Download determination of the acceleration of an elevator.

DETERMINATION OF THE ACCELERATION OF AN ELEVATOR.
INTRODUCTION:
In order for an object to accelerate, there must be a net force acting on it. We know that the direction of
the acceleration will be in the same direction as the direction of the net force. The equation for Newton’s
2nd law is F = ma or a = F/m.
Today we will be looking at the acceleration of an elevator when it starts and stops. Since acceleration
and force are vector quantities, we must distinguish between up and down. Our sign convention will be
positive for up and negative for down. Therefore the force of gravity, Fg is negative and the force
exerted by the elevator, FN is positive. FN is your “apparent weight” measured by the bathroom scale
you will be standing on while the elevator accelerates. Since there are only 2 forces acting on you in the
elevator, the net force, F is just the vector sum of the 2 forces.
FN (+)
F = FN + Fg
If acceleration is upwards, F = (+)
If acceleration is downwards, F = (-)
Fg (-)
OBJECTIVE : Determine the acceleration of an elevator using a bathroom scale.
MATERIALS: Bathroom scale, elevator
PROCEDURE:
1) Determine your actual weight by standing on a bathroom scale. If the scale is graduated in
pounds, convert pounds to kilograms by using the conversion 2.2 lb = 1 kg. Then calculate
the weight in Newtons by multiplying your mass in kg by 9.8 N/kg. Record these values in
the first 3 rows of the data table (each row should have the same numbers)
2) While standing on the scale in an elevator, press the up button. Measure the maximum
weight the scale reaches. Convert to Newtons by multiplying your scale reading by 4.45 N/lb.
Record the data in the “up/start” column.
3) When the elevator approaches the next stop, read the scale just as the elevator decelerates
to a stop. Determine the minimum weight attained while decelerating. Convert to Newtons.
Record the data in the “up/stop” column.
4) While standing on the scale in an elevator, press the down button. Read the minimum
weight attained just as the elevator accelerates. Convert to Newtons. Record the data in the
“down-start” column.
5) When the elevator approaches the next stop, read the scale just as the elevator decelerates
to a stop. Determine the maximum weight attained while decelerating. Convert to Newtons.
Record the data in the “down/stop” column.
*** Be sure to express forces and weights with the appropriate signs, + for up and - for down. ***
6)
F = FN + Fg
Take your values for FN and Fg to obtain your F values. (this
should be equal to the difference in your “weights” as measured by the scale)
7) Then, take your F values, divide them by your mass, and obtain the acceleration rate of
the elevator for each part of your journey.
UP/START
UP/STOP
DOWN/START
DOWN/STOP
1) Actual Weight (lb)
2) Mass (kg)
3)
Fg (N)
4)
FN (lb)
5)
FN (N)
6) F (N) (# 5 - # 3)
7) Acceleration = F/m
(#6/#2)
CONCLUSIONS:
1. When there is a net force on your body, what happens to your velocity. When does this
happen in this activity?
2. When there is not a net force on your body, what happens to your velocity? When does this
happen in this activity?
3. Describe the feeling you would get in each situation as feeling lighter, heavier or the same
weight.
4. How does the acceleration you experience in an elevator compare to that you might
experience in a car that accelerates from 0 to 60 miles per hour in 7 seconds? (Remember
to express accelerations in m/s2; 60 miles/hr = 26.8 m/s)
5. Draw a force diagram for each of the situations to show the magnitude of weight and normal
force on your body.