# Quiz: 2 simple physics questions

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#### roygoh

##### Senior Member
Two simple but interesting questions to refresh your O Level physics knowledge:

1. There is a man and a piece of rock the size of a basket ball in a little boat that is floating in a swimming pool. The man throws the rock into the water. Does the water level in the swimming pool rise or fall?

2. A deflated balloon is place on one side of a balance and a balloon filled with air (same composition as the ambient atmosphere) is placed on the other side. If both the balloons have identicle mass before one of them is inflated, which side does the balance tip towards?

- Roy

1. Same level
2. Neither

#### roygoh

##### Senior Member
Zerstorer said:
1. Same level
2. Neither
Haha...I kind of expected you to participate in this discussion....

Let's wait for more answers before I disclose my version.

1. Fall
2. Neither

#### doraemon7

##### New Member
1. rise.
2. balloon filled with air.

#### eddietkm

##### New Member
1. Same level as it makes not different if the rock is on the boat or at the bottom. But if the rock is not on the boat in the first place. It does make a difference.

2. Nothing will happen. Air is weightless. The total mass is the same.

#### Wai

##### Senior Member
#1. sounds like a trick question, since it is a swimming pool, assume that all the water pumps are working properly and the pool is 100% filled, then the water level will be maintained even you throw an elephant in

however, let's say the water level is half full, then the water level should drop.

#2. Deflated balloon should dip downward

#### roygoh

##### Senior Member
Wai said:
#1. sounds like a trick question, since it is a swimming pool, assume that all the water pumps are working properly and the pool is 100% filled, then the water level will be maintained even you throw an elephant in
Haha...that's my fault. It is not a trick question, so assume that the pumps are not working, and the swimming pool is originally half-filled, so the over-flow controls do not come into the equation at all. In short, assume a pool of water with no water flowing in or out of the pool during the experiment.

I will give my version of answers tomorrow and dicuss further.

#### wantunn

##### Senior Member
I failed Physics but I'll take a shot anyway.
1. The water level should fall. Since when it's in the boat, it's weight affects a bigger surface area (the bottom of the boat). When in the water, it displaces it's own volume only. Can probably work it out mathematically but I'll leave that to someone who passed physics.
Of course, if the rock is bigger then the boat then maybe it's a different answer.

2. I'm guessing the deflated one will tip down, in some hazy part of my mind it's related to surface area and friction from the surounding air but I'm too woozy to work it out now.

roygoh said:
Two simple but interesting questions to refresh your O Level physics knowledge:

1. There is a man and a piece of rock the size of a basket ball in a little boat that is floating in a swimming pool. The man throws the rock into the water. Does the water level in the swimming pool rise or fall?

2. A deflated balloon is place on one side of a balance and a balloon filled with air (same composition as the ambient atmosphere) is placed on the other side. If both the balloons have identicle mass before one of them is inflated, which side does the balance tip towards?

- Roy

#### zod

##### New Member
eddietkm said:
Air is weightless.

:bigeyes: :bigeyes: :bigeyes:

#### zod

##### New Member
1)if density of rock is lesser than or equal to density of water in swimming pool, water level will be same
if density of rock is more than density of water in swimming pool, water level will fall

2)if ambient atmosphere is used to inflate the balloon normally, then the air inside the balloon is "compressed" and balance will tip towards the balloon filled with air.
if the air inside the balloon is exact same composition as ambient atmosphere(ie same density and pressure) then balance will stay still.

***note that in 2nd case, if the opening of the balloon is opened, the balloon will not deflate, as pressure and density of air outside and inside the balloon is same. this also means that 2nd case is not practical

#### silver.wolf

##### Senior Member
My take:

1) water level remains the same

2) inflated balloon will tip upwards

#### wantunn

##### Senior Member
zodnm said:
:bigeyes: :bigeyes: :bigeyes:
Yah.. haven't you heard? Air pressure is an urban legend

#### eric69

##### New Member
1) It depends on whether the level in the swimming pool is relative to what reference point. If it is with reference to the swimming pool's bottom, then the level will rise. If it is relative to the boat, then the level is the same.

2) Balance will tip towards the inflated balloon. Becos the air in the inflated balloon is "compressed air" and it is heavier.

#### gooseberry

##### Senior Member
roygoh said:
Two simple but interesting questions to refresh your O Level physics knowledge:

1. There is a man and a piece of rock the size of a basket ball in a little boat that is floating in a swimming pool. The man throws the rock into the water. Does the water level in the swimming pool rise or fall?

2. A deflated balloon is place on one side of a balance and a balloon filled with air (same composition as the ambient atmosphere) is placed on the other side. If both the balloons have identicle mass before one of them is inflated, which side does the balance tip towards?

- Roy
1. Hmmm... really dependant on density/weight of the rock - with the rock in the boat with the man, the boat will displace a certain volume of water; with the rock dropped in the pool, the boat will displace less volume of water, but the rock will now also displace voume of water equivalent to the rock's size. So whether the reduced displacement of the boat is equivalent to the displacement caused by the rock is dependant on the rock's density.

2. balance should be the same.

#### zod

##### New Member
eric69 said:
2) Balance will tip towards the inflated balloon. Becos the air in the inflated balloon is "compressed air" and it is heavier.
:thumbsup: :thumbsup:
i din thought of that

#### Zerstorer

##### Senior Member
eric69 said:
2) Balance will tip towards the inflated balloon. Becos the air in the inflated balloon is "compressed air" and it is heavier.
Good point.

#### onlock

##### New Member
1. Same level.

2. Balance should be the same.

#### szekiat

##### Senior Member
1)The boat is floating due to upthrust being equal to the total wt of the boat+man+rock system. U did not mention if the boat was floating on the surface or partially submerged, but i assumed the latter, this being a practical scenario. Therefore, when the rock is removed from the boat, the water level relative to the boat should fall since the upthrust required to support the boat is now less. Hence the vol of water displaced is less too. (anyone lost?) when the rock is dropped into the water, the total volume of the rock water system would have increased. However, this is irrelevent to the water level on the boat since we assume that the boat is still on the surface and is only affected by the water around it. However, the water level relative to the side of the pool will have increased since the new vol is greater with the introduction of a rock.

2)Since even air has weight, won't the total moments due to the balloon with air be greater than that of the one without? If so then i think the one without will tip up.

#### showtime

##### New Member
1. fall
2. ballon inflated with air will tip downwards

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