Objectives

In this experiment you will use a specialized setup to measure the force that acts on a mass on a spinning motorized apparatus to keep it moving in a circle, and see how the force relates to the speed and radius of rotation.

It can be shown that the net force on an object in uniform circular motion has a net force acting toward the center of rotation with magnitude

Resources

• Centripetal Motion Apparatus
• Force sensor and photogate (for detecting rotations/sec)
• Lab power supply
• Circular masses
• Science Workshop interface and DataStudio software
• DataStudio setup file CentripetalMotion.ds

How to Use the Centripetal Motion Apparatus

Please read this carefully, as it can be helpful for your safety.

The apparatus you will use in this lab consists of a rotating, motorized arm to which weights can be attached. There is a weight holder at each end of the arm. One of the weight holders is fixed. The other slides freely along a groove in the rotating arm and is attached to a force sensor with a steel wire that runs over a pulley at the axis of the rotating arm. By attaching weights to the sliding mass holder, the centripetal acceleration provided by the tension in the steel wire when the arms rotate can be measured by the force sensor. A small flag underneath the arm passes through a photogate each rotation. Since each rotation is 360°, or 2π radians, the time between each photogate triggering gives the angular velocity of the arms.

The fixed mass holder is provided as a counterbalance to prevent the apparatus from wobbling out of control at high speeds. It is a good idea to have equal amounts of mass on each end of the arm. The fixed mass holder itself has a mass of 2.5 grams, and the sliding mass holder has a mass of 4.0 grams. The apparatus has the potential to spin very rapidly and cause injury. Before turning on the power supply for the first time, turn the voltage and current all the way down. Then, turn on the power supply, turn the current knob to 10-20%, and slowly increase the voltage to get a feel for the speed of the rotating arms. Attach weights securely when using the apparatus.