14.4: Unit 10 Lab Extension- Collisions

Inelastic Collisions

• lab sheet and writing utensil
• calculator
• “frictionless” track + two carts with velcro bumpers and magnetic or rubber bumpers
• two motion sensors + computer with sensor control and analysis software (or one motion sensor and one self-tracking motion cart).

Observation

Two objects colliding and sticking together looks just like an explosion in reverse.

Question

When two objects collide and stick together, also known as a perfectly inelastic collision, are kinetic energy and momentum conserved?

Hypothesis

Based on what you know about kinetic energy and momentum during an explosion, form a hypothesis about kinetic energy and momentum conservation during a perfectly inelastic collision.

Test

Now perform this experiment on your carts and track by giving the carts an initial velocity with a light push. Use the Velcro bumpers so that the carts stick together (you may instead use the magnetic bumpers arranged so that they attract). You may start with one cart stationary or give both carts an initial velocity.

Record the measured initial (before collision) and final (after collision) velocities of each cart here, being sure to record them as positive or negative according to your own choice of directions:

Measure and record the mass of each cart:

Momentum Analysis

Calculate the initial total momentum immediately before the collision.

Calculate the final total momentum immediately after the collision

Momentum Conclusion

Do the results above support or refute your momentum hypothesis. Explain.

Kinetic Energy Analysis

Calculate the initial total kinetic energy immediately before the collision.

Calculate the final total kinetic energy immediately after the collision

Kinetic Energy Conclusion

Does the result above support or refute your kinetic energy hypothesis. Explain.

If kinetic energy was not conserved, then where did it go?

Elastic Collisions

Observation

Now attach the magnetic bumpers to your so that they repel each other and then softly collide them. What do you observe about this collision in contrast to the perfectly inelastic collision?

Question

Does this type of collision conserve kinetic energy and momentum?

Hypothesis

Form a hypothesis about kinetic energy and momentum conservation during a collision between the carts with repelling magnetic bumpers.

Test

Softly collide the carts so that they “bounce” without actually touching Record the initial and final velocities for each cart here:

Record the mass for each cart here:

Momentum Analysis

Calculate the total initial momentum of the carts:

Calculate the total final momentum of the carts:

Momentum Conclusion

Do the results above support or refute your momentum hypothesis? Explain.

Kinetic Energy Analysis

Calculate the total initial kinetic energy of the carts:

Calculate the total final kinetic energy of the carts:

Kinetic Energy Conclusion

Do the results above support or refute your hypothesis? Explain.

Was this a perfectly elastic collision? Explain.

If not, how much kinetic energy was “missing?”

Calculate a coefficient of restitution for this collision.

If this were a perfectly elastic collision then we should be able to calculate the final velocities using the elastic collision equations found at the very bottom of this web page. Use your measured initial velocities and cart masses in the elastic collision equations to calculate the expected final velocity for each cart. Show your work below.

Calculate a percent difference between the expected and observed final velocities for each cart. Was this collision elastic enough that the elastic collision equations were still accurate to within 10 % ?