Physics Help Forum Find initial velocity of projectile

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 Apr 10th 2018, 05:47 AM #1 Junior Member   Join Date: Apr 2018 Posts: 3 Find initial velocity of projectile ballistic pendulum initially at rest. projectile with mass m=10g hits object M=2kg and remains stuck and it goes up to height h=20cm. no air friction. im foreign and i tried to translate it i hope it s clear, i'm in ninth grade and i guess i didnt really understand the theory. i tried to solve it but i dont think its correct. can someone also explain the solution to me please? edit:i think i figured it out Last edited by archie13; Apr 10th 2018 at 06:19 AM.
 Apr 10th 2018, 09:24 AM #2 Senior Member   Join Date: Aug 2010 Posts: 404 If mass m1 has speed v, then it has kinetic energy m1v^2. Mass m2 has 0 speed so initial kinetic energy 0. Taking potential energy to be at be bottom of the pendulum, where the two masses first strike, the total energy of the system is m1v^2. At height h= 20 cm, the system potential energy is h(m1+ m2)g and, since two masses stop there, the system kinetic energy is 0. Since the total energy is conserved, we must have m1v^2= h(m1+ m2)g. Solve that equation for v. (Because the two objects stick together their collision is NOT "elastic" and kinetic energy is not conserved but the total energy of the system is conserved.) It is hard to tell what "m1" and "m2" are because, in your textbook, the two masses are called "m1" and "m2" in the picture but "m" and "M" in the text. archie13 likes this. Last edited by HallsofIvy; Apr 10th 2018 at 02:58 PM.
Apr 10th 2018, 11:13 PM   #3
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 Originally Posted by HallsofIvy If mass m1 has speed v, then it has kinetic energy m1v^2. Mass m2 has 0 speed so initial kinetic energy 0. Taking potential energy to be at be bottom of the pendulum, where the two masses first strike, the total energy of the system is m1v^2. At height h= 20 cm, the system potential energy is h(m1+ m2)g and, since two masses stop there, the system kinetic energy is 0. Since the total energy is conserved, we must have m1v^2= h(m1+ m2)g. Solve that equation for v. (Because the two objects stick together their collision is NOT "elastic" and kinetic energy is not conserved but the total energy of the system is conserved.) It is hard to tell what "m1" and "m2" are because, in your textbook, the two masses are called "m1" and "m2" in the picture but "m" and "M" in the text.
Thanks a lot!'Taking potential energy to be at be bottom of the pendulum' here you mean that its 0 then?

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Apr 10th 2018, 11:18 PM   #4
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 Originally Posted by archie13 Thanks a lot!'Taking potential energy to be at be bottom of the pendulum' here you mean that its 0 then? Trimis de pe al meu SM-J530F folosind Tapatalk
And m2=m
m1=M

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Apr 11th 2018, 05:41 AM   #5
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 Originally Posted by archie13 Thanks a lot!'Taking potential energy to be at be bottom of the pendulum' here you mean that its 0 then? Trimis de pe al meu SM-J530F folosind Tapatalk
Yes, I intended to write 'Taking potential energy to be 0 at the bottom of the pendulum'

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