Lab Investigation 4: Conservation of Energy

Overview: This investigation is divided into two different parts. Each part engages the student in guided-inquiry activities.

In Part I, a spring-loaded cart is placed on an incline and the cart's motion is observed once the spring is released. Students design their own experiment to test how the angle of the ramp changes the motion of the cart for the same compression of the spring.

In Part II, students design their own experiment to

determine how changes in the compression of the spring change the amount of increase of the gravitational potential energy of the Earth-cart system.

Question: How does the compression of a spring affect themotion of a cart?

Challenge: In this investigation, students experiment with the concept of the conservation of energy by qualitatively investigating the relationship between elastic potential energy and gravitational potential energy. Students take a spring-loaded cart and release it so that it travels up a ramp. In addition to making observations and measurements, they make predictions as to what would happen if the angle of the ramp changed. Then, students experiment quantitatively with the relationship between the compression of the spring and the gravitational potential energy of the Earth-cart system. They do this by repeating measurements of the cart on the ramp for different compressions of the spring.

Background: The principle of conservation of energy states that energy cannot be created or destroyed, and the total energy of a closed system remains constant. The gravitational potential energy of the Earth-cart system can be calculated with the equation U_g=mgy. Remind them that if energy is indeed conserved, then the work on the spring from compressing it must give it some spring potential energy (U_s).

Energy exists in the compression of the spring (spring potential energy [U_s]), in the movement of the cart (kinetic energy [K]), and in the Earth-cart system (gravitational potential energy [U_g]).

Equipment:

Low-friction spring-loaded plunger cart

Ramp

Meterstick

Stopwatch

Assorted masses

Books or blocks (to create incline)

Poster-size whiteboards for sharing group work

Part I: Qualitative Investigation of Potential Energy

Design an experiment to qualitatively describe the relationship between compression of the spring and the gravitational potential energy.

Part II: Quantitative Investigation of Potential Energy

Design an experiment in which they collect data in order to quantitatively support their claim.

Part III(Bonus)(Done on a separate document): Improving the Experimental Design

1. What role does friction play in the experiment?

2. How can you minimize or take into account the frictional effects?

3. If the spring could only be compressed by two values (or if the spring could be compressed for multiple values), how would your experiment change?

4. How does the amount of compression of the plunger change the manner in which you measure the distance the cart moved and/or the maximum height?

5. How would the results change if the angle of the ramp were to change?

6. Should the experiment be done at multiple angles?

7. Which angle produces the most reliable results?

8. Do the wheels have an impact on the experimental results? Would the experiment work better with large wheels or small wheels?

9. Does the mass of the cart affect the experimental results? Which mass car would produce the most reliable results?