closed]

As a newbie in physics I imagine this device and I would like to know why it can't increase its energy. Note I study the sum of energy during one second (or less) after set friction ON. The device: ...
303 views

### I need help figuring out what is wrong in this aspiring perpetuum mobile

Credits: My question is motivated from a question from another user (https://physics.stackexchange.com/q/143377/), I just reformulated what I think he tried to ask into, what seem to me, simpler terms....
241 views

### Sum of energy from torques of several disks in double rotation [closed]

Here the study: An external system (not drawn) give energy for rotate disks around themselves and around green axis. All disks have energy at start, at $t=0$ friction is ON and at $t=0$ external ...
When a yo-yo is released from a height $h$, the gravitational potential energy is converted to kinetic energy. However, the yo-yo obviously has less acceleration than $g$, $9.8\frac{m}{s}$. This means ...