Lab: Rubber Band Cannon Energy

Name:______Date:______

Objectives:

1)  Find the energy stored within the rubber band cannon for various displacements.

2)  Find the spring constant in the rubber band.

3)  Recognize the translation of energy in the cannon when firing.

4)  Calculate the distance needed to pull back to hit various targets.

Overview: In this lab your group is responsible for finding how fast the canon shoots for various rubber band displacements and then fire the cannon at a 45 degree angle to hit targets. To begin this process you will first have to find the spring constant of the rubber bands so that you may solve for the energy stored at different displacements. Using these energies you will then find the exit velocity of the projectile for the displacements. Finally, a series of kinematic calculations will have to be performed to find the firing speed so that the targets may be hit.

Pre-Lab Calc:

1)  Find the exit velocity of the following projectile if the cannon is angled at 45 degrees and shot 20 meters.

2)  Using the exit velocity, find the kinetic energy of the projectile if it has a mass of 200 grams.

3)  How far must the rubber band be pulled back if it has a spring constant of 35N/m?

Procedure:

Part 1: Finding the Energy in the Cannon

Rubber Band Displacement
(meters) / Force (N) / Spring Constant (N/m) / Elastic Potential Energy / Kinetic Energy at Release / Velocity (m/s)
.05 / This value does not change. The spring constant (k) is the slope of a force vs. displacement graph.
.10
.15
.20
.25
.30
.35
.40
.45
.50
.55
.60
.65
.70

1)  Find the mass of the rubber band projectile:______(kg)

2)  With the projectile detached from the rubber band, connect a 50N force scale to it and pull back until there is tension. Record the force for 15 cm (.15m), 20 cm, 25cm etc... Continue to record the force until 70cm or until the band will not strength any more.

3)  Plot Force (y-axis) vs. rubber band displacement (x-axis). Don’t forget the units!!!

4)  Find the slope of the line of the force vs. distance graph. This is your spring constant. Record this value in the table. Show the calculation below.

5)  Using PEe = 1/2kx2 equation, find the energy stored in the band for each displacement and the KE as well. Show one sample calculation below.

6) Using the Kinetic energy equation KE =1/2mv2, find the speed the projectile will leave for each displacement. Show one sample calculation.

7) Plot exit velocity vs rubber band displacement.

Force vs. Rubber Band Displacement:

Velocity Vs Rubber Band Displacement

Part 2: Calculate the Exit Velocity for the Targets

1)  Shot 1:

Calculation

How close were you? 1 2 3 4 5

2)  Shot 1:

Calculation

How close were you? 1 2 3 4 5

3)  Shot 1:

Calculation

How close were you? 1 2 3 4 5

4)  Shot 1:

Calculation

How close were you? 1 2 3 4 5

Questions and Analysis:

1)  Describe the translation of energy from being pulled back to the point where the projectile hits the target.

2)  To increase the velocity of the projectile would it be better to double the rubber band displacement or decrease the projectiles mass by half. EXPLAIN WHY.

3)  Discuss three sources of error in this lab. Operator errors do not count.