UNIT: CATAPULT

ACT-Based Science: Kinetic vs. Potential Energy in Catapult Launches

OBJECTIVE:

Students compare stored potential energy to kinetic energy output.

Materials Needed:

Rubber bands

Different-sized projectiles

Spring scale

Student Directions:

Goal:
You will measure and compare the amount of potential energy stored in a stretched rubber band before launch and the kinetic energy of a projectile after launch. This will help you understand energy conversion and how efficiently energy is transferred in a catapult system.

Step 1: Prepare Materials

Gather the following:

Rubber bands (same or different types)

A spring scale (to measure applied force)

Projectiles of varying mass (e.g., small rubber balls, marbles, foam balls)

Ruler or meter stick

Stopwatch (or use a phone timer)

Calculator

Data recording sheet or notebook

Step 2: Measure Potential Energy

Attach one rubber band to a stable catapult base.

Hook the spring scale to the rubber band and pull it back to a fixed stretch distance (e.g., 10 cm).

Record the force required to pull it to that distance in Newtons (N).

Calculate elastic potential energy (PE) using the formula:

where:

k is the spring constant (can be estimated by dividing force by distance stretched in meters)

x is the stretch distance in meters

Record all measurements in your notebook or a data table.

Step 3: Launch and Measure Kinetic Energy

Launch the projectile using the stretched rubber band.

Measure:

Projectile mass (kg) using a scale

Launch speed (m/s) using a motion tracker app or calculate using:

where distance is horizontal travel and time is measured with a stopwatch.

Calculate kinetic energy (KE) using the formula:

where:

m is the mass of the projectile in kg

v is the launch velocity in m/s

Step 4: Compare and Analyze

Create a comparison table for each test:

Trial	Stretch Distance (cm)	Force (N)	PE (J)	Mass (kg)	Velocity (m/s)	KE (J)	% Efficiency (KE ÷ PE × 100)
1	10	3.0	1.5	0.05	5.0	0.625	41.7%

Calculate efficiency:

Step 5: Discuss Your Results

With your group or class, discuss the following:

Which projectile had the most efficient energy transfer?

What might cause energy loss in the system (e.g., air resistance, friction)?

How could you improve the design to increase energy transfer efficiency?

Use science vocabulary:

Potential energy

Kinetic energy

Energy conversion

Efficiency

Force

Mass

Velocity

Extension (Optional):

Repeat the experiment with different stretch distances or projectile sizes.

Graph your data:

X-axis: Stretch distance or projectile mass

Y-axis: Kinetic energy or efficiency percentage