Since sound is more easily transmitted between close molecules, it travels faster in the denser substance. Sound moves faster through denser air because the molecules are closer together in dense air and sound can be more easily passed on.
You have to find the acceleration first. so you would use the equation a=Vf-V0/t. This would be 0-20/0.5 which equals to be a=-40. then you plug this in to the force equation wich is F=ma. F=55 x -40. F= -2200
