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Newton’s Laws in an Elevator Elevators are great devices for the study of Newton’s First and Second Laws as during a single trip the elevator moves at constant velocity and at other times it accelerates! Consider a trip from the first floor up to the 10th floor. Newton’s Laws in an Elevator Consider a trip from the first floor up to the 10th floor. The complete motion might look something like… Newton’s Laws in an Elevator Accelerating (slowing down) There are three parts to the motion Constant Velocity (no acceleration) Accelerating (speeding up) Newton’s Laws in an Elevator Draw a force diagram of the rider (not the elevator!) FN FN Fg a Fg Accelerating (speeding up) Since the elevator and the rider are accelerating upward, the net force must act upward How would you feel? Heavier, lighter or normal? Heavier! You don’t feel the force of gravity, rather you feel how much the floor is supporting you! Newton’s Laws in an Elevator FNet ma FNet FN Fg FN a Fg Accelerating (speeding up) FN Fg ma Don’t forget! Fg mg FN mg ma The acceleration is positive in this case because it and the net force act upward Newton’s Laws in an Elevator Draw a force diagram of the rider (not the elevator!) Constant Velocity (no acceleration) FN FN Fg Fg Since the elevator and the rider are not accelerating (constant velocity) You would feel normal! Newton’s Laws in an Elevator Accelerating (slowing down) Draw a force diagram of the rider (not the elevator!) a FN Fg FN Fg Since the elevator and the rider are accelerating downward, the net force must act downward How would you feel? Heavier, lighter or normal? Lighter! You don’t feel the force of gravity, rather you feel how much the floor is supporting you! Newton’s Laws in an Elevator Accelerating (slowing down) FNet FN Fg a FN Fg FNet ma FN Fg ma Don’t forget! Fg mg FN mg ma The acceleration is negative in this case because it and the net force act downward Newton’s Laws in an Elevator What if you were standing on a scale while in the elevator? The scale supports you, so the scale would read the Normal Force it exerts on you! Rest Weight Feel Lighter Feel Heavier
