The Rubber Band Lab

Purpose

The goal of this activity is to verify the physics relations that apply to a launched rubber band.  You will also get practice with kinematics, conservation of energy, work, projectile motion, etc.

Procedure for Launching

Stretch a rubber band around the end of a meter stick and stretch it a few centimeters.  The amount of stretch can be judged by noting the length of the rubber band before and after stretching.  Release the end of the band in order to launch it from the end.  CAUTION:  Take great care not to aim or launch the rubber band in the direction of any classmate.  And do not stretch the rubber band more than 6 cm.

Part A – Launch Velocity

Stretch the rubber band 3.0 cm (for example if the rubber band is 8.0 cm long then pull it back to the 11 cm mark.)  Launch the rubber band straight up and determine how far it rises above its release from the meter stick.  It is convenient to place the meter stick vertically on the floor next to a wall and observe the highest position attained by the rubber band relative to existing features on the wall.  You may want to try this several times using the same “3.0 cm stretch” to ensure a consistent value for the height to which it will rise. 

Measure the mass of the rubber band and the vertical displacement when fired vertically.

Now use the measured vertical displacement to determine the initial velocity.  Use conservation of energy.

Determine how much energy is obtained by the rubber band.

Calculate the average amount of force required to stretch the rubber band 3 cm.  Use the work energy theorem. 

Measured Values:		Calculated Values:
mass	vertical displacement	launch velocity	energy of rubber band	average force of rubber band

Part B – Projectile Motion

Use the launch velocity that was determined in part A to predict the range of the rubber band if it is fired horizontally from atop a table.  You will need to first measure the height of the table to enable you to make this determination.  Measure along the floor and place a target at your calculated range.  Then fire the rubber band horizontally from atop the table using the same “3 cm stretch” and see how well you can hit your target.  You may want to try several times.  Remember to use caution not to hit anyone!  What are some likely sources of error?

Measured Values:		Calculated Values:
table height, h	actual range, x	velocity, v (from part A)	predicted time in air, t	predicted range, x

Part C – Effect of Increasing the Stretch of the Band

Suppose the rubber band is stretched twice as far (i.e. it is stretched 6 cm), what will be the effect on the range if fired horizontally?  Assume the rubber band follows Hooke’s Law F = kx.  Use this assumption and previous observations to calculate a new launch energy and velocity and a new range and then test the prediction.

Launch:		Predicted Values:
“spring” constant, k	launch energy of rubber band	new velocity, v	predicted time in air, t	predicted range, x