IB Physics SLGOHS

GRADE: ______/20

(1 point per step; 6 free points!)

The “Patty-cake” Explosion Lab: The Conservation of Momentum (Chp. 6)

Purpose

Investigate and compare the forces and change in momentum acting on different masses during an explosion.

Materials

  • Two piano dollies
  • Metric measuring tape
  • Masking tape (or chalk)
  • Stopwatch
  • Two persons (the greater the mass differential the better!)
  • One write-up for the group; ATTACH THIS COVER SHEET to the front of the write-up!

Procedure & Analysis

(Note: you should create a table, chart or other organizational structure to record every comment or calculation made in this lab.)

  1. Measure a distance 10 m apart and mark the distances with masking tape or chalk then measure the mid-point of these distances with masking tape or chalk.
  2. Place the piano dollies together with contact point over the mid-point of the measured distance (in the middle!).
  3. The persons sit on each piano dolly facing each other, almosttouching hands with arms coiled (bent ready to push).
  4. Guess which person will reach the mark first. Explain.
  5. On the timer’s count, the persons should push on each others hands creating a “patty-cake” explosion. Each will be propelled in opposite directions. The timer will measure the time it takes for one person to reach the marked distance (from the starting point in the middle).
  6. What can be said about the change in momentum for the system before and after the explosion? Explain.
  7. Calculate the mass of each person in kilograms 1 kg = 2.2 lbs (yes Bart you must trust that the person is telling the truth about their weight!).
  8. Calculate the velocity of the person whose time was measured by the timer: (v = d/t)
  9. Calculate the velocity of the person whose time was not measured by the timer using the equation for conservation of momentum:

m1v1i + m2v2i = m1v1f + m2v2f

  1. Suppose the time of contact for the push was measured to be 0.050 s, calculate the force of the “patty-cake” explosion using the impulse-momentum theorem: F∆t = mvf - mvi

Application

  1. Explain why a target shooter might prefer to shoot a more massive gun.
  1. A bullet with a mass of 5.00 x 10-3 kg is loaded into a gun. The loaded gun has a mass of 0.52 kg. The bullet is fired, causing the empty gun to recoil at a speed of 2.1 m/s. What is the speed of the bullet?

Conclusion

  1. Based on your observations in this lab, what can be said of the conservation of momentum?
  2. How does Newton’s 3rd law of motion play a role in this lab?

For Fun!!!

If there is time, have two people sit on the piano dollies holding a tight rope. Have one person pull on the rope. What is the effect? In what way is thisthe same as the “patty-cake” explosion?