# Need help on two easy highschool questions

#### isak

OK. I couldn't find help anywhere so i registered this forum i hope you help me.

Question 1 : A wood plank with 0,7 g/cm^3 density is having drilled to create a gap. Then this gap is being filled with a material has 2,5 g/cm3 density. The wood now weighs 360g more.
Then what's the volume of the gap ? I found it 144 cm3 but the test says 200cm3 .

question 2: A stone that has 280g of mass and 4g/cm^3 of density is being dropped into a glassware. What would be the increased amount of weight of the glassware ?
i thought its 280 because the rock weighs so but the testbook says its increasing 264 g

#### studiot

First one
You should have the following equation

If V is the volume of the hole then

2.5V - 0.7V = 360

#### isak

First one
You should have the following equation

If V is the volume of the hole then

2.5V - 0.7V = 360
thanks for the idea .i put them into the equation which is d=m/v next 360/1,8=200
still couldnt understand the second question

#### Woody

Question 2 does seem a bit odd,
Are they perhaps including the mass of the air displaced from the top of the container?

#### isak

Question 2 does seem a bit odd,
Are they perhaps including the mass of the air displaced from the top of the container?
Ok nevermind
The question is actually translated and illustrated version of the original one in my highschool test book. i started to think there is no way of it and the teacher that wrote that question probably or must wrote wrong. Maybe the answer is wrong. i don't know i can't take my time for thinking over that . IT just makes no sense to me .
thanks yall

#### Global Tutor

I see what you are missing in the plank problem. I'll do a video that explains the solution and post a link later today

isak

#### Global Tutor

As promised here is the solution. Feel free to share:

isak

#### isak

As promised here is the solution. Feel free to share:
i understood how quickly i can do it with diagrams and thinking of a replacement in the mind. You've explained a lot easier. Thank you Sir!

It's a pleasure

#### benit13

For question 2, you need to recall a few important facts:

1. The reading on the scales is actually determined from the force pushing down on it, which will be the result of a force balance.
2. If an object is put into a fluid (like water or oil) then the fluid will impose a force on it equal to the weight of the displaced fluid. This is called buoyancy and the force is called the buoyant force (or just buoyancy).
3. Ultimately, the rock will float or sink based on its density relative to the fluid density. If the rock's density is greater, it will sink. If the fluid's density is higher, it will float.

So... think about the following when you're trying to solve the problem:

i) when the rock is in the olive oil, will it sink or float?
ii) when the rock is in the water, will it sink or float?
iii) what fluids are being displaced?
iv) what is the volume of displaced fluid equal to?
v) what is the buoyancy force?

It's a pretty tough question actually, but it doesn't require specialist knowledge or anything... we'll help you through it