Look up!

Classical Mechanics Level 1

As shown in the picture, water flows from the bottle into the cup. What explains this phenomena?

Details

Note, the picture is upside down. The Earth is in the top of the photo and the bottom of the plane is facing the atmosphere.

Gravity pulls the water down toward the stationary cup. A normal force from the bottle pushes the water out. A bumpy ride due to turbulence forces water from the bottle. The plane and water accelerate toward the Earth, but the plane accelerates faster.

3 solutions

Sravanth C.
May 1, 2015

For a simple explanation, let's consider the following thought experiment:

Fill a bucket with some water.
Tie a string to it, and rotate it in a vertical circle.
The water doesn't fall! How?

It seems as if the water in the bucket is defying gravity. But no, Gravity is still at work even when the bucket and water are above your head. The water's inertia wants to keep the water traveling in a straight path, but gravity is acting on the water, causing it to fall in a downward path that will eventually hit the earth.

However, while the water is falling, the bucket is falling along with it, catching the water. What keeps the bucket and water moving in a nice circular path is the bucket's string.

The string acts as the centripetal force that pulls the bucket and water into the center and keeps them from following their paths of inertia , giving the illusion that a centrifugal force is pulling the water away from the center.

A similar explanation holds for the water bottle trick.

I should mention that the picture is actually upside down, as evidenced by the ground at the top. Hence the title of "Look up". Here is the image displayed "right side up", with the water flowing upwards due to centripetal acceleration.

Calvin Lin Staff - 6 years, 1 month ago

It's a marvelous feat of physics!

Sravanth C. - 6 years, 1 month ago

I agree with that!

Swapnil Das - 6 years, 1 month ago

the semi transparent curved image of the ground could be an internal reflection of the ground below.. on the glass top of plane. how else could the convex-concave match of Earth's horizon and plane's dome be in same direction?

Ananya Aaniya - 6 years, 1 month ago

I was thinking that it was due to centripetal force. But I did not assume the plane was in a barrel roll.

William Washington - 6 years, 1 month ago

Tartar sauce. I've been tricked! Good (y)!

Figel Ilham - 6 years ago

@Calvin Lin Sir, this solution to the problem is wrong. This doesn't happen due to centripetal force as there isn't any circular motion. This happens due to a Pseudo force. When we see w.r.t a non-inertial frame then or Newton's laws to be applicable we consider a Pseudo force.

Rajdeep Dhingra - 5 years, 11 months ago

I posted the solution on the basis of the previous options, which gave centripetal force as the answer. It was mentioned that the plane was in a barrel roll, which made me give such an explanation. Yeah, you're right the pseudo force can give a better explanation.

anyways thank you very much! You can post the solution as you suggested. $\huge\ddot\smile$

Sravanth C. - 5 years, 11 months ago

I could not tell the plane was inversed! trick question, but I am still right if it was upright!

Billy Fondren - 6 years ago

Centripetal force is not a real force, it's rather a made-up force to explain circular motion, so you should not call it as an external force. Also, the string is not the only force that plays as centipetal force, but the gravity does as well; it is possible that when the bucket is at the top of its circular path, the string exerts no force on the bucket and water.

Hung Pham - 6 years, 1 month ago

It is a real force. Centrifugal isn't real.

Rajdeep Dhingra - 5 years, 11 months ago

Evan Dagg
May 28, 2015

The water is falling towards the Earth as normal, however, the plane (and thus the man and the cup) are upside down though descending to the Earth faster than the water is. Basically, the cup is catching the water. The water appears to be flowing upside down because the bottle is likewise descending faster than the water pouring out of it is.

Alvin Anassi
May 21, 2015

Centripetal force is the only answer, even with the misguided answer given. Water in a bucket doesn't fall to catch the water, that water's inertia forces it to the "bottom"of the bucket and the tangential force of a person swinging the bucket is the engine that maintains the centripetal force. In the example given, the plane is that bucket, gravity is the string and the pilot is the "person" swinging the "bucket". When the pilot stops his arched trajectory, the phenomena stops

Look up!

3 solutions

0 pending reports