An engineer claims to have a new kind of "safety" air-brake for elevators. If the cable should snap, a patented velocity-sensor turns on the brakes once the elevator hits 40 mph. The brakes are powerful enough to stop a 700 lb elevator in 1/20 of a second. How much force do the brakes exert to stop the elevator like this? Why isn't the answer 700 lbs? Isn't the force of gravity only 700 lbs? I'm not sure where to begin with this one, I understand the concept of the elevator is being held taut by the elevator cord, and that on the sides, there is Ff (f=friction), but I am not really sure what to do with the information being given to me in this problem. Could someone give me a general idea how I should attempt this problem? Thanks in advance.