a sample of gas is confined in a cylinder with a moveable piston of mass m that is initially held fixed. there is a block of mass 2m on top of the piston, as shown in the figure. a free-body diagram for the block is also shown, which includes the normal force fn exerted on the block by the piston and the weight wb of the block. the upward force that the gas exerts on the piston has magnitude fgas. what is the acceleration of the block immediately after the piston is released?