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 Feb 20 awarded Yearling Aug 4 awarded Autobiographer May 26 awarded Fanatic Mar 17 awarded Enthusiast Dec 9 awarded Critic Dec 9 revised Doesn't a box holding a vacuum weigh the same as a box full of air? added 11 characters in body Dec 9 revised Doesn't a box holding a vacuum weigh the same as a box full of air? added 55 characters in body Dec 9 answered Doesn't a box holding a vacuum weigh the same as a box full of air? Dec 9 comment Doesn't a box holding a vacuum weigh the same as a box full of air? Hm, just to give a hint: You are wondering about the Archimedes' principle here. Sep 21 awarded Yearling Dec 1 comment How to get energy of collision if you know force of gravity of an object($m \rightarrow F=mg$)? As you pointed out right: $W = F \Delta s$ and the energy is derived form integrating over the path. If we assume that the gravitational field remains constant (so $g$ is independent on the position of the considered object) and that an object falls down form the height $h$ we have to integrate $mg\Delta s$ from $h$ to $0$, which yields $-mgh$. If you are familiar with concept of energy conservation, this(the potential energy) is equivalent to the kinetic energy the mass wins in a free fall from height $h$, which is $E_k = \frac{1}{2}mv^2$ Sep 28 comment Does a material exist that reduces a magnetic field without being affected by the magnetic field itself? Also, if it is a type II superconductor, there will be losses due to flux pinning Sep 28 comment Does a material exist that reduces a magnetic field without being affected by the magnetic field itself? The point is, however, when the superconductor enters into the magnetic field, the field lines are deviated, which will be related to work... so actually, entering the region between the two magnets will cost some energy. If the plate leaves the area after, the energy will be won back. Sep 28 comment Does a material exist that reduces a magnetic field without being affected by the magnetic field itself? Check out this Sep 27 comment Acceleration/tension of object. 1 on horizontal surface one hanging By the way, I strongly advice you to not replace the masses and accelerations by the actual values, but to keep the symbols (so do not replace $m_1$ with 9). This really helps to not confuse things, also it allows you to check whether the units are right. Sep 27 comment Acceleration/tension of object. 1 on horizontal surface one hanging Effectively, the net force on $m_1 = 9kg$ is given by $m_1g - T$, thus the 2nd law of Newton for $m_1$ writes: $m_1g - T = m_1a$. Now what remains to do, is to write Newton's 2nd law for the other mass $m_2 = 5kg$ to find out what is $T$ Sep 27 answered Acceleration/tension of object. 1 on horizontal surface one hanging Sep 26 awarded Supporter Sep 26 revised Does a material exist that reduces a magnetic field without being affected by the magnetic field itself? added 182 characters in body Sep 26 answered Does a material exist that reduces a magnetic field without being affected by the magnetic field itself?