You are sitting in a boat on a lake, you have with you in the boat a large rock. You throw the rock out of the boat and it sinks to the bottom of the lake, does the water level of the lake go up, down or stay the same?
Whilst in the boat the rock (and you) are floating, this means that the amount of water displaced by the rock is the mass of the rock. When the rock sinks to the bottom of the lake the amount of water displaced by the rock is the volume of the rock.
As the rock sinks to the bottom of the lake, we know that the rock is more dense than water.
As the rock is more dense than water, the rock will displace more (a greater volume of) water when floating than when at the bottom of the lake. This means that after throwing the rock out of the boat the water level of the lake as a whole will go down.
JB
Related Physics A Level answers
All answers ▸A cyclist rides 10km. In the first 5km, they climb a hill, averaging 10km/h. In the second 5km, they descend the hill, averaging 30km/h. What is their average speed over the full 10km?
Calculate the kinetic energy of a proton moving at 95% of the speed of light. (c = 3x10^8 m/s, m_p = 1.67x10^-27 kg) [4 marks]
The LHC accelerates protons to a speed of 0.999999991c around a 27km ring. Due to relativistic effects, their mass increases. Given that the magnetic fields used are 8T, calculate this mass. What is the total energy of an LHC beam containing 3e14 protons?
Derive an expression to show that for satellites in a circular orbit T^2 ∝ r^3 where T is the period of orbit and r is the radius of the orbit.