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 Another (mostly) calculus question FlatAssembler Member Religious Views: Atheist Posts: 864 Threads: 84 Joined: July 26, 2017 Reputation: 5 Another (mostly) calculus question April 26, 2019 at 12:57 pm In one of the solutions to the problems in my physics textbooks, it's written (without further explanation, as if it were somehow obvious) that the gravitational potential energy of a vertical rod is given by the formula U=1/2*m*g*l. How does that make any sense? The sum of the gravitational energy of the infinitesimally small parts of the rod is obviously U=integral(m*g*l,l,0,l)=1/2*m*g*l^2. Fireball Refractory Member Religious Views: Atheist Posts: 7973 Threads: 17 Joined: September 8, 2015 Reputation: 78 RE: Another (mostly) calculus question April 26, 2019 at 2:13 pm Don't need calculus unless you're a masochist. If the rod is standing on the ground, its center of mass is 1/2 way up the bar. PE=mgl for the center of mass, thus mgl/2 for the bar. If you get to thinking you’re a person of some influence, try ordering somebody else’s dog around. onlinebiker Senior Member Religious Views: cynical S.O.B. Posts: 8944 Threads: 399 Joined: October 3, 2018 Reputation: 17 RE: Another (mostly) calculus question April 26, 2019 at 2:25 pm (April 26, 2019 at 2:13 pm)Fireball Wrote: Don't need calculus unless you're a masochist. If the rod is standing on the ground, its center of mass is 1/2 way up the bar. PE=mgl for the center of mass, thus mgl/2 for the bar. Practically yes. But - if picking nits -- Earth' s gravitational attraction is stronger at the " bottom" ( in relation to the earth) than it is at the top - so the center of mass will be below the centerpoint of the rod... brewer I've got industrial disease Religious Views: atheist Posts: 24976 Threads: 513 Joined: June 16, 2015 Reputation: 90 RE: Another (mostly) calculus question April 26, 2019 at 4:18 pm What if the rod is made of Viagra? I don't have an anger problem, I have an idiot problem Fireball Refractory Member Religious Views: Atheist Posts: 7973 Threads: 17 Joined: September 8, 2015 Reputation: 78 RE: Another (mostly) calculus question April 26, 2019 at 5:37 pm (April 26, 2019 at 2:25 pm)onlinebiker Wrote: (April 26, 2019 at 2:13 pm)Fireball Wrote: Don't need calculus unless you're a masochist. If the rod is standing on the ground, its center of mass is 1/2 way up the bar. PE=mgl for the center of mass, thus mgl/2 for the bar. Practically yes. But - if picking nits -- Earth' s gravitational attraction is stronger at the " bottom" ( in relation to the earth) than it is at the top - so the center of mass will be below the centerpoint of the rod... That difference is orders of magnitude smaller than the error in the measurement of the length of the rod. If you get to thinking you’re a person of some influence, try ordering somebody else’s dog around. Cod Fit as a butchers dog Religious Views: Atheist Posts: 2380 Threads: 43 Joined: October 30, 2017 Reputation: 48 RE: Another (mostly) calculus question April 26, 2019 at 5:41 pm (April 26, 2019 at 5:37 pm)Fireball Wrote: (April 26, 2019 at 2:25 pm)onlinebiker Wrote: Practically yes. But - if picking nits -- Earth' s gravitational attraction is stronger at the " bottom" ( in relation to the earth) than it is at the top - so the center of mass will be below the centerpoint of the rod... That difference is orders of magnitude smaller than the error in the measurement of the length of the rod. I've been told the length of the rod doesn't matter. Fireball Refractory Member Religious Views: Atheist Posts: 7973 Threads: 17 Joined: September 8, 2015 Reputation: 78 RE: Another (mostly) calculus question April 26, 2019 at 7:54 pm (April 26, 2019 at 5:41 pm)IWNKYAAIMI Wrote: (April 26, 2019 at 5:37 pm)Fireball Wrote: That difference is orders of magnitude smaller than the error in the measurement of the length of the rod. I've been told the length of the rod doesn't matter. Girth! Amirite!? If you get to thinking you’re a person of some influence, try ordering somebody else’s dog around. onlinebiker Senior Member Religious Views: cynical S.O.B. Posts: 8944 Threads: 399 Joined: October 3, 2018 Reputation: 17 RE: Another (mostly) calculus question April 26, 2019 at 10:30 pm (April 26, 2019 at 5:37 pm)Fireball Wrote: (April 26, 2019 at 2:25 pm)onlinebiker Wrote: Practically yes. But - if picking nits -- Earth' s gravitational attraction is stronger at the " bottom" ( in relation to the earth) than it is at the top - so the center of mass will be below the centerpoint of the rod... That difference is orders of magnitude smaller than the error in the measurement of the length of the rod. Agreed... But - somebody had to pick the nits... Fireball Refractory Member Religious Views: Atheist Posts: 7973 Threads: 17 Joined: September 8, 2015 Reputation: 78 RE: Another (mostly) calculus question April 26, 2019 at 10:49 pm (April 26, 2019 at 10:30 pm)onlinebiker Wrote: (April 26, 2019 at 5:37 pm)Fireball Wrote: That difference is orders of magnitude smaller than the error in the measurement of the length of the rod. Agreed... But - somebody had to pick the nits... You been looking at the back of my head, again? If you get to thinking you’re a person of some influence, try ordering somebody else’s dog around. Smaug Junior Member Religious Views: ~ Posts: 273 Threads: 2 Joined: November 19, 2014 Reputation: 9 RE: Another (mostly) calculus question May 7, 2019 at 3:41 am (This post was last modified: May 7, 2019 at 5:11 am by Smaug.) (April 26, 2019 at 12:57 pm)FlatAssembler Wrote: In one of the solutions to the problems in my physics textbooks, it's written (without further explanation, as if it were somehow obvious) that the gravitational potential energy of a vertical rod is given by the formula U=1/2*m*g*l. How does that make any sense? The sum of the gravitational energy of the infinitesimally small parts of the rod is obviously U=integral(m*g*l,l,0,l)=1/2*m*g*l^2.You've made a mistake in assuming the potential energy of an infinitesimally small element of the rod in the integral. It actually is the following: dU=dm*g*x where dm is an infinitely small element of mass and x is its height above an arbitrary zero energy level. Since dm=rho(x)*dx where rho(x) is lineal density of the rod, it can be rewritten as dU=rho(x)*g*x*dx . If the rod is uniform (rho(x)=m/l=const) we arrive at dU=(m/l)*g*x*dx and the integral is the following: U = int((m/l)*g*x dx,  x=0 to x=l) = (m*g*l^2)/(2*l) = 1/2*m*g*l. Also it's a good practice to give distinct names to your parameters and variables. In Quote:U=integral(m*g*l , l ,0,l) you use l for both the length of the rod (a constant parameter in this context) and the height above the ground (an integration variable). Even though in this very case they are measured along the same line they are different things. While it may not look as a big deal it will inevitably lead to confusion sooner or later. P. S. I have to note that for an inclined rod the calculation will not be quite the same bacause dm=rho(x)*dx=m/l*dx is only valid if the rod is parallel to the axis x. Otherwise you have to introduce an inclination angle. « Next Oldest | Next Newest »

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