PHYS 1405 – Conceptual Physics I
Apparent Weight
Leader: _________________________ Recorder: __________________________
Skeptic: _________________________ Encourager: ________________________
Materials
Laptop Atwood Machine
LabPro Mass Hanger
String Right Angle Clamp
¾” Table Ring Stand Foam
Introduction
The process of “weighing” something is a little tricky. If you think about it, we almost never weigh something directly. Instead we measure another force which is equal to the weight that we want to measure. For example, if we stand on a bathroom scale, we compress a spring. The scale actually measures the force that compresses the spring. You can change the reading of a bathroom scale by rocking back and forth on the scale. Of course you aren’t changing your weight when you rock back and forth on a scale; you are merely changing what the scale reads. Thus a bathroom scale doesn’t measure the weight of an object directly. Instead it measures the apparent weight.
Q1. What is the weight of an object?
Q2. What is the apparent weight of an object?
In this activity we will investigate the affect that the acceleration of an object will have on its apparent weight.
Procedure
1. Setup
Clamp the Atwood’s machine to the ring stand with the pulleys aligned horizontally. Connect the LabPro to the laptop with the USB cable. (Make sure you use the USB ports in the back of the laptop, not the power USB port on the side.) Use the electronic balance to determine the mass of the force probe. Convert the mass to kg by moving the decimal three places to the left. Don’t forget units.
Force Probe mass = _____________________ kg
Make sure that the switch on the force probe is set to the +/- 10 N position. Connect the force probe to CH 1 of the LabPro.
Attach the string to the force probe and the mass hanger, drape the string over the pulleys, and adjust the mass on the hanger so that the system remains in equilibrium. Place the piece of foam on the table underneath the force probe to protect it from hitting the table.
2. Start LoggerPro
Open LoggerPro
and click on the Open icon 
,
then double click on the folder Probes &
Sensors, Double Click on the folder titled Dual Range Force Sensor, then double click on the file Force 10 N Dual Range. Click OK on the Confirm Sensors that will
open.
3. Zero the Force Probe
As usual we will need to zero the force probe immediately before each data run. You always want to zero the probe in the same orientation that you will use it, so when zeroing the probe for this experiment hold it by the body of the probe with the hook up, and with no tension on the string. To zero the probe, click on the Zero… button next to the collect button. Zero the probe in the same way immediately before each data run.
4. Data Acquisition
Make sure that the string is draped
over the pulley and that the mass on the hanger is adjusted so that the system
doesn’t accelerate. If either the mass
hanger or the force probe is swinging, gently stop it. Click on the Collect button and measure the
force for a few seconds. Click and drag
over a range of the force data. Use the
STAT button 
and determine the mean value of the force that
you measured. Don’t forget units.
Force = _____________
Q3. What force is the force probe actually measuring? (Hint: what is pulling on the hook of the force probe?)
Q4. Explain why in this case the force you answered in Q3 equals the weight of the force probe.
Q5. In this case, does the force probe measure its own weight directly or its apparent weight? Explain.
While holding the system in place, add 20 g to the mass hanger. Test that the force probe now accelerates up. If it doesn’t add a little more mass. Note try to prevent the probe from crashing into the pulleys.
Zero the force probe as before. Click on collect. Allow the probe to collect data for a few second before releasing the system.
Q6. Sketch a graph of the appearance of the Force vs. time graph.
Q7. Examine your data closely. When you released the system, force measured by the probe changes a little bit. In what way?
Q8. On your sketch in Q6 indicate three regions: i) the force before you released the system; ii) while it was moving; iii) When it hits the pulley (or else you stopped it.)
In LoggerPro click and drag over the data while the probe was falling (but before it hits the pulley) and use the STAT button to record the mean value of the force. Don’t forget units.
Force = _____________
Q9. Imagine you are standing on a bathroom scale in an elevator. If the elevator is at rest, will the bathroom scale read your weight?
Q10. If the elevator accelerates up, how is that situation like the procedure you just performed?
Q11. If the elevator accelerates up, will the bathroom scale read a value that is the same, less, or more than your weight? Explain.
Remove the additional weight that you added so that the system is in equilibrium again.
Now while holding the system in place, remove 20 g from the mass hanger. Test that the force probe now accelerates down. If it doesn’t subtract a little more mass. Try to prevent the mass hanger from crashing into the pulley.
Zero the force probe as before. Click on collect. Allow the probe to collect data for a few seconds before releasing the system.
Q12. Examine your data. When you released the system, the force measured by the probe changes a little? In what way?
Click and drag over the data after you released the system but before it hits the pulley and use the STAT button to record the mean value. Don’t forget units.
Force = _____________
Q13. Imagine you are back in the elevator again. If the elevator accelerates down, how is that situation like the procedure you just performed?
Q14. If the elevator accelerates down, will the bathroom scale read a value that is the same, less, or more than your weight? Explain.
While holding the system, remove 50 g more. Zero the force probe as before. Click on collect, wait for a few seconds and release the system.
Q15. Examine your data. When you released the system, the force measured by the probe changed. In what way?
Q16. Was the amount the force changed greater in this case than previously (Q12)?
Click and drag over the data after you released the system but before it hits the pulley and use the STAT button to record the mean value. Don’t forget units.
Force = _____________
While holding the system, remove 50 g more. Zero the force probe as before. Click on collect, wait for a few seconds and release the system.
Q17. Examine your data. When you released the system, the force measured by the probe changed. In what way?
Q18. Was the amount the force changed greater in this case than previously (Q12 and Q16)?
Click and drag over the data after you released the system but before it hits the pulley and use the STAT button to record the mean value. Don’t forget units.
Force = _____________
Q19. If you continued removing mass, what do you think would be the trend of the measured force while the system was moving?
P20. If you removed all of the mass from the hanger, what do you think the force probe would read while the system was moving?
While holding the system, remove the mass hanger from the string. Zero the force probe as before. Click on collect, wait for a few seconds and release the system.
Click and drag over the data after you released the system but before it hits the ground and use the STAT button to record the mean value. Don’t forget units.
Force = _____________
Q21. Did you obtain a value close to zero? You may not get exactly zero because of inaccuracy of the force probe and slight effects of air resistance and resistance of the pulley.
Q22.You decide to try an experiment. (By the way, don’t try this.) You loosely tape a bathroom scale to your feet and have a burly friend drop you from a tree. What does the scale read as you fall? Explain.
Summary
Questions
After each question fill in the blank with same, greater, less or 0.
S23. You are standing on a bathroom scale in an elevator. Initially the elevator is at rest. How does the reading on the scale compare to your weight? __________________
S24. The elevator speeds up at the start of the ride.
How does the reading on the scale compare to your weight? __________________
S25. The elevator continues up at a constant velocity.
How does the reading on the scale compare to your weight? __________________
S26. You are reaching your floor and the elevator slows.
How does the reading on the scale compare to your weight? __________________
S27. Unfortunately, just before reaching your floor the cable on the elevator breaks. As you plummet how does the reading on the scale compare to your weight? ____________
Circle the best choice in S28 and S29
S28. Professor Williams has a great idea for a book called Elevator Weight Management. If someone wants to lose weight they continuously ride an elevator that accelerates upward/downward.
S29. If someone wants to gain weight they continuously ride an elevator that accelerates upward/downward.
S30. Do you actually gain or lose weight simply by riding an elevator? Explain.