Extended objects can have translational and rotational motion. To describe the motion of an unconstrained object, such as a football in flight, it is most convenient to treat the motion as a combination of translational motion of the center of mass and rotational motion about the center of mass. The motion of an object constrained to rotate about a fixed axis, such as a door rotating about a vertical axis defined by its hinges, is often more conveniently described as a pure rotation about this axis. If this axis is not a symmetry axis, the object exerts a force on the axis, and an external force is required to keep the net force on the axis zero and the axis fixed.
In this laboratory you will become familiar with describing rotational motion. You will investigate the relationships between angular acceleration, moment of inertia, angular momentum and torque. Finally you will examine a simple model of a forearm.
Open a Microsoft Word document to keep a log of your experimental procedures, results and discussions. This log will form the basis of your lab report. Address the points highlighted in blue. Answer all questions.
Stand up and make a fist. Gently swing your arm in a vertical circle, pivoting at the shoulder and keeping the rest of the arm straight. Observe the motion of the arm. Pay attention to the motion of the elbow and of the hand.
Make the following measurements and record the data in your log:
Use your data to answer the following questions.
Motion of the hand
Motion of the elbow
Use an on-line simulation from the University of Colorado PhET group to
investigate the relationships between angular acceleration, moment of inertia,
angular momentum and torque.
Link to the simulation: http://phet.colorado.edu/en/simulation/torque
Click the Intro tab explore the interface.
Click the Torque tab.
Click the Moment of Inertia tab.
Click the Angular Momentum tab.
Take a meter
stick with a paper clip to which you can attach a weight. (If you cannot find
one, there are meter sticks, clips and weights in room 203, Nielsen Physics.)
Grab the stick on one end, and hold it horizontal, with the weight close to your
hand. Slide the weight out to other end. Does it become
harder to hold the stick horizontal? Why? You are holding up the same
In this experiment, we will use the meter stick as an artificial forearm.
The arm (excluding the shoulder and wrist) is composed to two major segments. The upper arm is attached to the shoulder. The forearm is attached to the upper arm at the elbow. Let us only concentrate on situations where the upper arm is in the vertical position. Then the forearm can move in two directions, upwards or downwards.
The upper-arm and the forearm can be thought of as two rigid levers, joined at the elbow which acts as a pivot point. Muscles attached to these levers provide the force required to articulate their motion. Since muscles can only provide force by contraction, they must always work in pairs. As the contracting muscle, (red), tightens, it applies a force to the arm. At the same time, the opposing muscle, (black), relaxes, thus allowing the arm to move. We have a pair of simple levers.
When we lift a load with our biceps muscle, this muscle does positive work.
The triceps muscle does positive work when we push down on something.
There is also a third force on the forearm, the force the upper arm's bone exerts on the forearm at the elbow joint. This force does no work, because the elbow joint is not moving.
3 clips are attached to a meter stick, one at each end, and one in the middle as shown in the figure below. The stick hangs from the force sensor. The force sensor is used to measure the combined weight of the stick and the clips in units of Newton (N). The minus sign indicates that the weight points downward. Enter the magnitude of the weight into the table below.
The center clip is moved to a position ~25 cm away from the left edge of the stick. Now the stick is no longer horizontal. You have to push down on the left end of the stick to bring the stick to a horizontal position. To find out how hard you have to push down to bring the stick to a horizontal position, a mass hanger is attached to the clip on the left end. Masses are loaded onto the mass hanger until the stick is horizontal. The combined weight of the masses and hanger equals the force with which you have to push down.
Record the magnitude of the reading of the force sensor when the stick is again horizontal in the table.
The table already lists the relevant distances.
|weight of meter stick and clips|
|force sensor reading when stick is again horizontal|
|weight suspended from left clip|
|distance from left clip to CM of the meter stick||0.47m|
|distance from force sensor hook to CM of meter stick||0.27 m|
|distance d from force sensor hook tothe left clip|
Paste your table into your log.
Modeling the meter stick as a forearm and the force sensor as the biceps, compare the force that the biceps has to exert to keep the forearm horizontal to the force it has to exert to just support the weight of the forearm. Comment on the relative magnitude of these forces.
List all the forces (magnitude and direction) acting on the forearm (meter stick) and calculate all the torques (magnitude and direction) exerted by those forces about the pivot point, when the stick is horizontal and in equilibrium. Do all the torques cancel out?
Convert your log into a lab report.
Laboratory 8 Report
Save your Word document (your name_lab8.docx), go to Canvas, Assignments, Lab 8, and submit your document.