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Physics 20: eLab Activity

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Energy in a Collision

Background:

A car travelling down the road is an excellent example of kinetic energy. If that car were to collide with another, what would happen to the kinetic energy? In a typical car collision, non-conservative forces are likely to cause the energy of the car collision system to change, preventing energy conservation.

In this lab, you will re-enact a car collision. The collision can be described as an "inelastic collision" in the sense that the two cars will stick together upon impact and travel off together after the collision. Your analysis of the velocity pre and post collision will determine if energy was conserved in the car collision system.

The Question:

Is kinetic energy conserved in an inelastic collision?

Variables:

This experiment involves these variables: velocity pre-collision, velocity post-collision, mass of carts. Identify and state the manipulated, responding, and controlled variables in this investigation.

Materials:

  • motion sensor
  • 250 g masses
  • USB link
  • triple beam balance or electronic scale
  • two dynamics carts with Velcro ends
  • dynamics system (if you do not have a dynamics system, you can substitute any smooth ramp)

Procedure:

Step 1:

Connect the USB Link to the computer. Connect the motion sensor to the USB link.

Place the motion sensor on the end of the dynamics track. Make sure the motion sensor is set to "cart". Place one cart near the motion sensor, and another further down the track. Arrange them so that when the carts meet each other, the Velcro on the ends of the carts will stick:

Step 2:

Configure the DataStudio software to display a position-time graph for the motion sensor. This is the default display for the sensor, so it may already be present on the computer screen. Use the setup window in DataStudio to increase the sampling rate of the motion sensor to 40 Hz.

Press START in DataStudio, then propel the cart that is nearer to the motion sensor toward the other cart. Keep your hand and finger low and away from the motion sensor, so that motion sensor detects the cart's motion, and not your hand.

After the carts have collided and moved down the track, press STOP in DataStudio.

Step 3:

In the position time graph, use the FIT tool to apply a linear fit to the data. You will do this twice; once for pre-collision, and once for post-collision. The pre- and post- collision areas on the graph should be easy to identify; look for a spot where the slope of the graph changes dramatically. This is where the collision took place:

The first linear fit slope indicates the velocity of the cart pre-collision, and the second linear fit slope indicates the velocity of the carts together, post-collision. Record these velocities in an appropriate table. Determine the mass of the two carts, and use the mass and velocity to calculate the kinetic energy before (initial) and after (final) collision. Calculate the percentage difference between the initial and final kinetic energy values.

Trials
Initial Velocity (m/s)
Final Velocity (m/s)
Initial KE (J)
Final KE (J)
KE % Difference
No addtional mass
250 g added to second cart
500 g added to second cart
250 g added to first cart
500 g added to first cart

Step 4:

Repeat steps 2 and 3 while adding mass to the system using cart masses.

As indicated in the table above, change the mass of the first cart, then the second cart, by 250 and 500 g.

Analyzing and Interpreting:

1. How did the kinetic energy compare before and after collision? If there was a significant difference between before and after, offer an explanation for the difference.
2. What was the effect of adding mass to the carts? Explain.

Forming Conclusions:

3. Write an answer to the following question:
  • Is kinetic energy conserved in an inelastic collision?

Applying and Connecting:

4. Many modern vehicles are equipped with "crumple zones" - these are areas of the vehicle that are designed to crush during impact. For example, cars in a head-on collision may have the engine compartments severely damaged, but the rest of the vehicles stay relatively intact.

Making a vehicle more rigid would mean the vehicle would be less damaged by a collision. So why do vehicle designers include crumple zones?

Extending:

5. Dynamics carts often come equipped with either springs or magnets to facilitate elastic collisions. Design and conduct an experiment that investigates the conservation of energy in an elastic collision.
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