A couple of you calculated force as pressure/area, instead of pressure x area. Very careless error, but also shows that you haven't really memorized your equation properly:
Pressure = Force / Area
Qn 1(a) Ans: 420N, 1(b) Ans: 1.5 x 10^6 N
Generally no problem with this question
Qn 2 Ans: 4400N
Some of you have a difficulty understanding the question. You need to recognize that the air inside the airplane is atmospheric pressure, i.e. 760mm mercury, while the air outside has a lower air pressure, i.e. 210mm mercury. The net force on the window is then the difference in pressure multiplied by the area. The question also wants you to use pgh to calculate pressure, instead of just using 10^6 Pa.
Q 3
I wrongly assigned this question. This is an upthrust question, will go through together with the other upthrust questions.
Q 4 Ans: 800 kgm^-3
No problem for most of you, except many of you left the answer in gcm^-3 instead of SI units. Always leave final answer in SI units unless otherwise specified.
Q 5 (a)(i) Ans: 345 kPa
(a) (ii) Ans: 245 kPa
(b) (i) Ans: 690 N
Generally not much problem for the first 3 sub-questions. Question b(ii) is very unclear what it wants from you, therefore I basically marked everybody correct. This was what the original question wanted to ask from you:
Consider the gas as an object. The force acting on the gas from the top is only 276N (are you able to get this value yourself?), but the downward force the gas is acting on the bottom of the tank is 690 N. Where does the extra 414 N of force come from?
Most of you answered something along the lines of oh Pascal's Principle states that the pressure of the tank must be the same, and hence the larger area below means a greater force is applied, since F = PA. While this statement is true, it doesn't explain where the extra 414 N comes from. So what's the real explanation?
The extra force comes from the top of the tank, the horizontal part adjacent to the shaft. What happens when you compress the gas (and hence increasing the pressure), the gas pushes outwards against the container walls. By Newton's 3rd law, all the container walls push back on the gas, exerting force on the gas. If you look at the container walls at the top of the container, the force exerted is downward, and this force accounts for the missing 414 N of force.
Second Q5 Ans: 31N
Not many people got this correct, even though the working is simple. Many identified the key concept is that the Pressure at A = Pressure at B. However, many forget to include the pgh of 8m of fluid when calculating the pressure at B.
Due Tuesday: Do the rest of the tutorial, including Q3 and the second Q5 if you have not done these questions.
Pressure = Force / Area
Qn 1(a) Ans: 420N, 1(b) Ans: 1.5 x 10^6 N
Generally no problem with this question
Qn 2 Ans: 4400N
Some of you have a difficulty understanding the question. You need to recognize that the air inside the airplane is atmospheric pressure, i.e. 760mm mercury, while the air outside has a lower air pressure, i.e. 210mm mercury. The net force on the window is then the difference in pressure multiplied by the area. The question also wants you to use pgh to calculate pressure, instead of just using 10^6 Pa.
Q 3
I wrongly assigned this question. This is an upthrust question, will go through together with the other upthrust questions.
Q 4 Ans: 800 kgm^-3
No problem for most of you, except many of you left the answer in gcm^-3 instead of SI units. Always leave final answer in SI units unless otherwise specified.
Q 5 (a)(i) Ans: 345 kPa
(a) (ii) Ans: 245 kPa
(b) (i) Ans: 690 N
Generally not much problem for the first 3 sub-questions. Question b(ii) is very unclear what it wants from you, therefore I basically marked everybody correct. This was what the original question wanted to ask from you:
Consider the gas as an object. The force acting on the gas from the top is only 276N (are you able to get this value yourself?), but the downward force the gas is acting on the bottom of the tank is 690 N. Where does the extra 414 N of force come from?
Most of you answered something along the lines of oh Pascal's Principle states that the pressure of the tank must be the same, and hence the larger area below means a greater force is applied, since F = PA. While this statement is true, it doesn't explain where the extra 414 N comes from. So what's the real explanation?
The extra force comes from the top of the tank, the horizontal part adjacent to the shaft. What happens when you compress the gas (and hence increasing the pressure), the gas pushes outwards against the container walls. By Newton's 3rd law, all the container walls push back on the gas, exerting force on the gas. If you look at the container walls at the top of the container, the force exerted is downward, and this force accounts for the missing 414 N of force.
Second Q5 Ans: 31N
Not many people got this correct, even though the working is simple. Many identified the key concept is that the Pressure at A = Pressure at B. However, many forget to include the pgh of 8m of fluid when calculating the pressure at B.
Due Tuesday: Do the rest of the tutorial, including Q3 and the second Q5 if you have not done these questions.
Posts
No comments:
Post a Comment