Force
Purpose
 Analyze force vectors in several situations of suspended
masses.
 Predicting x and y components of forces (Free body diagrams:
Done in the homework).
 Measuring x and y components of the forces.
 Encounter static and dynamic equilibrium situations.
• Static: acceleration = 0
• Dynamic: acceleration  0
Force
Equipment
Two force sensors (static situations)
One force sensor and one photo gate (dynamic situation)
Force
Static Suspension of Mass on Strings
tare buttons
Force Sensors must
be aligned with strings
for proper measurement
of the force.
 Force sensors must be aligned and mass must be temporarily removed
while taring the sensor.
 Taring must be redone whenever force sensor is realigned.
 Tare button is on the side of the force sensor.
Force
WRONG (not aligned)
Force sensor will not
measure the proper
tension in the string.
Force
Dynamic Situation (Swinging Mass)
Force sensor oriented so
string is aligned at low
point of motion.
Photogate at
low point of
motion
Photogate triggers force measurement at the right moment and measures
velocity (actually measures time and calculates v).
Force
Dynamic Situation: Free Body Diagram
At the lowest point of the swing:
F
F
Q
r
x
0
y
 T  mg  mac
T
y
q
mg sin q
mg cos q
ac = v2/r (direction of a is in the
direction of the net force)
mg
W=mg
x
(Note: This is just Newton’s second law: F = ma.
Only T and W are forces! “ m ac“ is not a force!)
Force
Hit the “Scale to
Fit” button here
if you cannot see
oscillations (F <0
if done correctly).
Data acquisition
starts here. This
is also the moment
in which the mass
swings through
the photo gate
for the first time.
Follow instructions
Force
Hints
 In the static situations (no acceleration) we always have:
F
x
0
and
F
y
0
 In the free body diagram the forces must always add up to
zero.
 For the dynamic situation you need only one force sensor: It
must be the force sensor that is plugged into port A of the
750 interface.
 Tare the force sensor before you hang the swinging mass on it.
Force
Hints
In Part I you will be asked to calculate the % difference between your
measured g and the accepted value of g (use 9.8 m/s2 for accepted value):
% difference 
g (accepted )  g (measured )
 100
g (accepted )
In Part III you will also be asked to calculate the % uncertainty in your
measurement of tension. With this we actually meant again the %
deviation from the expected (calculated) value of T:
"% uncertainty" 
T (calculated )  T (measured )
 100
T (calculated )
Force
Hints: Measuring the Angle of the String
String
Protractor
0°
30°
60°
90°
Q  90 - measured angle
In this case:
Q  90 - 60  30
small weight (acts as plumb level)
Force
Hints: Measuring the Angle of the String – Alternative Method
String
Q  measured angle
In this case:
Q  30