Quick Phyiscs Questions

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thread starter wierdmonkey4 Aug 30 2009 + PM | QUOTE | PERMALINK | REPORT

1. A uniform rope of weight 50 N hangs from a hook. A block of wright 100 N hangs from the rope. What is the tension on the rope? A. 50 N B. 75 N C. 100 N D. 150 N E. It varies from 100 N at the bottom of the rope to 150 N at the top. * i personally think its D. 150 N* 2. According to Newtons Law of universal gravitation, the force of attraction between any 2 masses is directly related to A. the distance b/w the masses B. The product of the 2 masses C. The velocity of the 2 masses D. Sum of the 2 masses *i thinks it A. the distance* 3. A 5 meter uniform plank of mass 100 kg rests ontop of a building with 2 meters extended over the edge ( so 3 meters of plank actually on top of building, and another 2 meters sticking off the edge). How far can a 50 kg person venture past the edge of the building on the plank before the plank begins to tip? A. 1/2 meter B. 1 meter C. 3/2 meter D. 2 meter E. Its impossible to make the plank tip since the person would have to be more than 2 meters from the edge of the building *i dont know the answer to this at all" If some1 could kindly help me with these problems... If possible, could you guys explain what you think the answer is and why, especially for #3. Please and thanks to everyone who tries

Nonexistent 08/30/09 + PM | QUOTE | PERMALINK | REPORT

Lucky you have a multiple choice assigment.

wierdmonkey4 08/30/09 + PM | QUOTE | PERMALINK | REPORT

yea, but its so damn hard

sutper 08/30/09 + PM | QUOTE | PERMALINK | REPORT

wierdmonkey4 : THanks for helping me with #3 A4No0b your answer to #2 still confuses me a bit. Sorry for being so dumb the formula of gravitation is G[m1][m2] / d^2. . the m's are for the masses of the 2 objects and d is for the distance between them. if everything stays the same but the m's get bigger, the answer you get to the equation gets larger. But if everything stays the same but distance is increased, the answer will get smaller. Direct variation means as somthing gets bigger this also gets bigger. So it is mass.

A4No0b Aug 30 2009 + PM | QUOTE | PERMALINK | REPORT

wierdmonkey4 : THanks for helping me with #3 A4No0b your answer to #2 still confuses me a bit. Sorry for being so dumb Let's say there's two equations: f(x) = x and g(x) = 1/x In f(x), as x rises, f(x) also rises. This is proportional and it's called a direct relationship. However in g(x), as x rises, g(x) decreases [example shown below]. This is inversely proportional and is called an indirect relationship. x = 1, g(x) = 1/1 = 1 x = 2, g(x) = 1/2 = 0.5 x = 4, g(x) = 1/4 = 0.25

wierdmonkey4 Aug 30 2009 + PM | QUOTE | PERMALINK | REPORT

A4No0b : Let's say there's two equations: f(x) = x and g(x) = 1/x In f(x), as x rises, f(x) also rises. This is proportional and it's called a direct relationship. However in g(x), as x rises, g(x) decreases [example shown below]. This is inversely proportional and is called an indirect relationship. x = 1, g(x) = 1/1 = 1 x = 2, g(x) = 1/2 = 0.5 x = 4, g(x) = 1/4 = 0.25 that makes alot more sense. THanks sutper : the formula of gravitation is G[m1][m2] / d^2. . the m's are for the masses of the 2 objects and d is for the distance between them. if everything stays the same but the m's get bigger, the answer you get to the equation gets larger. But if everything stays the same but distance is increased, the answer will get smaller. Direct variation means as somthing gets bigger this also gets bigger. So it is mass. THanks for your help too

DarkFairy18 08/30/09 + PM | QUOTE | PERMALINK | REPORT

1. you have the rope+mass=150N. Tension acts equally opposite to the rope which is down due to gravity so you have... Tension = 150N up 2. (A)Distance of the two masses 3. I think it's E. since the center of gravity is 2.5m and only 2m is out. The weight of the plank that is 3m in has 60kg x 9.8(gravity) that's about 600N down x 3m so 1800N. 2m part of the plank has 40kg+50kg is 90kg x gravity is 900N. This 900N can act 1m out or 2m out. At 2m it's 900x2 for 1800N which will equal the other part of the plank(no tipping). (I did not do any heavy calculation for this or much review so don't trust this answer if you disagree)

A4No0b Aug 30 2009 + PM | QUOTE | PERMALINK | REPORT

DarkFairy18 : 3. I think it's E. since the center of gravity is 2.5m and only 2m is out. The weight of the plank that is 3m in has 60kg x 9.8(gravity) that's about 600N down x 3m so 1800N. 2m part of the plank has 40kg+50kg is 90kg x gravity is 900N. This 900N can act 1m out or 2m out. At 2m it's 900x2 for 1800N which will equal the other part of the plank(no tipping). (I did not do any heavy calculation for this or much review so don't trust this answer if you disagree) The mistake here is that although you remembered where the center of gravity is, you didn't apply it. The center of gravity states that all of an object's mass is fixated/concentrated at that specific point. So instead of calculating the mass of the plank that is resting, you can theoretically substitute the entire mass of the plank at the 2.5 meter mark. Ergo, the torque keeping the plank rested is the entire 980 N, multiplied by the difference from the center of mass to the pivot point (0.5 m).