physics qs

[sshaltoni]

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1) Suppose that a 10-cm^3 cube of mass 21kg is submerged in a container of fluid density 0.2 kg/cm^3. The cube is tied by a string and is buoyed upwards such that the string is pulled taut. The tension in the string would be closest to:

4N
6N
8N
10N

Explanation at the back of the book:

The tension can be found by subtracting the weight from the buoyant force:

T=Vpg-mg=(10 cm^3)(0.2 kg/cm^3)(10m/s^2)-(1kg)(10m/s^2)= 10N


can anyone explain where they got the mass of 1kg from??

2)consider a coin in a container hanging from a massless string. string is cut and coin falls hitting the bottom of the container. in reference to the coin, which of the following quantities would have a positive numeric value?

kinetic energy
potential energy
momentum
impulse


Book explanation:

Kinetic energy requires a positive velocity to have a positive value. potential energy may have a positive value at rest. both momentum and impules require a positive velocity vector to have positive values.

i'm not satisfied with this explanation, it sucks in my opinion. if someone wants to take a stab at it and conceptually explain this better, that'd be oh so sweet. thanks.

 

[ASDIC]

the explanation is wrong:

"T=Vpg-mg=(10 cm^3)(0.2 kg/cm^3)(10m/s^2)-(1kg)(10m/s^2)= 10N"

They need to convert cm to meters since they are using g = 10 m/s2


and Kinetic energy is always positive. since mass can never be negative and velocity squared is always positive.

 

[bgreet]

I'm at a complete loss on both of those. I just don't see their explanations making sense.

 

[imsotired]

the explanation is wrong:

"T=Vpg-mg=(10 cm^3)(0.2 kg/cm^3)(10m/s^2)-(1kg)(10m/s^2)= 10N"

They need to convert cm to meters since they are using g = 10 m/s2


and Kinetic energy is always positive. since mass can never be negative and velocity squared is always positive.

The cm^3 cancel out so there is no need to convert cm to m. Newtons are in kg m/s^2 so the explanation is correct.

To the OP, I think that is probably a mistake from the book. The tension is the difference between the upward buoyant force and the downward force of the weight.

The second question is just understanding the numbers. I agree with ASDIC in that KE is ALWAYS positive since it is v^2. The others may be negative if there is a negative velocity (since velocity is a vector, it has direction and the negative and positive are conventions that show the directional component of speed).

 

[DieselPetrolGrl]

the explanation is wrong:

"T=Vpg-mg=(10 cm^3)(0.2 kg/cm^3)(10m/s^2)-(1kg)(10m/s^2)= 10N"

They need to convert cm to meters since they are using g = 10 m/s2


and Kinetic energy is always positive. since mass can never be negative and velocity squared is always positive.

and the denity is given in kg/cm^3!!!!
wtf conversions from hell ...i hope the real mcat is all kg/m/s
because i FORGET to convert

 

[DieselPetrolGrl]

The cm^3 cancel out so there is no need to convert cm to m. Newtons are in kg m/s^2 so the explanation is correct.

To the OP, I think that is probably a mistake from the book. The tension is the difference between the upward buoyant force and the downward force of the weight.

The second question is just understanding the numbers. I agree with ASDIC in that KE is ALWAYS positive since it is v^2. The others may be negative if there is a negative velocity (since velocity is a vector, it has direction and the negative and positive are conventions that show the directional component of speed).

BRILLIANT! i like canceling out better than convering anyways 😉

 

[Jonathan13180]

Ive seen these questions, and it still puzzles me how they got 1kg for the mass???

 

[sshaltoni]

yeah umm... why the 1kg though?

and for the second question, the book said impulse was the right answer, not KE. another typo?? i agree that KE is positive since velocity is squared. so, what's the deal?