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Homework answers / question archive / UNIFORM CIRCULAR MOTION LAB Objective: In this experiment we will determine the centripetal force acting on an object undergoing UCM and determine its relationship to other parameters such as mass, radius, and velocity
UNIFORM CIRCULAR MOTION LAB
Objective:
In this experiment we will determine the centripetal force acting on an object undergoing UCM and determine its relationship to other parameters such as mass, radius, and velocity.
Available Equipment:
Circular motion apparatus, stopwatch, mass hanger, lab masses, broom, and bowling ball
Theory: 4
Newton’s first law states that an object will remain in motion with a constant velocity unless acted upon by a net external force. When a net force acts on an object, the velocity of that object changes, therefore the object accelerates. Since velocity is a vector, it has both magnitude (the speed) and direction. If either the magnitude or the direction of the velocity changes, the object accelerates. The direction of the velocity vector is always tangent to the trajectory and thus when an object travels in a circular path, the direction of the velocity constantly changes. If the velocity changes, the object accelerates, and therefore there must be a net force acting on that object. Newton’s second law states that F_{net} = ma and from this equation one can see that the net force has the same direction as the acceleration.
An object undergoes uniform circular motion (UCM) when it moves with constant speed in a circle.
The acceleration of an object in UCM is the result of the change only in the direction of the velocity vector, it is always oriented toward the center of the circle and it is called centripetal (or radial) acceleration.
Therefore, the net force acting on an object in UCM is oriented toward the center of the motion and it is also called centripetal force. This is not a new force, it is the net force (or the sum of all forces) acting on the object.
An object of mass M following a circular path of radius R with a constant speed v experiences a centripetal (or radial) acceleration given by a_{cp} = v^{2}/R and a net force of F_{net} = ma_{cp} = m. v^{2}/R oriented toward the center of the circle. If the object completes a given number of revolutions N in a total time t, then the period of motion is given by T = total time/number of revolutions = t/N. Since the path is circular and the speed is constant, one can calculate the speed of the object as the total path traveled (the circumference of the circle) in one revolution: v = 2πR/T.
In this experiment, an object of mass M is suspended by a string and connected to a horizontal spring, as shown in the figure below. When the entire apparatus rotates with constant speed, the mass M executes a UCM. The elastic force in the spring applies the centripetal force to the mass M and forces it to follow a circular trajectory. If the mass M is spun just right, the string is perfectly vertical and thus the tension force in the spring is balanced by the weight of the mass M, and therefore the only force left unbalanced is the elastic force, which plays the role of net force, or centripetal force in this case. A schematic representation of the rotating apparatus and a free body diagram are provided below.
M = supporting mass
A = arm
W = weight
E = mounting eyelet
S_{1}, S_{2} = points at which the spring is attached
d = stretching distance
R = radius of rotation
P = post
Schematic representation of the apparatus
Free body diagram for the rotating mass M
F tension
F elastic
F Weight
F_{tension} = F_{weight} => F_{tension} = mg
F_{elasyic} = Ma_{cp} = F_{elastic} = M* v^{2}/R
EXPERIMENTAL PROCEDURE:
As you follow the experimental procedure and collect data, enter the data in the Data and Analysis Section.
BE SURE TO WEAR EYE PROTECTION.
1. Examine the string supporting mass M. If it is worn or frayed, replace it.
2. Decide what radius you want for your circular path. The radius can be measured using the scale on the bottom of the apparatus. For the first trial, an Intermediate value is recommended, for example
1618 cm.
3. With the spring unhooked, adjust the length of arm A so that VM hangs directly over the chosen value of R. The string that holds the mass M exerts only a vertical force on M, which counterbalances the gravitational force, Mg.
4. The height of mass M should be checked so that the two points where the spring attaches (S_{1} and S_{2}) are at the same vertical height. If not, the position of the mounting eyelet, E, can be changed by its locking screw adjustment. This ensures that the spring will be horizontal and the force it exerts will also be horizontal.
5. When the spring is hooked at S_{1} and S_{2}, it will need to stretch in order for M to hang over the chosen value of R. The position of S_{1}, or equivalently the distance d, will determine how much stretch is needed. Initially you might adjust the S_{1} screw to a mid range setting.
Activity 1: Determine the elastic force acting on the mass M from uniform circular motion (rotating)
6. The string that suspends mass M is not in a vertical] position now.
7. Rotate the post, slowly at first, then faster, until mass M hangs over to the desired radius R. Be careful not to over spin and allow mass M to strike the pulley.
8. Maintain a steady speed so that mass M hangs over the desired radius R by periodically applying a little “tweak” with your fingers. (The large counter weight W is present to increase the rotational inertia of the apparatus so that once the proper speed is attained it is easy to maintain it.) Keep your eyes in the plane of motion, but far enough away so that the rotating mass does not hit you. BE SURE TO WEAR EYE PROTECTION.
9. Have your lab partner time the motion for a large number of revolutions (50 revolutions for example).
Then switch job assignments and repeat the measurement (your instructor may require you to use the average value obtained from taking several measurements for improved accuracy).
10. Do not adjust any of the parameters on the apparatus. Move on to Step 11.
Activity 2: Determine the elastic force acting on mass M by balancing forces (stationary)
11. Based on your result of the elastic force from Activity 1, calculate the expected mass m that will be necessary to hang off the pulley to make mass M hang vertically. Record your calculation in the Data and Analysis Section.
12. Attach one end of a light string to mass M, pass it over the pulley, and attach a mass hanger to the other end of the string (see the figure below). Add lab masses to the mass hanger (combined mass m) until mass M hangs over the desired value of R.
The force diagram is shown in this figure.
For object M: in the vertical direction F_{tension} = F_{weight} = Mg
In the horizontal direction F_{tension2} = F_{elastic }F_{elastic} = mg
For hanging mass m: in the vertical direction F_{tension2} = Mg
13. Repeat the experiment, Steps 612, for configurations 23. (Only adjust the radius R, do not adjust the length of the screw d)
14. For configuration 4, while keeping the radius, R, set at 18cm, adjust the length of the screw, d.
Uniform Circular Motion Lab: Data and Analysis Section
After collecting your data, fill out the questions below. Do not forget to include units for each one of the values written.
Activities 1&2: Determine the elastic force acting on mass M from uniform circular motion and by balancing forces
Start with Configuration 1, Perform Activity 1 and Z, fill in the tables below, then move on to configuration 2 and continue the measurements.
Mass M
Show all of your calculations in the data table
Configuration 1 

Radius R 
Activity 1 
# of revolutions N 
Time for N revolutions 
Period T 
Speed v 
Centripetal Accel., a_{cp} 
Centripetal force F_{cp} 
Elastic force F_{elastic} 

16 cm 
50 
46.7s 






Activity 2 
Calculated hanging mass m 
Measured Hanging mass, m 
Weight of measured hanging mass 
Elastic force F_{elastic} 


290g 
2.8N 
2.8N 

(Don’t change the length of screw d) Configuration 2 

17 cm 
Activity 1 
# of revolutions N 
Time for N revolutions 
Period T 
Speed v 
Centripetal Accel., a_{cp} 
Centripetal force F_{cp} 
Elastic force F_{elastic} 

50 
36s 






Activity2 
Calculated hanging mass m 
Measured Hanging mass, m 
Weight of measured hanging mass 
Elastic force F_{elastic} 


490g 
4.8N 
4.8N 

(Don't change the length of screw d) Configuration 3 

18 cm 
Activity 1 
# of revolutions N 
Time for N revolutions 
Period T 
Speed v 
Centripetal Accel., a_{cp} 
Centripetal force F_{cp} 
Elastic force F_{elastic} 

50 
31s 






Activity 2 
Calculated hanging mass m 
Measured Hanging mass, m 
Weight of measured hanging mass 
Elastic force F_{elastic} 


690g 
6.8N 
6.8N 

*For configuration 4, keep the radius R at 18cm, change the length of the screw, d.

How did you the length The length of d increased The length of d decreased Of the screw d? (circle one) 

Configuration 4 

*18 cm 
Activity 1 
# of revolutions N 
Time for N revolution s 
Period T 
Speed v 
Centripetal Accel., a_{cp} 
Centripetal force F_{cp} 
Elastic force F_{elastic} 
50 
27.5s 






Activity 2 
Calculated hanging mass m 
Measured Hanging mass, m 
Weight of measured hanging mass 
Elastic force F_{elastic} 


920g 
9.0N 
9.0N 
1. For each configuration, examine the elastic forces determined from each activity. How do the elastic forces from Activities 1 and 2 compare?
a. Explain why they would be similar.
b. Explain why the elastic forces would differ between activities.
2. (a) Look at the apparatus. Predict How the elastic force (centripetal force) would change if the radius R is changed while the length of the screw d is fixed?
(b) Look at your results in Configurations 1, 2, and 3, do they agree with your prediction in (a)?
If they don’t agree, you need to reconcile your prediction and your results.
3. (a) Look at the apparatus. Predict How the elastic force (centripetal force) would change if the length of the screw d is changed while the radius R is fixed?
(b) Look at your results in Configurations 3 and 4, do they agree with your prediction in (a)?
If they don’t agree, you need to reconcile your prediction and your results.
4. In Activity 2, how did the measured hanging mass m compare with the calculated values?
Calculate the percentage difference = ? calculated value  measured value/1/2(calculated value + measured value) x 100% for each configuration.
5. In determining the elastic force in Activity 2, only one experimental quantity is involved, the hanging mass m. Its value in your experiment has uncertainty. Uncertainty is also called error but not mistake. Uncertainty is due to the limit to the accuracy of experimental results.
a. On your last configuration of Activity 2, from your obtained hanging mass m value, add 1 gram to the hanger, do you see visible impact on the motion of the object M?
b. If not, keep adding a few grams till you observe visible impact on the motion of the object M.
The added a few grams is the uncertainty of the hanging mass m. Write down the result.
Uncertainty of hanging mass m =
c. How was the value of mass M measured? What is its major source of uncertainty? What is its estimated uncertainty?
d. Should the uncertainties in questions b and c above be the same?
6. Examine the experimental quantities that were involved in determining the elastic force in
Activities 1 and 2. Which method contains greater uncertainty? Explain why.