[Youtube Review][Veritasium] Explained: Beaker Ball Balance Problem

(Recommended)Popular Videos : [Veritasium] Explained: Beaker Ball Balance Problem

 

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(Recommended)Popular Videos : [Veritasium] Explained: Beaker Ball Balance Problem

https://www.youtube.com/watch?v=stRPiifxQnM

 

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Top Comments : [Veritasium] Explained: Beaker Ball Balance Problem

Co*****************:

C: The mass of the water and beakers is equal, and the buoyant forces on both the ping pong and the acrylic ball are equal to the mass of their volume of water (since the net sinking force for the acrylic is counteracted by the string and the net floating force of the ping pong ball is corrected by the pushing down by your finger). Therefore the force down both beakers is equal to the force of gravity either would have had if it was filled with water instead of ping-pong/acrylic.

Ka*****:

C because you're now supplying to the ping-pong ball side the same downward force as the weight of the acrylic ball minus the strain on the string is supplying. I think.

T4****:

C:
Perfectly balanced, as all things should be

Ba*****:

B: Because of the force you put into the glass.

GD*:

Here's a physics answer: Synopsis: "In this setup - as long as the tension in the acrylic ball's string is greater than the weight of the ping pong ball, the acrylic ball's glass will always register as "heavier".

The weight registered by the scale is the normal force exerted upward by the scale. This force (on both sides) will increase as you add mass to the glasses of water. No matter the density of the object, the scale will read the normal force exerted upward by the scale. Therefore if there were no strings involved at all, the acrylic ball side will still be heavier.

The net downward force on the ping pong ball was equal to [(the weight of the ping pong ball) plus (the tension in the string) minus (the buoyant force)], whereas the net downward force on the acrylic ball was equal to [(the weight of the acrylic ball) minus (the tension in the string) minus (the buoyant force)].

For the ping pong ball, the tension in the string is exactly equal to the buoyant force so they cancel out. That means the net downward force on the ping pong ball is reduced to just its weight.
For the acrylic ball, the tension in the string is acting in the same direction as the buoyant force, so they do not cancel out. Instead they add together in the upward direction against the weight of the acrylic ball.

 In other words, as long as the tension in the acrylic ball's string is greater than the weight of the ping pong ball, the acrylic ball's glass will always register as "heavier".

Da************:

Balanced because both balls are now effectively density 1 displacements of same size. 'Holes' in the water. Assuming same size.

dr*****:

About 29k people watched the "question" video, but only about 7k wanted to know the answer...

Ea***************:

C. Because only difference is what is the force acting on balls (fingers for ping pong ball or gravity for acrylic) and they are both the same (because they just counteract Archimedes force).

Mi*****************:

C. Balanced
Say the ball was glued to the finger,which doesn't change the fact that the finger is pushing on it. It wouldn't matter what's inside the ping-pong ball; it could as well be an acrylic ball, like the one on the right. Which could be glued to whatever is holding it, which doesn't change the fact that it is being hold up.

Ja********:

my teacher just assigned a problem like this today, thanks for the help :D

Ju***********:

As in my comment on the previous video, I'll use the same variables.

M1 still has several forces acting upon it. Fb + Ft = mg
M2 now has this equation: Fb = mg
Rewrite it to get pg1V + Ft = m1g and p2gV = m2g. V cancels out.
So p1g + Ft = m1g and p2g = m2g
This is the same as p1g = m1g - Ft. 

So for the acrylic ball, the buoyant force is the same, just add the tension to the weight. In the ping pong ball, it's the same as before. Weight = buoyant force. 

So it's C, the acrylic ball, again.

Sa*************:

C. Balanced
The variables we will use here are, Mp: mass of ping-pong ball, Ma: mass of acrylic ball, Vb: volume of ball, Vw: volume of water, D: density of water, g: gravitational acceleration, T: tension force of string connected to the acrylic ball, f: the external force applied to the ping-pong ball, Fp: the total force on the ping-pong ball side of the balance, Fa: the total force on the acrylic ball side of the balance.
f = g*Vb*D – g*m
T = g*M – g*Vb*D
Fp = f + g*m + g*Vw*D = g*Vb*D + g*Vw*D
Fa = – T + g*M + g*Vw*D = g*Vb*D + g*Vw*D
Therefore,
Fp = Fa
Under ideal situations, the balance will not tip in either direction.

Ca*****:

B. You are adding more weight force with your fingers.

fr***********:

C. F1 = rgH1*S = rgH2*S = F2

Li*********:

I am usually good at this, but I'm mad confused, why does the ping pong balls string give an upwards force?

Je********:

I don't know. But since I see a lot of people saying C, I'm just going to assume it's not. Because these videos always seem to have some super counter intuitive twist at the end.

I love how much this stuff makes me giggle. The second the conundrum is revealed, it cracks me up. I don't know why.

Jo*****:

C, what you're doing to the ball with your finger is the same as what is going on with the suspended ball?

Un*************:

I got my answer wrong, and I didn't understand your explanation at first. Maybe the fact that you illustrated forces acting on different bodies at the same time made it very difficult to understand. Showing a full diagram of the forces acting on each body separately would have been less confusing in my opinion.

I think came up with an easier explanation. I consider the left side as a whole (the beaker + the water + the ping pong ball) and the right side as a whole (the beaker + the water + "something"). Both beakers are equal, and there is the same amount of water in both. So the difference is in that "something", which is equal to the buoyant force, since that is the amount of force that is being taken from the tension of string and added to the beaker. So, the difference in weight should be the weight of the displaced water minus the weight of the ping pong ball... 

Now, for the question at the end, I think the answer is B. Ping-pong ball, since the beakers are the same weight, the amount of water is the same, the buoyant force is the same (since now you added the buoyant force on the left with your fingers). BUT the Ping Pong ball in the left is still there. So that makes the difference between the two parts. (Edit: changed my mind to C. Balanced, explanation in comments)

(Please forgive any mistake on terminology, I am not used to physics in English)

And great video Derek! Very very thought provoking. I really liked it :D

Hi*********:

D: None of the above

Wo*******:

B, the left side being the "heaviest", seems the most logical to me. In the previous set-up we already concluded that the downward force on the left and right side respectively was the weight of the water and the pingpong ball and the weight of the water and the water displaced by the acryllic ball. As we don't change anything to the right set-up, this force stays the same.

The left force can be "reached" as follows: using the previous set-up, remove the tether. The system of the left beaker is still an independent system, except for gravity, so the downward force is still the weight of the water and the pingpong ball. Now, to keep the ball submerged, we need to push it downward with a force equal to the weight of the water displaced by the pingpong ball.* Therefore, this force is added to the left side.

In conclusion, we have these total forces:
left side: weight(water) + weight(pingpong ball) + weight(displaced water by pingpong ball)
right side: weight(water) + weight(displaced water by acryllic ball)
As the pingpong ball and the acryllic ball have the same volume, after comparing, we see that the force on the left side is equal to the force on the right side with the weight of the pingpong ball added to it. Considering weights are positive, the left force has to be greater than the right force, so the balance should tilt so that the left side lowers.

* EDIT: The text above was my first guess. After reading Vizaru's response to my comment I should now mention what I said at the * was wrong: the force required to just keep it submerged is as strong as the buoyant force pushing it upwards, which is not the weight of the displaced water, but the weight of the displaced water MINUS the weight of the pingpong ball itself. Using this as the added left force, you can conclude the result should actually be balanced, so my new answer is C. Thanks to Vizaru for pointing this out!

an**********:

C. It will remain Balanced.

**Weight on Acrylic Ball Side = Weight of Beaker +Weight of Water + Weight of Water Displaced by Ball
**Weight on PingPong Ball Side = Weight of Beaker +Weight of Water+ Weight of PingPong Ball + Force Applied by finger.

==> Force Applied by Finger = Weight of Water Displaced by Ball - Weight of PingPong Ball.

Sm*************:

I'm going to guess ping pong ball again (I got this wrong)... because when you push down on the pingpong ball you can feel the force pushing against you.

ph****************:

c because the same forces you explained with the acrylic ball (the displacement of water pushes down) are both equal since that depends on the amount of water displaced which depends on the volume of the ball and both balls are the same.

Go*******:

B since the ping pong ball is hollow it has air inside it, and since air rises in liquid the ping pong ball would be feeling an upward force. But since that upward force is canceled by the finger it will then push down instead.

ep***********:

C, because I don't like conflict and that seems to be the most diplomatic solution.

Os******:

C. Balanced. Think about it like this. The water doesn't have eyes. The only thing it can tell is that both balls are the same size, and they are not moving. In other words, it treats both balls the same.

ro********:

When I will push the ping pong ball inside the water & it just submerges without letting my finger getting wet then on releasing the balance it will remain BALANCED because both the balls are now displacing the exact amount of water so on both side the EQUAL buoyant force will be acting upon as a reaction in the downward direction along with the water weight.
 
In the previous case I feel that the explanation given is INCOMPLETE as the buoyant force acting on ping pong ball is transmitted to the base via thread in upward direction which cancels the reaction buoyant force in water (which is pointing in the downward direction) ..... So on LEFT hand side (ping pong side) only water weight acting & on Right side (Acrylic ball side) both water weight AS WELL AS reaction buoyant force acting on the downward direction.

Check this OUT

Br********:

C. Assuming the explanation of the first experiment is correct and that it is the tension from the string opposing the bouyant force that causes the acrylic ball to drop, removing the tension from the string should have the forces between both balls be the same and the system in balance.

Ar****:

C, but only if you found a way to keep the ball perfectly stable.
If you were to dip the ball slightly deeper your contraption to keep the ball in place would displace water too, resulting in B, the Ping Pong ball sinking.
And if you were to slightly elevate the ball above the water the opposite would of course occur, where less water gets didsplaced leading to A.

Di******************:

C, because on both sides there is now a downforce that is equal of the weight of the replaced water. on the left side because you are pushing with your finger and on the right side because the ball pushes down like you explained in the video.

Wi*******:

C. 
The forces on the scales are the forces on the beaker. The beaker applies a force to the water necessary to balance the other forces on the water, and feels this same force in return.
Acrylic: Forces on the water are its weight, air pressure, and the buoyant force, all down. These are balanced by the beaker pushing up, so that is the weight
Ping Pong: Forces on the water are the same three pushing down: water weight, air pressure, and buoyancy force. the buoyancy forces are the same for both as in the last experiment. These three are countered by their beaker.
Same forces on both beakers : balanced scale
The force of the finger is carefully matched to balance the buoyancy and weight of the ping pong ball, this finger force does not act directly on the water or beaker. It only allows the buoyancy force to fully push against the water, which the acrylic ball does with its weight

ca**********:

C: the effect is determined by the force acting on the beaker rather than the ball that remains in eqm. With the ping-pong the reaction force counters the buoyancy force on the ball however the displacement force of the water is not reduced/countered by any force (unlike last time since the tension also pulled on the beaker bottom) so the weight increases by the weight of water displaced. With the acrylic ball the weight of itself was fully supported by the tension before and therefore has no impact on the beaker (even though the buoyancy on the ball reduced the tension it is still in eqm) and the displacement force acts on the water fully as before and since it displaces the same weight of water as the ping-pong ball the weight of the beaker is increased by the same amount hence they remain balanced.

Mi*:

 @Veritasium  do you know when you'll be posting the solution for this second part of the problem?

St********:

shame to stumble across a video like this 5 years later and find that there's still no link to the next video

Jo***********:

I don't understand this solution at all haha
My 1st year university physics knowledge has failed me :P

Dh*********:

C. Would be balanced.
Explanation:
The force that you are exerting on the ping pong ball and its own weight are balanced out by the buoyant force which is equal to the buoyant force experienced by the acrylic ball since they displace the same volume. By Newton's third law, the force experienced by the beaker due to the balls is the same in both of them and so the Normal force experienced by the balance, or the effective mass of the system will be the same on both sides.
An alternate solution to the first problem:
Since the ping-pong ball+beaker+water system doesn't really depend on the systems internal arrangement, one could simply cut the string to which the ping-pong ball is tethered. Which would make it rise up to the top and float. Now the buoyant force will just be the weight of the ball which is M*g or d*V*g (where d is the density of the ping-pong ball).
In the second beaker, the buoyant force is just the weight of the water displaced which is D*V*g (where D is the density of water). Since density of water is greater than density of the ping pong ball, the opposite force, i.e., the force acting on the beaker (by newton's third law) will be greater for the second beaker and it will sink.
Good video! Would love to see more such experiments!

Ni**********:

My answer: B ... The upward buoyant force and the force of gravity from the water pushing on the ball (newton's 3rd law) is counteracted exactly by you pushing on it, leaving the downward buoyant force... and the weight of the ping pong ball. The acrylic ball suspended on a string leaves ONLY the downward  buoyant force. So the scale will tip in favor of the ping pong ball beaker to exactly the mass of the ping pong ball.

Jo**********:

C.

If you imagine the acrylic ball as being exactly the same weight as water, the ball would just float there and there would be no extra tension on the string. Any excess weight the acrylic ball has will be bourne by the string. Hence, the right beaker's weight is the weight of the water + weight of the acrylic ball if it were made out of water.

Meanwhile, Derek pushing down on the ping pong ball just enough to keep it submerged effectively makes the ping pong ball as heavy as it would be if it were the same weight as the water. (Any less pressure wouldn't submerge the ball. Any more pressure would push the ball to the bottom of the beaker.) So the total weight of the left beaker is the weight of the water + the weight of the ping pong ball if it were made out of water.

Because both balls have the same volume, both beakers should weigh the same. Hence, the scales would be balanced.

sl****************:

Assuming that he can submerge the whole ping pong ball without submerging his fingertips, the scale will remain balanced. This is why:
As he mentioned, on the side of the acrylic ball the scale will see the weight of the cup and the water and the buoyancy of the ball (density of water*Volume of ball).
On the other side the scale will see the weight of the cup and the water, the weight of the ping pong ball (equal to the weight of the water it displaces as it floats, name this Volume1) plus the force he has to apply in order to submerge the rest of the ball (the volume left to be submerged is Volume2=Volume of ball -Volume 1). By adding everything you get that the scale on the ping pong ball side will see the same as on the other side, i.e. the weight of the cup plus the weight of the water and the buoyancy of the fully submerged ping pong ball.

ri************:

C). My explenation goes like this: in the case of the ping pong ball Derek is now providing the downward force equal to the weight of the water it displaces, so in the end it's as if the becker on the left got heavier by adding the water that would fit inside the ping pong ball. While for the acrylic ball the same explenation as before is still valid, the ball weighs a certain amount and it's suspended by the string, then you dip in water and the string no longer has to suspend the whole weight of the ball because of the boeing force acting on the ball.

 

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[Veritasium] We gathered comments about popular videos and looked at them in summary, including play time, and order of popularity.

It's a good video or channel, but if you're sad because it's too long, please leave a YouTube channel or video link and I'll post it on this blog.

 

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