Download PHYS 1405 – Conceptual Physics I The Equilibrium Rule Leader

PHYS 1405 – Conceptual Physics I
The Equilibrium Rule
Leader: _____________________________ Recorder: ___________________________
Skeptic: _____________________________ Encourager: _________________________
Materials
Meter stick with loops
Mass Hanger (50 g)
2 x Spring Scales (5 N capacity)
Masking tape
2 x Ring stands with right angle clamps
Introduction
The term mechanical equilibrium means that an object has a net force of zero on it. We
know from Newton’s first law, that when an object has a net force of zero acting on it, it
will continue in a state of uniform motion. If it is at rest, it will remain at rest and if the
object is moving at constant velocity, it will remain moving at constant velocity. In this
activity we will explore Newton’s 1st Law from the point of view of finding the forces
necessary to produce equilibrium on an object. The apparatus we will use will consist of
a meter stick with loops of thread attached, hereafter known as “the bar”, two spring
scales and a weight which can be suspended from different places on the bar.
Procedure and Questions
1. Examine one of the provided spring scales. Note that there are different scales on
opposite sides. Record the units that each scale reads.
___________ and ___________.
2. The type of quantity that can be measured with a spring scale then is either
___________ or __________.
In the following, we will use the newton (N) scale on the spring scale. When using a
spring scale to make a measurement, make sure that the scale hangs straight up and
down and the tongue of the spring scale is free to move.
Holding the spring scale vertically with nothing suspended from it, make sure that it
reads 0 N. If it doesn’t then adjust the white nut on top of the scale until it does read 0 N.
Contact your instructor if you have trouble with this step.
3. Use one of the spring scales to determine the weight, in newtons, of the bar. Record
your result in the following space. Include units.
Weight of Bar: ___________________
PHYS 1405, p. 1/1
4. Use one of the spring scales to determine the weight, in newtons, of the mass hanger.
Record your result in the following space. Include units.
Weight of Mass Hanger: _____________
5. You will suspend the bar simultaneously from the two spring scales. Predict what the
reading on each spring scale will be when you support the bar with two spring scales.
Record your prediction below. Include units.
Prediction: Scale 1:__________________ Scale 2:__________________
For the following questions, you will be taking readings from each spring scale. Use
masking tape to label one of the scales as #1 and the other as #2. We will refer to
readings from scale 1 as F1 and readings from scale 2 as F2.
6. Suspend the bar on the side with two hooks from the two scales and record the
readings from both scales. Before taking your reading make sure that each spring scale is
suspended vertically and that the tongue of the spring scale can move easily. Include
units.
F1 : _____________________________
F2 : _____________________________
7. Was you prediction correct?
8. Add F1 + F2.
F1 + F2 = _____________________________
9. How does the sum of the scale readings compare to the weight of the bar?
10. Add the measured weights of the bar and mass hanger? Include units.
Combined weight of bar and hanger = ______________________________
11. Predict how the readings on the scales will change (compared to the bar only) if the
mass hanger is suspended from the center of the bar.
Prediction: F1 = ___________F2 = ______________
PHYS 1405, p. 2/2
12. Suspend the mass hanger from the hook in the center and record the readings of both
spring scales.
F1 : _____________________________
F2 : _____________________________
13. Add F1 + F2.
F1 + F2 = _____________________________
14. How does the sum of the readings on the scale compare to the combined weight of the
bar and the hanger?
15. Move the mass hanger to a different hook and record both readings.
F1 : _____________________________
F2 : _____________________________
16. Did the readings on the individual scales change?
17. Add F1 + F2.
F1 + F2 = _____________________________
18. Did the sum of the readings change?
19. How does the sum of the readings compare to the combined weight of the mass and
hanger?
20. Move the mass hanger to a different hook (should be the only one you haven’t used)
and record both readings.
F1 : _____________________________
F2 : _____________________________
21. Did the readings on the individual scales change?
PHYS 1405, p. 3/3
22. Add F1 + F2.
F1 + F2 = _____________________________
23. Did the sum of the readings change?
24. How does the sum of the readings compare to the combined weight of the mass and
hanger?
25. What forces are acting down on the bar (i.e. pulling the bar down)?
26. What forces are acting up on the bar (i.e. pulling the bar up)?
27. Is the bar in equilibrium? Explain.
28. Complete the following statement.
The sum of the forces up on the bar ______________ the sum of the forces down on the
bar. Thus if an object is in equilibrium, the net sum of the forces acting on it is _______.
This is an alternative way of expressing Newton’s First Law of Motion and is known as
the Equilibrium Rule.
Work the following example.
29. A horizontal bar of weight 75 N is supported by two spring scales at either end. A
weight of 25 N is suspended from the bar. a) Draw a free body diagram. b) If one scale
reads 60 N, find the reading on the other spring scale.
PHYS 1405, p. 4/4